Tag: Climate change

The end-Triassic mass extinction and ocean acidification

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Triassic reef limestones in the Dolomites of northern Italy. Credit: © Matteo Volpone

Four out of six mass extinctions that ravaged life on Earth during the last 300 Ma coincided with large igneous events marked by basaltic flood volcanism. But not all such bursts of igneous activity match significant mass extinctions. Moreover, some rapid rises in the rate of extinction are not clearly linked to peaks in igneous activity. Another issue in this context is that ‘kill mechanisms’ are generally speculative rather than based on hard data. Large igneous events inevitably emit very large amounts of gases and dust-sized particulates into the atmosphere. Carbon dioxide, being a greenhouse gas, tends to heat up the global climate, but also dissolves in seawater to lower its pH. Both global warming and more acidic oceans are possible ‘kill mechanisms’. Volcanic emission of sulfur dioxide results in acid rain and thus a decrease in the pH of seawater. But if it is blasted into the stratosphere it combines with oxygen and water vapour to form minute droplets of sulfuric acid. These form long-lived haze, which reflects solar energy beck into space. Such an increased albedo therefore tends to cool the planet and create a so-called ‘volcanic winter’. Dust that reaches the stratosphere reduces penetration of visible light to the surface, again resulting in cooling. But since photosynthetic organisms rely on blue and red light to power their conversion of CO­2­ and water vapour to carbohydrates and oxygen, these primary producers at the base of the marine and terrestrial food webs decline. That presents a fourth kill mechanism that may trigger mass extinction on land and in the oceans: starvation.

Palaeontologists have steadily built up a powerful case for occasional mass extinctions since fossils first appear in the stratigraphic record of the Phanerozoic Eon. Their data are simply the numbers of species, genera and families of organisms preserved as fossils in packages of sedimentary strata that represent roughly equal ‘parcels’ of time (~10 Ma). Mass extinctions are now unchallengeable parts of life’s history and evolution. Yet, assigning specific kill mechanisms involved in the damage that they create remains very difficult. There are hypotheses for the cause of each mass extinction, but a dearth of data that can test why they happened. The only global die-off near hard scientific resolution is that at the end of the Cretaceous. The K-Pg (formerly K-T) event has been extensively covered in Earth-logs since 2000. It involved a mixture of global ecological stress from the Deccan large igneous event spread over a few million years of the Late Cretaceous, with the near-instantaneous catastrophe induced by the Chicxulub impact, with a few remaining dots and ticks needed on ‘i’s and ‘t’s. Other possibilities have been raised: gamma-ray bursts from distant supernovae; belches of methane from the sea floor; emissions of hydrogen sulfide gas from seawater itself during ocean anoxia events; sea-level changes etc.

The mass extinction that ended the Triassic (~201 Ma) coincides with evidence for intense volcanism in South and North America, Africa and southern Europe, then at the core of the Pangaea supercontinent. Flood basalts and large igneous intrusions – the Central Atlantic Magmatic Province (CAMP) – began the final break-up of Pangaea. The end-Triassic extinction deleted 34% of marine genera. Marine sediments aged around 201 Ma reveal a massive shift in sulfur and carbon isotopes in the ocean that has been interpreted as a sign of acute anoxia in the world’s oceans, which may have resulted in massive burial of oxygen-starved marine animal life. However, there is no sign of Triassic, carbon-rich deep-water sediments that characterise ocean anoxia events in later times. But it is possible that bacteria that use the reduction of sulfate (SO42-) to sulfide (S2-) ions as an energy source for them to decay dead organisms, could have produced the sulfur isotope ‘excursion’. That would also have produced massive amounts of highly toxic hydrogen sulfide gas, which would have overwhelmed terrestrial animal life at continental margins. The solution ofH2S in water would also have acidified the world’s oceans.

Molly Trudgill of the University of St Andrews, Scotland and colleagues from the UK, France, the Netherlands, the US, Norway, Sweden and Ireland set out to test the hypothesis of end-Triassic oceanic acidification (Trudgill, M. and 24 others 2025. Pulses of ocean acidification at the Triassic–Jurassic boundary. Nature Communications, v. 16, article 6471; DOI: 10.1038/s41467-025-61344-6). The team used Triassic fossil oysters from before the extinction time interval. Boron-isotope data from the shells are a means of estimating variations in the pH of seawater. Before the extinction event the average pH in Triassic seawater was about the same as today, at 8.2 or slightly alkaline. By 201 Ma the pH had shifted towards acidic conditions by at least 0.3: the biggest detected in the Phanerozoic record. One of the most dramatic changes in Triassic marine fauna was the disappearance of reef limestones made by the recently evolved modern corals on a vast scale in the earlier Triassic; a so-called ‘reef gap’ in the geological record. That suggests a possible analogue to the waning of today’s coral reefs that is thought to be a result of increased dissolution of CO2 in seawater and acidification, related to global greenhouse warming. Using the fossil oysters, Trudgill et al. also sought a carbon-isotope ‘fingerprint’ for the source of elevated CO2, finding that it mainly derived from the mantle, and was probably emitted by CAMP volcanism. So their discussion centres mainly on end-Triassic ocean acidification as an analogy for current climate change driven by CO2 largely emitted by anthropogenic burning of fossil fuels. Nowhere in their paper do they mention any role for acidification by hydrogen sulfide emitted by massive anoxia on the Triassic ocean floor, which hit the scientific headlines in 2020 (see earlier link).

Modelling climate change since the Devonian

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A consortium of geoscientists from Australia, Britain and France, led by Andrew Merdith of the University of Adelaide examines the likely climate cooling mechanisms that may have set off the two great ‘icehouse’ intervals in the last 541 Ma (Merdith, A.S. et al. 2025. Phanerozoic icehouse climates as the result of multiple solid-Earth cooling mechanisms. Science Advances, v. 11, article eadm9798: DOI: 10.1126/sciadv.adm9798). They consider the first to be the global cooling that began in the latter part of the Devonian culminating in the Carboniferous-Permian icehouse. The second is the Cenozoic global cooling to form the permanent Antarctic ice cap around 34 Ma and culminated in cyclical ice ages on the northern continents after 2.4 Ma during the Pleistocene. They dismiss the 40 Ma long, late Ordovician to early Silurian glaciation that left its imprint on North Africa and South America –  then combined in the Gondwana supercontinent. The data about two of the parameters used in their model – the degree of early colonisation of the continents by plants and their influence on terrestrial weathering are uncertain in that protracted event.  Yet the Hirnantian glaciation reached 20°S at its maximum extent in the Late Ordovician around 444 Ma to cover about a third of Gondwana: it was larger than the present Antarctic ice cap. For that reason, their study spans only Devonian and later times.

Fluctuation in evidence for the extent of glacial conditions since the Devonian: the ‘ice line’ is grey. The count of glacial proxy occurrences in each 10° of latitude through time is shown in the colour key. Credit: Merdith et al., Fig 2A.

Merdith et al. rely on four climatic proxies. The first of these comprises indicators of cold climates, such as glacial dropstones, tillites and evidence in sedimentary rocks of crystals of hydrated calcium carbonate (ikaite – CaCO3.6H2O) that bizarrely forms only at around 0°C . From such occurrences it is possible to define an ‘ice line’ linking different latitudes through geological time. Then there are estimates of global average surface temperature; low-latitude sea surface temperature; and estimates of atmospheric CO2. The ‘ice-line’ data records an additional, long period of glaciation in the Jurassic and early Cretaceous, but evidence does not extend to latitudes lower than 60°. It is regarded by Merdith et al. as an episode of ‘cooling’ rather than an ‘icehouse’. Their model assesses sources and sinks of CO2­ since the Devonian Period.

The main natural source of the principal greenhouse gas CO2 is degassing through volcanism expelled from the mantle and breakdown of carbonate rock in subducted lithosphere. Natural sequestration of carbon involves weathering of exposed rock that releases dissolved CO2 and ions of calcium and magnesium.   A recently compiled set of plate reconstructions that chart the waxing and waning of tectonics since the Devonian Period allows them to model the tectonically driven release of carbon over time, with time scales on the order of tens to hundreds of Ma. The familiar Milanković forcing cycles on the order of tens to hundreds of ka are thus of no significance in Merdith et al.’s  broader conception of icehouse episodes  Their modelling shows high degassing during the Cretaceous, modern levels during the late Palaeozoic and early Mesozoic, and low emissions during the Devonian. The model also suggests that cooling stemmed from variations in the positions and configuration of continents over time.  Another crucial factor is the tempo of exposure of rocks that are most prone to weathering. The most important are rocks of the ocean lithosphere incorporated into the continents to form ophiolite masses. The release of soluble products of weathering into ocean basins through time acts as a fluctuating means of ‘fertilising’ so that more carbon can be sequestered in deep sediments in the form of organisms’ unoxidised tissue and hard parts made of calcium carbonates and phosphates. Less silicate weathering results in a boost to atmospheric CO2.

Only two long, true icehouse episodes emerge from the empirical proxy data, expressed by the ‘ice-line’ plots. Restricting the modelling to single global processes that might be expected to influence degassing or carbon sequestration produces no good fits to the climatic proxy data. Running the model with all the drivers “off” produces more or less continuous icehouse conditions since the Devonian. The model’s climate-related outputs thus imply that many complex processes working together in syncopation may have driven the gross climate vagaries over the last 400 Ma or so. A planet of Earth’s size without such complexity would throughout that period have had a high-CO2 warm climate. According to Andrew Merdith its fluctuation from greenhouse to icehouse conditions in the late Palaeozoic and the Cenozoic were probably due to “coincidental combination of very low rates of global volcanism, and highly dispersed continents with big mountains, which allow for lots of global rainfall and therefore amplify reactions that remove carbon from the atmosphere”.

Geological history is, almost by definition, somewhat rambling. So, despite despite the large investment in seeking a computed explanation of data drawn from the record, the outcome reflects that in a less than coherent account. To state that many complex processes working at once may have driven climate vagaries over the last 400 Ma or so, is hardly a major advance: palaeoclimatologists have said more or less the same for a couple of decades or more, but have mainly proposed single driving mechanisms. One aspect of Merdith et al.’s  results seems to be of particular interest. ‘Icehouse’ conditions seem to be rare events interspersed with broader ice-free periods. We evolved within the mammal-dominated ecosystems on the continents during the latest of these anomalous climatic episodes. And we and those ecosystems now rely on a cool world. As the supervisor of the project commented, ‘Over its long history, the Earth likes it hot, but our human society does not’.

Readers may like to venture into how some philosophers of science deal with a far bigger question; ‘Is intelligent life a rare, chance event throughout the universe?’ That is, might we be alone in the cosmos? In the same issue of Science Advances is a paper centred on just such questions (Mills, D.B. et al. 2025. A reassessment of the “hard-steps” model for the evolution of intelligent life. Science Advances, v. 11, article eads5698; DOI: 10.1126/sciadv.ads5698). It stems from cosmologist Brandon Carter’s ‘Anthropic Principle’ first developed at Nicolas Copernicus’s 500th birthday celebrations in 1973. This has since been much debated by scientists and philosophers – a gross understatement as it knocks the spots off the Drake Equation. To take the edge off what seems to be a daunting task, Mills et al. consider a corollary of the Anthropic Principle, the ‘hard steps model’. That, in a nutshell, postulates that the origin of humanity and its ability to ponder on observations of the universe required a successful evolutionary passage through a number of hard steps. It predicts that such intelligence is ‘exceedingly rare’ in the universe. Icehouse conditions are respectable candidates for evolutionary ‘hard steps’, and in the history of Earth there have been five of them.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

The earliest known human-Neanderthal relations

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The first anatomically modern humans (AMH) known to have left their remains outside of Africa lived about 200 ka ago in Greece and the Middle East. They were followed by several short-lived migrations that got as far as Europe, leaving very few fossils or artefacts. Over that time Neanderthals were continually present. Migration probably depended on windows of opportunity controlled by pressures from climatic changes in Africa and sea level being low enough to leave their heartland: perhaps as many as 8 or 9 before 70 ka, when continuous migration out of Africa began. The first long-enduring AMH presence in Europe began around 47 ka ago.

For about 7 thousand years thereafter – about 350 generations – AMH and Neanderthals co-occupied Europe. Evidence is growing that the two groups shared technology. After 40 ka there are no tangible signs of Neanderthals other than segments of their DNA that constitute a proportion of the genomes of modern non-African people. They and AMH must have interbred at some time in the last 200 ka until Neanderthals disappeared. In the same week in late 2024 two papers that shed much light on that issue were published in the leading scientific journals, Nature and Science, picked up by the world’s news media. Both stem from research led by researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. They focus on new DNA results from the genomes of ancient and living Homo sapiens. One centred on 59 AMH fossils dated between 45 and 2.2 ka and 275 living humans (Iasi, L. M. N. and 6 others 2024. Neanderthal ancestry through time: Insights from genomes of ancient and present-day humans Science, v. 386, p. 1239-1246: DOI: 10.1126/science.adq3010. PDF available by request to leonardo_iasi@eva.mpg.de). The other concerns genomes recovered from seven AMH individuals from the oldest sites in Germany and Czechia. (Sümer, A. P. and 44 others 2024. Earliest modern human genomes constrain timing of Neanderthal admixture. Nature, online article; DOI: 10.1038/s41586-024-08420-x. PDF available by request to arev_suemer@eva.mpg.de ).

Leonardo Iasi and colleagues from the US and UK examined Neanderthal DNA segments found in more than 300 AMH  genomes, both ancient and in living people, by many other researchers. Their critical focus was on lengths of such segments. Repeated genetic recombination in the descendants of those individuals who had both AMH and Neanderthal parents results in shortening of the lengths of their inherited Neanderthal DNA segments. That provides insights into the timing and duration of interbreeding. The approach used by Iasi ­et al­. used sophisticated statistics to enrich their analysis of Neanderthal-human gene flow. They were able to show that the vast majority of Neanderthal inheritance stems from a single period of such gene flow into the common ancestors of all living people who originated outside Africa. This genetic interchange seems to have lasted for about 7 thousand years after 50 ka. This tallies quite closely with the period when fossil and cultural evidence supports AMH and Neanderthals having co-occupied Europe.

Reconstruction of the woman whose skull was found at Zlatý kůň, Czechia. Credit: Tom Björklund / Max Planck Institute for Evolutionary Anthropology.

The other study, led by Arev Sümer,  has an author list of 44 researchers from Germany, the US,  Spain, Australia, Israel, the UK, France, Sweden, Denmark and Czechia. The authors took on a difficult task: extracting full genomes from seven of the oldest AMH fossils found in Europe, six from a cave Ranis in Germany and one from about 230 km away at Zlatý kůň in Czechia. Human bones, dated between 42.2 and 49.5 ka, from the Ranis site had earlier provided mitochondrial DNA that proved them to be AMH. A complete female skull excavated from Czechia site, dated at 45 ka had previously yielded a high quality AMH genome. Interestingly that carried variants associated with dark skin and hair, which perhaps reflect African origins. Neanderthals probably had pale skins and may have passed on to the incomers genes associated with more efficient production of vitamin D in the lower light levels of high latitudes and maybe immunity to some diseases. Both sites contain a distinct range of artefacts known as the Lincombian-Ranisian-Jerzmanowician technocomplex. This culture was once regarded as having been made by Neanderthals, but is now linked by the mtDNA results to early AMH. Such artefacts occur across central and north-western Europe. The bones from both sites are clearly important in addressing the issue of Neanderthal-AMH cultural and familial relationships.

The new, distinct genetic data from the Ranis and Zlatý kůň individuals reveals a mother and her child at Ranis. The female found at Zlatý kůň had a fifth- to sixth-degree genetic relationship with Ranis individuals: she may have been their half first cousin once removed. This suggests a wider range of communications than most people in medieval Europe would have had. The data from both sites suggests that the small Ranis-Zlatý kůň population – estimated at around 200 individuals – diverged late from the main body of AMH who began to populate Asia and Australasia at least 65 ka ago. Their complement of Neanderthal genetic segments seems to have originated during their seven thousand-year presence in Europe. Though they survived through 350 generations it seems that their genetic line was not passed on within and outside of Europe. They died out, perhaps during a sudden cold episode during the climatic decline towards the Last Glacial Maximum. We know that because their particular share of the Neanderthal genome does not crop up in the wider data set used by Iasi et al., neither in Europe and West Asia nor in East Asia. That they survived for so long may well have been due to their genetic inheritance from Neanderthals that made them more resilient to what, for them, was initially an alien environment. It is not over-imaginative to suggest that both populations may have collaborated over this period. But neither survived beyond about 40 ka..

Widely publicised as they have been, the two papers leave much more unanswered than they reveal. Both the AMH-Neanderthal relationship and the general migration out of Africa are shown to be more complex than previously thought by palaeoanthropologists. For a start, the descendants today of migrants who headed east carry more Neanderthal DNA that do living Europeans, and it is different. Where did they interbreed? Possibly in western Asia, but that may never be resolved because warmer conditions tend to degrade genetic material beyond the levels that can be recovered from ancient bones. Also, some living people in the east carry both Neanderthal and Denisovan DNA segments. Research Centres like the Max Planck Institute for Evolutionary Anthropology will clearly offer secure employment for some time yet!

How changes in the Earth System have affected human evolution, migration and culture

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Refugees from the Middle East migrating through Slovenia in 2015. Credit: Britannica

During the Pliocene (5.3 to 2.7 Ma) there evolved a network of various hominins, with their remains scattered across both the northern and southern parts of that continent. The earliest, though somewhat disputed hominin fossil Sahelanthropus tchadensis hails from northern Chad and lived  around 7 Ma ago, during the late Miocene, as did a similarly disputed creature from Kenya Orrorin tugenensis (~5.8 Ma). The two were geographically separated by 1500 km, what is now the Sahara desert and the East African Rift System.  The suggestion from mtDNA evidence that humans and chimpanzees had a common ancestor, the uncertainty about when it lived (between 13 to 5 Ma) and what it may have looked like, let alone where it lived, makes the notion debateable. There is even a possibility that the common ancestor of humans and the other anthropoid apes may have been European. Its descendants could well have crossed to North Africa when the Mediterranean Sea had been evaporated away to form the thick salt deposits that now lie beneath it: what could be termed the ‘Into Africa’ hypothesis. The better known Pliocene hominins were also widely distributed in the east and south of the African continent. Wandering around was clearly a hominin predilection from their outset. The same can be said about humans in the general sense (genus Homo) during the Early Pleistocene when some of them left Africa for Eurasia. Artifacts dated at 2.1 Ma have been found on the Loess Plateau of western China, and Georgia hosts the earliest human remains known from Eurasia. Since them H. antecessor, heidelbergensis, Neanderthals and Denisovans roamed Eurasia. Then, after about 130 ka, anatomically modern humans progressively populated all continents, except Antarctica, to their geographic extremities and from sea level to 4 km above it.

There is a popular view that curiosity and exploration are endemic and perhaps unique to the human line: ‘It’s in our genes’. But even plants migrate, as do all animal species. So it is best to be wary of a kind of hominin exceptionalism or superior motive force. Before settled agriculture, simply diffusion of populations in search of sustenance could have achieved the enormous migrations undertaken by all hominins: biological resources move and hunter gatherers follow them. The first migration of Homo erectus from Africa to northern China by way of Georgia seems to taken 200 ka at most and covered about ten thousand kilometres: on average a speed of only 50 m per year! That achievement and many others before and later were interwoven with the evolution of brain size, cognitive ability, means of communication and culture. But what were the ultimate drivers? Two recent papers in the journal Nature Communications make empirically-based cases for natural forces driving the movement of people and changes in demography.

The first considers hominin dispersal in the Palaearctic biogeographic realm: the largest of eight originally proposed by Alfred Russel Wallace in the late 19th century that encompasses the whole of Eurasia and North Africa (Zan, J. et al. 2024. Mid-Pleistocene aridity and landscape shifts promoted Palearctic hominin dispersals. Nature Communications, v. 15, article 10279; DOI: 10.1038/s41467-024-54767-0). The Palearctic comprises a wide range of ecosystems: arid to wet, tropical to arctic. After 2 Ma ago, hominins moved to all its parts several times. The approach followed by Zan et al. is to assess the 3.6 Ma record of the thick deposits of dust carried by the perpetual westerly winds that cross Central Asia. This gave rise to the huge (635,000 km2) Loess Plateau. At least 17 separate soil layers in the loess have yielded artefacts during the last 2.1 Ma. The authors radiocarbon dated the successive layers of loess in Tajikistan (286 samples) and the Tarim Basin (244 samples) as precisely as possible, achieving time resolutions of 5 to 10 ka and 10 to 20 ka respectively. To judge variations in climate in these area they also measured the carbon isotopic proportions in organic materials preserved within the layers. Another climate-linked metric that Zan et al. is a time series showing the development of river terraces across Eurasia derived from the earlier work of many geomorphologists. The results from those studies are linked to variations through time in the numbers of archaeological sites across Eurasia that have yielded hominin fossils, stone tools and signs of tool manufacture, many of which have been dated accurately.

The authors use sophisticated statistics to find correlations between times of climatic change and the signs of hominin occupation. Episodes of desertification in Palaearctic Eurasia clearly hindered hominins’ spreading across the continent either from west to east of vice versa. But there were distinct, periodic windows of climatic opportunity for that to happen that coincide with interglacial episodes, whose frequency changed at the Mid Pleistocene Transition (MPT) from about 41 ka to roughly every 100 ka. That was suggested in 2021 to have arisen from an increased roughness of the rock surface over which the great ice sheets of the Northern Hemisphere moved. This suppressed the pace of ice movement so that the 41 ka changes in the tilt of the Earth’s rotational axis could no longer drive climate change during the later Pleistocene, despite the fact that the same astronomical influence continued. The succeeding ~100 ka pulsation may or may not have been paced by the very much weaker influence of Earth changing orbital eccentricity. Whichever, after the MPT climate changes became much more extreme, making human dispersal in the Palearctic realm more problematic. Rather than hominin’s evolution driving them to a ‘Manifest Destiny’ of dominating the world vastly larger and wider inorganic forces corralled and released them so that, eventually, they did.

Much the same conclusion, it seems to me, emerges from a second study that covers the period since ~ 9 ka ago when anatomically modern humans transitioned from a globally dominant hunter-gatherer culture to one of ‘managing’ and dominating ecosystems, physical resources and ultimately the planet itself. (Wirtz, K.W et al. 2024. Multicentennial cycles in continental demography synchronous with solar activity and climate stability. Nature Communications, v. 15, article 10248; DOI: 10.1038/s41467-024-54474-w). Like Zan et al., Kai Wirtz and colleagues from Germany, Ukraine and Ireland base their findings on a vast accumulated number (~180,000) of radiocarbon dates from Holocene archaeological sites from all inhabited continents. The greatest number (>90,000) are from Europe. The authors applied statistical methods to judge human population variations since 11.7 ka in each continental area. Known sites are probably significantly outweighed by signs of human presence that remain hidden, and the diligence of surveys varies from country to country and continent to continent: Britain, the Netherlands and Southern Scandinavia are by far the best surveyed. Given those caveats, clearly this approach gives only a blurred estimate of population dynamics during the Holocene. Nonetheless the data are very interesting.

The changes in population growth rates show distinct cyclicity during the Holocene, which Wirtz et al. suggest are signs of booms and busts in population on all six continents. Matching these records against a large number of climatic time series reveals a correlation. Their chosen metric is variation in solar irradiance: the power per unit area received from the Sun. That has been directly monitored only over a couple of centuries. But ice cores and tree rings contain proxies for solar irradiance in the proportions of the radioactive isotopes 10Be and 14C contained in them respectively. Both are produced by the solar wind of high-energy charged particles (electrons, protons and helium nuclei or alpha particles) penetrating the upper atmosphere. The two isotopes have half-lives long enough for them to remain undecayed and thus detectable for tens of thousand years. Both ice cores and tree rings have decadal to annual time resolutions. Wirtz et al. find that their crude estimates of booms and busts in human populations during the Holocene seem closely to match variations in solar activity measured in this way. Climate stability favours successful subsistence and thus growth in populations. Variable climatic conditions seem to induce subsistence failures and increase mortality, probably through malnutrition.

A nice dialectic clearly emerges from these studies. ‘Boom and bust’ as regards populations in millennial and centennial to decadal terms stem from climate variations. Such cyclical change thus repeatedly hones natural selection among the survivors, both genetically and culturally, increasing their general fitness to their surroundings. Karl Marx and Friedrich Engels would have devoured these data avidly had they emerged in the 19th century. I’m sure they would have suggested from the evidence that something could go badly wrong – negation of negation, if readers care to explore that dialectical law further . . . And indeed that is happening. Humans made ecologically very fit indeed in surviving natural pressures are now stoking up a major climatic hiccup, or rather the culture and institutions that humans have evolved are doing that.

Divining the possible climatic impacts of slowing North Atlantic current patterns

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Meltwater channels and lake on the surface of the Greenland ice sheet

In August 2024 Earth-Logs reported on the fragile nature of thermohaline circulation of ocean water. The post focussed on the Atlantic Meridional Overturning Circulation (AMOC), whose fickle nature seems to have resulted in a succession of climatic blips during the last glacial-interglacial cycle since 100 ka ago. They took the form of warming-cooling cycles known as Dansgaard-Oeschger events, when the poleward movement of warm surface water in the North Atlantic Ocean was disrupted. An operating AMOC normally drags northwards warm water from lower latitudes, which is more saline as a result of evaporation from the ocean surface there. Though it gradually cools in its journey it remains warmer and less dense than the surrounding surface water through which it passes: it effectively ‘floats’. But as the north-bound, more saline stream steadily loses energy its density increases. Eventually the density equals and then exceeds that of high-latitude surface water, at around 60° to 70°N, and sinks. Under these conditions the AMOC is self-sustaining and serves to warm the surrounding land masses by influencing climate. This is especially the case for the branch of the AMOC known as the Gulf Stream that today swings eastwards to ameliorate the climate of NW Europe and Scandinavia as far as Norway’s North Cape and into the eastern Arctic Ocean.

The suspected driving forces for the Dansgaard-Oeschger events are sudden massive increases in the supply of freshwater into the Atlantic at high northern latitudes, which dilute surface waters and lower their density. So it becomes more difficult for surface water to become denser on being cooled so that it can sink to the ocean floor. The AMOC may weaken and shut down as a result and so too its warming effect at high latitudes. It also has a major effect on atmospheric circulation and moisture content: a truly complicated climatic phenomenon. Indeed, like the Pacific El Niño-Southern Oscillation (ENSO), major changes in AMOC may have global climatic implications.  QIyun Ma of the Alfred Wegner Institute in Bremerhaven, Germany and colleagues from Germany, China and Romania have modelled how the various possible locations of fresh water input may affect AMOC (Ma, Q. et al. 2024. Revisiting climate impacts of an AMOC slowdown: dependence on freshwater locations in the North Atlantic. Science Advances, v. 10, article eadr3243; DOI: 10.1126/sciadv.adr3243). They refer to such sudden inputs as ‘hosing’!

Location of the 4 regions in the northern North Atlantic used by Ma et al. in their modelling of AMOC: A Labrador Sea; B Irminger Basin; C NE Atlantic; D Nordic Seas. Colour chart refers to current temperature. Solid line – surface currents, dashed line – deep currents

First, the likely consequences under current climatic conditions of such ‘hosings’ and AMOC collapses are: a rapid expansion of the Arctic Ocean sea ice; delayed onset of summer ice-free conditions; southward shift of the Intertropical Convergence Zone (ITCZ) –  a roughly equatorial band of low pressure where the NE and SE trade winds converge, and the rough location of the sometimes windless Doldrums. There have been several attempts to model the general climatic effects of an AMOC slowdown. Ma et al. take matters a step further by using the Alfred Wegener Institute Climate Model (AWI-CM3) to address what may happen following ‘hosing’ in four regions of the North Atlantic: the Labrador Sea (between Labrador and West Greenland); the Irminger Basin (SE of East Greenland, SW of Iceland); the Nordic Seas (north of Iceland; and the Greenland-Iceland-Norwegian seas) and the NE Atlantic (between Iceland, Britain and western Norway).

Prolonged freshwater flow into the Irminger Basin has the most pronounced effect on AMOC weakening, largely due to a U-bend in the AMOC where the surface current changes from northward to south-westward flow parallel to the East Greenland Current. The latter carries meltwater from the Greenland ice sheet whose low density keeps it near the surface. In turn, this strengthens NE and SW winds over the Labrador Sea and Nordic Seas respectively, which slow this part of the AMOC. In turn that complex system slows the entire AMOC further south. Since 2010 an average 270 billion tonnes of ice has melted in Greenland each year. This results in an annual 0.74 mm rise in global sea level, so the melted glacial ice is not being replenished. When sea ice forms it does not take up salt and is just as fresh as glacial ice. Annual melting of sea ice therefore temporarily adds fresh water to surface waters of the Arctic Ocean, but the extent of winter sea ice is rapidly shrinking. So, it too adds to freshening and lowering the density of the ocean-surface layer. The whole polar ocean ‘drains’ southwards by surface currents, mainly along the east coast of Greenland potentially to mix with branches of the AMOC. At present they sink with cooled more saline water to move at depth. To melting can be added calving of Greenlandic glaciers to form icebergs that surface currents transport southwards. A single glacier (Zachariae Isstrom) in NE Greenland lost 160 billion tonnes of ice between 1999 and 2022. Satellite monitoring of the Greenland glaciers suggests that a trillion tonnes have been lost through iceberg formation during the first quarter of the 21st century. Accompanying the Dansgaard-Oeschger events of the last 100 ka were iceberg ‘armadas’ (Heinrich events) that deposited gravel in ocean-floor sediments as far south as Portugal.

 The modelling done by Ma et al. also addresses possible wider implications of their ‘hosing’ experiments to the global climate. The authors caution that this aspect is an ‘exploration’ rather than prediction. Globally increased duration of ‘cold extremes’ and dry spells, and the intensity of precipitation may ensue from downturns and potential collapse of AMOC. Europe seems to be most at risk. Ma et al. plea for expanded observational and modelling studies focused on the Irminger Basin because it may play a critical role in understanding the mechanisms and future strength of the AMOC.

 See also: Yirka, R. 2024. Greenland’s meltwater will slow Atlantic circulation, climate model suggests. Phys Org, 21 November 2024

A major breakthrough in carbon capture and storage?

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Carbon capture and storage is in the news most weeks and is increasingly on the agenda for some governments. But plans to implement the CCS approach to reducing and stopping global warming increasingly draws scorn from scientists and environmental campaigners. There is a simple reason for their suspicion. State engagement, in the UK and other rich countries, involves major petroleum companies that developed the oil and gas fields responsible for unsustainably massive injection of CO2 into the atmosphere. Because they have ‘trousered’ stupendous profits they are a tempting source for the financial costs of pumping CO2 into porous sedimentary rocks that once contained hydrocarbon reserves. Not only that, they have conducted such sequestration over decades to drive out whatever petroleum fluids remaining in previously tapped sedimentary strata. For that second reason, many oil companies are eager and willing to comply with governmental plans, thereby seeming to be environmentally ‘friendly’. It also tallies with their ambitions to continue making profits from fossil-fuel extraction. But isn’t that simply a means of replacing the sequestered greenhouse gas with more of it generated by burning the recovered oil and natural gas; i.e. ‘kicking the can down the road’? Being a gas – technically a ‘free phase’ – buried CO2 also risks leaking back to the atmosphere through fractures in the reservoir rock. Indeed, some potential sites for its sequestration have been deliberately made more gas-permeable by ‘fracking’ as a means of increasing the yield of petroleum-rich rock. Finally, a litre of injected gas can drive out pretty much the same volume of oil. So this approach to CCS may yield a greater potential for greenhouse warming than would the sequestered carbon dioxide itself.

Image of calcite (white) and chlorite (cyan) formed in porous basalt due to CO2-charged water-rock interaction at the CarbFix site in Iceland. (Credit: Sandra Ósk Snæbjörnsdóttir)

Another, less widely publicised approach is to geochemically bind CO2 into solid carbonates, such as calcite (CaCO­3), dolomite (CaMgCO3), or magnesite (MgCO3). Once formed such crystalline solids are unlikely to break down to their component parts at the surface, under water or buried. One way of doing this is by the chemical weathering of rocks that contain calcium- and magnesium-rich minerals, such as feldspar (CaAl2Si2O8), olivine ([Fe,Mg]2SiO4) and pyroxene ([Fe,Mg]CaSi2O6) . Mafic and ultramafic rocks, such as basalt and peridotite are commonly composed of such minerals. One approach involves pumping the gas into a Icelandic borehole that passes through basalt and letting natural reactions do the trick. They give off heat and proceed quickly, very like those involved in the setting of concrete. In two experimental field trials 95% of injected CO2 was absorbed within 18 months. Believe it or not, ants can do the trick with crushed basalt and so too can plant roots. There have been recent experiments aimed at finding accelerants for such subsurface weathering (Wang, J. et al. 2024. CO2 capture, geological storage, and mineralization using biobased biodegradable chelating agents and seawater. Science Advances, v. 10, article eadq0515; DOI: 10.1126/sciadv.adq0515). In some respects the approach is akin to fracking. The aim is to connect isolated natural pores to allow fluids to permeate rock more easily, and to release metal ions to combine with injected CO2.

Chelating agents are biomolecules that are able to dissolve metal ions; some are used to remove toxic metals, such as lead, mercury and cadmium, from the bodies of people suffering from their effects. Naturally occurring ones extract metal ions from minerals and rocks and are agents of chemical weathering; probably used by the aforesaid ants and root systems. Wang and colleagues, based at Tohoku University in Japan, chose a chelating agent GLDA (tetrasodium glutamate diacetate –  C9H9NNa4O8) derived from plants, which is non-toxic, cheap and biodegradable. They injected CO2 and seawater containing dissolved GDLA into basaltic rock samples. The GDLA increases the rock’s porosity and permeability by breaking down its minerals so that Ca and Mg ions entered solution and were thereby able to combine with the gas to form carbonate minerals. Within five days porosity was increased by 16% and the rocks permeability increased by 26 times. Using electron microscopy the authors were able to show fine particles of carbonate growing in the connected pores. In fact these carbonate aggregates become coated with silica released by the induced mineral-weathering reactions. Calculations based on the previously mentioned field experiment in Iceland suggest that up to 20 billion tonnes of CO2 could be stored in 1.3 km3 of basalt treated in this way: about 1/25000 of the active rift system in Iceland (3.3 x 104 km2 covered by 1 km of basalt lava). In 2023 fossil fuel use emitted an estimated 36.6 bllion tons of CO2 into the atmosphere.

So, why do such means of efficiently reducing the greenhouse effect not receive wide publicity by governments or the Intergovernmental Panel on Climate Change? Answers on a yellow PostIt™ please . . .

A new timeline for modern humans’ colonisation of Europe

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Aurignacian sculptures: ‘Lion-Man’ and ‘Venus’ from the Hohlenstein-Stadel and Hohle Fels caves in Germany.

The earliest culture (or techno-complex) that can be related to anatomically modern humans (AMH) in Europe is called the Aurignacian. It includes works of art as well as tools made from stone, bone and antler. Perhaps the most famous are the ivory sculptures of ‘Lion-Man’ and Venus of the Hohlenstein-Stadel  and Hohle Fels caves in Germany,  and also the stunning cave art, of Chauvet Cave in France. Aurignacian artefacts that are dated at 43 to 26 ka occur at sites throughout Europe south of about 52°N. It was this group of people who interacted with the original Neanderthal population of Europe and finally replaced them completely. There is a long standing discussion over who ‘invented’ the stone tools, both human groups apparently having used similar styles of manufacture (Châtelperronian). Likewise, as regards the subsistence methods deployed by each; in one approach Neanderthals may have largely restricted their activities to roughly fixed ranges, whereas the incomers were generally seasonal nomads. As yet it has not been possible to show if the interbreeding between the two, which ancient and modern genetic data show, preceded the Aurignacian influx or continued when the met in Europe. Whatever, Neanderthals as a distinct human group had disappeared from the geological record by 40 ka. (Note that the three thousand years of coexistence is as long as the time between now and the end of the Bronze Age, about 150 generations at least.) But that aspect of European human development is not the only bone of contention about the spread into Europe. How did the Aurignacian people fare during and after their entry into Europe?

Despite continuing discovery of AMH sites in Europe, and reappraisal of long-known ones, there are limits to how much locations, dates, bones and artifacts can tell us. The actual Aurignacian dispersal of people across Europe is confounded by the limited number of proven occupation sites. These were people who, like most hunter gatherers, must have moved continually in response to variations in the supply of resources that depend on changing climatic conditions. They probably travelled ‘light’, occupied many temporary camp sites but few places to which they returned generation after generation. Temporary ‘stopping places’ are difficult to find, showing little more than evidence of fire and a ‘litter’ of shards from retouched stone tools (debitage), together with discarded bones that show marks left by butchery. A group of archaeologists and climate specialists from the University of Cologne, Germany have tried to shed some light on the completely ‘invisible’ aspects of Aurignacian dispersal and subsistence using what they have called – perhaps a tribute to Frank Sinatra! – the ‘Our Way Model’ (Shao, Y. et al. 2024. Reconstruction of human dispersal during Aurignacian on pan-European scale. Nature Communications, v. 15, Article 7406; DOI: 10.1038/s41467-024-51349-y. Click link to download a PDF).

The reality of hunter-gatherer life during a period of repeated rapid change in climate would clearly have been complex and sometimes precarious. To grasp it also needs to take account of human population dynamics as well as climatic and ecological drivers. The team’s basic strategy was to combine climate and archaeological data to model the degree to which human numbers may have fluctuated and the extent and direction of their migration. Three broad factors would have driven both: environmental change; culture – social change, curiosity, technology; and human biology. Really, environmental change is the only one that can be addressed with any degree of precision through records of climate change, such as Greenland ice cores. Archaeological data from known sites should provide some evidence for technological change, but only for two definite phases in Aurignacian culture (43-38 ka and 38-32 ka). Dating of   Aurignacian sites establishes some time calibration for episodes of occupation, abandonment and resettlement. Issues of human biology can be addressed to some extent from ancient genetics, where suitable bones are available. However, the ‘Our Way Model’ is driven by climate modelling and archaeology. It outputs an historical estimate of ‘human existence potential’ (HEP) that includes predictions of carbon storage in plants and animals – i.e.  potential food resources – expressed as regional population density in Europe. The technical details are complex, but Shao et al.’s conclusions are quite striking.

Maps of estimated anatomically modern human population density during the first six thousand years of Aurignacian migration and palaeoclimate record from the Greenland NGRIP ice core, with shaded warm episodes – red spots indicate the time of the population estimates above. (Credit: Shao et al. Fig. 1)

Climate change in the later stages of cooling towards the last glacial maximum at ~20 ka was cyclical, with ten Dansgaard-Oeschger cold stadial events capable of ‘knocking back’ both population density and the extent of settlement. In the first two millennia expansion from the Levant into the Balkans was slow. From 43 to 41 ka the pace quickened, taking the Aurignacian culture into Western Europe, with an estimate total European AMH population of perhaps 60 thousand. A third phase (41 to 39 ka) shrank the areas and densities of population during a prolonged cold period. The authors suggest that survival was in Alpine refuge areas that AMH people had occupied previously. Starting at around 38 ka, a lengthy climatic warm period allowed the culture to spread to its maximum extent reaching southern Britain and the north and east of the Iberian Peninsula. Perhaps by then the AMH population had evolved better strategies to adapt to increasing frigid conditions. But by that time the Neanderthals had disappeared from Europe freeing up territory and food resources. That too may have contributed to the expansion and the sustenance of an AMH total population of between 80 and 100 thousand during the second phase of the Aurignacian.

It’s as well to remember that this work is based on a model, albeit sophisticated, based on currently known data. Palaeoanthropology is extremely prone to surprises as field- and lab work progresses …

See also: New population model identifies phases of human dispersal across Europe. EurekaAlert, 4 September 2024; Kambani, K. 2024. The Dynamics of Early Human Dispersal Across Europe: A New Population Model. Anthropology.net, 4 September 2024.

The gross uncertainty of climate tipping points

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That the Earth has undergone sudden large changes is demonstrated by all manner of geoscientific records. It seems that many of these catastrophic events occurred whenever steady changes reach thresholds that trigger new behaviours in the interlinked atmosphere, hydrosphere, atmosphere, biosphere and lithosphere that constitute the Earth system. The driving forces for change, both steady and chaotic, may be extra-terrestrial, such as the Milankovich cycles and asteroid impacts, due to Earth processes themselves or a mixture of the two. Our home world is and always has been supremely complicated; the more obviously so as knowledge advances.  Abrupt transitions in components of the Earth system occur when a critical forcing threshold is passed, creating a ‘tipping point’. Examples in the geologically short term are ice-sheet instability, the drying of the Sahara, collapse of tropical rain forest in the Amazon Basin, but perhaps the most important is the poleward transfer of heat in the North Atlantic Ocean. That is technically known as the Atlantic Meridional Overturning Circulation with the ominous acronym AMOC.

Simplified Atlantic Meridional Overturning Circulation (AMOC). Red – warm surface currents; cyan – cold deep-water flow. (Credit: Stefano Crivellari)

As things stand today, warm Atlantic surface water, made more saline and dense by evaporation in the tropics is transferred northwards by the Gulf Stream. Its cooling at high latitudes further increases the density of this water, so at low temperatures it sinks to flow southwards at depth. This thermohaline circulation continually pulls surface water northwards to create the AMOC, thereby making north-western European winters a lot warmer than they would be otherwise. Data from Greenland ice cores show that during the climatic downturn to the last glacial maximum, the cooling trend was repeatedly interrupted by sudden warming-cooling episodes, known as Dansgaard-Oeschger events, one aspect of which was the launching of “armadas” of icebergs to latitudes as far south as Portugal (known as Heinrich events), which left their mark as occasional gravel layers in the otherwise muddy sediments on the deep Atlantic floor (see: Review of thermohaline circulation; February 2002).

These episodes involved temperature changes over the Greenland icecap of as much as 15°C.  They began with warming on this scale within a matter of decades followed by slow cooling to minimal temperatures, before the next turn-over. Various lines of evidence suggest that these events were accompanied by shutdowns of AMOC and hence the Gulf Stream, as shown by variations in the foraminifera species in sea-floor sediments. The culprit was vast amounts of fresh water pouring into the Arctic and northernmost Atlantic Oceans, decreasing the salinity and density of the surface ocean water. In these cases that may have been connected to repeated collapse of circumpolar ice sheets to launch Heinrich’s iceberg armadas. A similar scenario has been proposed for the millennium-long Younger Dryas cold spell that interrupted the onset of interglacial conditions. In that case the freshening of high-latitude surface water was probably a result of floods released when glacial barriers holding back vast lakes on the Canadian Shield burst.

At present the Greenland icecap is melting rapidly. Rising sea level may undermine the ice sheet’s coastal edges causing it to surge seawards and launch an iceberg armada. This may be critical for AMOC and the continuance of the Gulf Stream, as predicted by modelling: counter-intuitive to the fears of global warming, at least for NW Europe. In August 2024 scientists from Germany and the UK published what amounts to a major caution about attempts to model future catastrophes of this kind (Ben-Yami, M. et al 2024, Uncertainties too large to predict tipping times of major Earth system components from historical data, Science Advances, v. 10, article  eadl4841; DOI 10.1126/sciadv.adl4841). They focus on records of the AMOC system, for which an earlier modelling study predicted that a collapse could occur between 2025 and 2095: of more concern than global warming beyond the 1.5° C currently predicted by greenhouse-gas climate models .

Maya Ben-Yami and colleagues point out that the assumptions about mechanisms in Earth-system modelling and possible social actions to mitigate sudden change are simplistic.  Moreover, models used for forecasting rely on historical data sets that are sparse and incomplete and depend on proxies for actual variables, such as sea-surface and air temperatures. The further back in geological time, the more limited the data are. The authors assess in detail data sets and modelling algorithms that bear on AMOC. Rather than a chance of AMOC collapse in the 21st century, as suggested by others, Ben Yami et al. reckon that any such event  lies between 2055 and 8065 CE, which begs the question, “Is such forecasting  worth the effort?”, however appealing it might seem to the academics engaged in climatology. The celebrated British Met Office and other meteorological institutions, use enormous amounts of data, the fastest computers and among the most powerful algorithms on the planet to simulate weather conditions in the very near future. They openly admit a limit on accurate forecasting of no more than 7 day ahead. ‘Weather’ can be regarded as short-term climate change.

It is impossible to stop scientists being curious and playing sophisticated computer games with whatever data they have to hand. Yet, while it is wise to take climate predictions with a pinch of salt because of their gross limitations, the lessons of the geological past do demand attention. AMOC has shut down in the past – the last being during the Younger Dryas – and it will do so again. Greenhouse global warming probably increases the risk of such planetary hiccups, as may other recent anthropogenic changes in the Earth system. The most productive course of action is to reduce and, where possible, reverse those changes. In my honest opinion, our best bet is swiftly to rid ourselves of an economic system that in the couple of centuries since the ‘Industrial Revolution’ has wrought these unnatural distortions.

Ocean-floor sediments reveal the influence of Mars on long-term climate cycles

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In 1976 three scientists from Columbia and Brown (USA) and Cambridge (UK) Universities published a paper that revolutionised the study of ancient climates (Hays J.D., Imbrie J. and Shackleton N.J. 1976. Variations in the Earth’s Orbit: Pacemaker of the Ice Ages. Science, v. 194, p. 1121-1132;  DOI: 10.1126/science.194.4270.1121). Using variations in oxygen isotopes from foraminifera through two cores of sediments beneath the floor of the southern Indian Ocean they verified Milutin Milankovich’s hypothesis of astronomical controls over Earth’s climate. This centred on changes in Earth’s orbital parameters induced by gravitational effects from the motions of other planets: its orbit’s eccentricity, and the tilt and precession of its rotational axis. Analysis of the frequency of isotopic variations in the resulting time series yielded Milankovich’s predictions of ~100, 41 and 21 ka periodicities respectively. The time spanned by the cores was that of the last 500 ka of the Pleistocene and thus the last 5 glacial-interglacial cycles. Subsequently, the same astronomical climate forcing  has been detected  for various climate-induced changes in the earlier sedimentary record, including the glacial cycles of the Carboniferous and Neoproterozoic, Jurassic climate changes due to oceanic methane emissions and many other types of cyclicity during the Phanerozoic.

One hemisphere of Mars captured by ESA’s Mars Express. Credit: ESA / DLR / FU Berlin /

As well as time series based on isotopic and other geochemical changes in marine cores, other variables such as thickness of turbidite beds or cyclical repetitions of short rock sequences such as the ‘cyclothems’ of Carboniferous age (repetitions of a  limestone, sandstone, soil, coal sequence) have also been subject to frequency analysis. Sedimentary features that have not been tried are gaps or hiatuses in stratigraphic sequences where strata are missing from a deep-sea sequence. These signify erosion of sediment due to vigorous bottom currents in sequences otherwise dominated by continuous deposition under low-energy conditions. Three geoscientists from the University of Sydney, Australia and the Sorbonne University, France, have subjected records of gaps in Cenozoic sedimentation from 293 deep-sea drill cores to time-series analysis to discover what such ‘big data’ might reveal as regards climate fluctuations on the order of millions of years (Dutkiewicz, A., Boulila, S. & Müller, R.D. 2024. Deep-sea hiatus record reveals orbital pacing by 2.4 Myr eccentricity grand cycles. Nature Communications, v. 15, article 1998; DOI: 10.1038/s41467-024-46171-5).

In theory gravitational interrelationships between all the orbiting planets should have an effect on the orbital parameters of each other, and thus the amount of received solar radiation and changes in global climate. As well as the Milankovich effect, longer astronomical ‘grand cycles’ may therefore have been reflected somehow in Earth’s climatic history (Laskar, J. et al. 2004. A long-term numerical solution for the insolation quantities of the Earth. Astronomy & Astrophysics, v. 428, p. 261-285; DOI: 10.1051/0004-6361:20041335). Based on Laskar et al.’s calculations Adriana Dutkiewicz and colleagues sought evidence for two predicted ‘grand cycles’ that result from orbital interactions between Earth and Mars. These are a 2.4 Ma period in the eccentricity of Earth’s orbit and one of 1.2 Ma in the tilt of its axis.

The authors were able to detect cyclicity in the hiatus time series that is close to the 2.4 Ma Mars-induced waxing and waning of solar heating. Warming would increase mixing of ocean water through cyclones and hurricanes. That would then induce more energetic deep ocean currents and more erosion on the deep ocean floor: more gaps in sedimentation. Cooler conditions would ‘calm’ deep ocean currents so that deposition would outweigh evidence of erosion. The 1.2 Ma axial tilt cyclicity is not apparent in the data. Interestingly, the ~2.4 Ma cyclicity underwent a significant deviation at the Palaeocene-Eocene Boundary’ (56Ma), seemingly predicted by Laskar et al’s  astronomical solutions as a chaotic orbital transition between 56 and 53 Ma. Dutkiewicz et al. also chart the relations between the sedimentary-hiatus time series and major tectonic, oceanographic, and climatic changes during the Cenozoic Era, and found that terrestrial processes did disrupt the Mars-related orbital eccentricity cycles.

The findings suggest that long-term astronomical climate forcing needs to be borne in mind for better understanding the future response of the ocean to global warming. Also, if Mars had such an influence so must have Venus, which is more massive and closer. That remains to be investigated, and also the effects of the giant planets. In the very distant past there behaviour may have resulted in unimaginable astronomical changes. According to the bizarrely named Nice Model a back and forth shuffling of the Giant Planets was probably responsible for the Late Heavy Bombardment 4.1 to 3.8 billion years (Ga) ago. Such errant behaviour may even have triggered the flinging of some of the Sun’s original planetary complement out of the solar system and changed the outward order of the existing eight. Fortunately, the present planetary set-up seems to be stable …

See also: Dutkiewicz, A., & Müller, R. D. 2022. Deep-sea hiatuses track the vigor of Cenozoic ocean bottom currents. Geology, v. 50, p. 710–715; DOI: 10.1130/G49810.1; Mars drives deep-ocean circulation in Earth’s oceans, study suggests. Sci News, 13 March 2024.

The ‘Anthropocene Epoch’ bites the dust?

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The International Commission on Stratigraphy (ICS) issues guidance for the division of geological history that has evolved from the science’s original approach: that was based solely on what could be seen in the field. That included: variations in lithology and the law of superposition; unconformities that mark interruptions through deformation, erosion and renewed deposition; the fossil content of sediments and the law of faunal succession; and more modern means of division, such as geomagnetic changes detected in rock over time. That ‘traditional’ approach to relative time is now termed chronostratigraphy, which has evolved since the 19th century from the local to the global scale as geological research widened its approach. Subsequent development of various kinds of dating has made it possible to suggest the actual, absolute time in the past when various stratigraphic boundaries formed – geochronology. Understandably, both are limited by the incompleteness of the geological record – and the whims of individual geologists. For decades the ICS has been developing a combination of both approaches that directly correlates stratigraphic units and boundaries with accurate geochronological ages. This is revised periodically, the ICS having a detailed protocol for making changes.  You can view the Cenozoic section of the latest version of the International Chronostratigraphic Chart and the two systems of units below. If you are prepared to travel to a lot of very remote places you can see a monument – in some cases an actual Golden Spike – marking the agreed stratigraphic boundary at the ICS-designated type section for 80 of the 93 lower boundaries of every Stage/Age in the Phanerozoic Eon. Each is a sonorously named Global Boundary Stratotype Section and Point or GSSP (see: The Time Lords of Geology, April 2013). There are delegates to various subcommissions and working groups of the ICS from every continent, they are very busy and subject to a mass of regulations …

Chronostratigraphic Chart for the Cenozoic Era showing the 5 tiers of stratigraphic time division. The little golden spikes mark where a Global Boundary Stratotype Section and Point monument has been erected at the boundary’s type section.

On 11 May 2011, the Geological Society of London hosted a conference, co-sponsored by the British Geological Survey, to discuss evidence for the dawn of a new geological Epoch: the Anthropocene, supposedly marking the impact of humans on Earth processes. There has been ‘lively debate’ about whether or not such a designation should be adopted. An Epoch is at the 4th tier of the chronostratigraphic/geochronologic systems of division, such as the Holocene, Pleistocene, Pliocene and Miocene, let alone a whole host of such entities throughout the Phanerozoic, all of which represent many orders of magnitude longer spans of time and a vast range of geological events. No currently agreed Epoch lasted less than 11.7 thousand years (the Holocene) and all the others spanned 1 Ma to tens of Ma (averaged at 14.2 Ma). Indeed, even geological Ages (the 5th tier) span a range from hundreds of thousands to millions of years (averaged at 6 Ma). Use ‘Anthropocene’ in Search Earth-logs to read posts that I have written on this proposal since 2011, which outline the various arguments for and against it.

In the third week of May 2019 the 34-member Anthropocene Working Group (AWG) of the ICS convened to decide on when the Anthropocene actually started. The year 1952 was proposed – the date when long-lived radioactive plutonium first appears in sediments before the 1962 International Nuclear Test-Ban Treaty. Incidentally, the AWG proposed a GSSP for the base of the Anthropocene in a sediment core through sediments in the bed of Crawford Lake an hour’s drive west of Toronto, Canada.   After 1952 there are also clear signs that plastics, aluminium, artificial fertilisers, concrete and lead from petrol began to increase in sediments. The AWG accepted this start date (the Anthropocene ‘golden spike’) by a 29 to 5 vote, and passed it into the vertical ICS chain of decision making. This procedure reached a climax on Monday 4 March 2024, at a meeting of the international Subcommission on Quaternary Stratigraphy (SQS): part of the ICS. After a month-long voting period, the SQS announced a 12 to 4 decision to reject the proposal to formally declare the Anthropocene as a new Epoch. Normally, there can be no appeals for a losing vote taken at this level, although a similar proposal may be resubmitted for consideration after a 10 year ‘cooling off’ period. Despite the decisive vote, however, the chair of the SQS, palaeontologist Jan Zalasiewicz of the University of Leicester, UK, and one of the group’s vice-chairs, stratigrapher Martin Head of Brock University, Canada have called for it to be annulled, alleging procedural irregularities with the lengthy voting procedure.

Had the vote gone the other way, it would marked the end of the Holocene, the Epoch when humans moved from foraging to the spread of agriculture, then the ages of metals and ultimately civilisation and written history. Even the Quaternary Period seemed under threat: the 2.5 Ma through which the genus Homo emerged from the hominin line and evolvd. Yet a pro-Anthropocene vote would have faced two more, perhaps even more difficult hurdles: a ratification vote by the full ICS, and a final one in August 2024 at a forum of the International Union of Geological Sciences (IUGS), the overarching body that represents all aspects of geology.  

There can be little doubt that the variety and growth of human interferences in the natural world since the Industrial Revolution poses frightening threats to civilisation and economy. But what they constitute is really a cultural or anthropological issue, rather than one suited to geological debate. The term Anthropocene has become a matter of propaganda for all manner of environmental groups, with which I personally have no problem. My guess is that there will be a compromise. There seems no harm either way in designating the Anthropocene informally as a geological Event. It would be in suitably awesome company with the Permian and Cretaceous mass extinctions, the Great Oxygenation Event at the start of the Proterozoic, the Snowball Earth events and the Palaeocene–Eocene Thermal Maximum. And it would require neither special pleading nor annoying the majority of geologists. But I believe it needs another name. The assault on the outer Earth has not been inflicted by the vast majority of humans, but by a tiny minority who wield power for profit and relentless growth in production. The ‘Plutocracene’ might be more fitting. Other suggestions are welcome …

See also: Witze, A. 2024. Geologists reject the Anthropocene as Earth’s new epoch — after 15 years of debate. Nature, v. 627, News article; DOI: 10.1038/d41586-024-00675-8; Voosen, P. 2024. The Anthropocene is dead. Long live the Anthropocene. Science, v. 383, News article, 5 March 2024.

Changing Atlantic Ocean currents may threaten Gulf Stream warming of Europe

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Climate during the last Ice Age was continually erratic. Generally fine-grained muds cored from the floor of the North Atlantic Ocean show repeated occurrences of layers containing gravelly debris. These have been ascribed to periods when ice sheets on Greenland and Scandinavia calved icebergs at an exceptionally fast rate, to release coarse debris as they melted while drifting to lower latitudes. These ‘iceberg armadas’ (known as Heinrich events) left their unmistakable signs as far south as Portugal. Their timing correlates with short-lived (1 to 2 ka) warming-cooling episodes (Dansgaard-Oeschger events) recorded in Greenland ice cores that involved variations in air temperature of up to 15°C. The process that resulted in these sudden climate shifts seems to have been changing ocean circulation brought about by vast amounts of fresh water flooding into the Arctic and North Atlantic Oceans. This lowered seawater density to the extent that its upper parts could not sink when cooled. It is this thermohaline circulation that drags warmer surface water northwards, known as the Atlantic Meridional Overturning Circulation (AMOC), part of which is the Gulf Stream. When it fails or slows the result is plummeting temperatures at high latitudes. The last major AMOC shutdown was after 8 ka of warming that followed the last glacial maximum. Between 12.9 and 11.7 ka major glaciers grew again north of about 50°N in the period known as the Younger Dryas, almost certainly in the aftermath of a flood to the Arctic Ocean of glacial meltwater from the Canadian Shield. Around 8.2 thousand years ago human re-colonisation of Northern Europe was set back by a similar but lesser cooling event.

The Atlantic Meridional Overturning Circulation (AMOC). Red – warm surface currents; cyan – cold deep-water flow. (Credit: Stefano Crivellari)

Three researchers at Utrecht University, the Netherlands have issued an early warning that the AMOC may have reached a critical condition (Van Westen, R.M., Kliphuis, M & Dijkstra, H.A. 2024. Physics-based early warning signal shows that AMOC is on tipping course. Science Advances, v. 10, article adl1189; DOI: 10.1126/sciadv.adk1189). Previous modelling of AMOC has suggested that only rapid, massive decreases in the salinity of North Atlantic surface water near the Arctic Circle could shut down the Gulf Stream in the manner of Younger Dryas and Dansgaard-Oeschger events. René van Westen and colleagues have simulated the effects of steady, long-term addition of fresh water from melting of the Greenland ice sheet. They ran a sophisticated Earth System model for six months on the Netherlands’ Snellius super computer. Their model used a slowly increasing influx of glacial meltwater to the Atlantic at high northern latitudes.

The various feedbacks in the model eventually shut down the AMOC, predicted to result in cooling of NW Europe by 10 to 15 °C in a matter of a few decades. Yet to achieve that required the model to simulate more than 2000 years of change. It took 1760 years for a persistent AMOC transport of 10 to 15 million m3 s-1 to drop over a century or so and reach near-zero. That collapse involved around 80 times more melting of Greenland’s ice sheet than at present. Yet their modelling does not take into account global warming: including that factor would have exceeded their budgeted supercomputer time by a long way. Melting of the Greenland ice sheet is, however, accelerating dramatically

Van Westen et al. have shown the possibility that steadily increasing ice-sheet melting can, theoretically, ’flip’  the huge current system associated with the Atlantic Ocean, and with it regional climate patterns. The tangible fear today is of a more than 1.5°C increase in global surface temperature, yet a warming-induced failure of AMOC may cause local annual temperatures to fall by up to ten times that. Rather than the currently heralded disappearance of sea-ice from the Arctic Ocean, it may spread in winter to as far south as the North Sea. The only way of forecasting in detail what may actually happen – and where – is ever-more sophisticated and costly modelling of ocean currents and ice melting in a warming world. Uncertain as it stands, the work by van Westen and colleagues may well be ignored: perhaps as a ‘thing we dinnae care to speak aboot’.

See also: Le Page, M. 2024. Atlantic current shutdown is a real danger, suggests simulation. New Scientist, 9 February 2024; Watts, J. 2024. Atlantic Ocean circulation nearing ‘devastating’ tipping point, study finds. The Guardian, 9 February 2024.

Why did the largest ever primate disappear?

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Chinese apothecary shops sell an assortment of fossils. They include shells of brachiopods that when ground up and dissolved in water allegedly treat rheumatism, skin diseases, and eye disorders. Traditional apothecaries also supply  ‘dragons’ teeth’, said by Dr Subhuti Dharmananda, Director of the Institute for Traditional Medicine in Portland, Oregon to treat epilepsy, madness, manic running about, binding qi (‘vital spirit’) below the heart, inability to catch one’s breath, and various kinds of spasms, as well as making the body light, enabling one to communicate with the spirit light, and lengthening one’s life. Presumably have done a roaring trade in ‘dragons’ teeth’ since they were first mentioned in a Chinese pharmacopoeia (the Shennong Bencao Jing) from the First Century of the Common Era. In 1935 the anthropologist Gustav von Koenigswald came across two ‘dragons’ teeth’ in a Hong Kong shop. They were unusually large molars and he realised they were from a primate, but far bigger (20  × 22 mm) than any from living or fossil monkeys, apes or humans.

Eventually, in 1952 (he had been interned by Japanese forces occupying Java), von Koenigswald formally described the teeth and others that he had found. Their affinities and size prompted him to call the former bearer the ‘Huge Ape’ (Gigantopithecus). By 1956 Chinese palaeontologists had tracked down the cave site in Guangxi province where the teeth had been sourced, and a local farmer soon unearthed a complete lower jawbone (mandible) that was indeed gigantic. More teeth and mandibles have since been found at several sites in Southern and Southeast Asia, with an age range from about 2.0 to 0.3 Ma. Anatomical differences between teeth and mandibles suggest that there may have been 4 different species. Using mandibles as a very rough guide to overall size it has been estimated that Gigantopithecus may have been up to 3 m tall weighing almost 600kg.

Above: Size comparison of G. blacki with a 1.8 m tall human male; NB G.blacki probably walked on all fours, as do living orangutans when they rarely descend from the forest canopy. (Credit: Frido Welker) Below: Mandible of Gigantopithecus blacki from India (Credit: Prof. Wei Wang, Photo retouched by Theis Jensen)

Plaque on some teeth contain evidence for fruit, tubers and roots, but not grasses, which suggest suggest that Gigantopithecus had a vegetarian diet based on forest plants. Mandibles also showed affinities with living and fossil orangutans (pongines). Analysis of proteins preserved in tooth enamel confirm this relationship (Welker, F. and 17 others 2019. Enamel proteome shows that Gigantopithecus was an early diverging pongine. Nature, v.576, p. 262–265; DOI: 10.1038/s41586-019-1728-8). It was one of the few members of the southeast Asian megafauna to go extinct at the genus level during the Pleistocene. Its close relative Pongo the orangutan survives as three species in Borneo and Sumatra. Detailed analysis of material from 22 southern Chinese caves that have yielded Gigantopithecus teeth has helped resolve that enigma (Zhang, Y. and 20 others 2024. The demise of the giant ape Gigantopithecus blacki. Nature, v. 625; DOI: 10.1038/s41586-023-06900-0).

At the time Gigantopithecus first appeared in the geological record of China (~2.2 Ma), it ranged over much of south-western China. The early Pleistocene ecosystem there was one of diverse forests sufficiently productive to support large numbers of this enormous primate and also the much smaller orangutan Pongo weidenreichi.  By 295 to 215 ka, the age of the last known Gigantopithecus fossils, its range had shrunk dramatically. The teeth show marked increases in size and complexity by this time, which suggests adaptation of diet to a changing ecosystem. That is confirmed by pollen analysis of cave sediments which reveal a dramatic decrease in forest cover and increases in fern and non-arboreal flora at the time of extinction. One physical sign of environmental stress suffered by individual late G. blacki is banding in their teeth defined by large fluctuations of barium and strontium concentrations relative to calcium. The bands suggest that each individual had to change its diet repeatedly over its lifetime. Closely related orangutans, on the other hand survived into the later Pleistocene of China, having adapted to the changed ecosystem, as did early humans in the area. It thus seems likely that Gigantopithecus was an extreme specialist as regards diet, and was unable to adapt to changes brought on by the climate becoming more seasonal. Today’s orangutans in Indonesia face a similar plight, but that is because they have become restricted to forest ‘islands’ in the midst of vast areas of oil palm plantations. Their original range seems to have been much the same as that of Gigantopithecus, i.e. across south-eastern Asia, but Pongo seems to have gone extinct outside of Indonesia (by 57 ka in China) during the last global cooling and when forest cover became drastically restricted.

Climate and tectonics since 250 Ma

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A central feature of the Earth’s climate system is the way that carbon bound in two gases – carbon dioxide (CO2) and methane (CH4) – controls the amount of incoming solar energy that is retained by the atmosphere. Indeed, without one or the other our home world would have been locked in frigidity since shortly after its formation: a sterile, ice-covered planet. The ‘greenhouse effect’ has been ever-present because the material from which the Earth accreted contained carbon as well as every other chemical element from hydrogen to uranium. Naturally reactive, it readily combines with hydrogen and oxygen to form methane and carbon dioxide, which would have escaped the inner Earth as gases to enter the earliest atmosphere as a ‘comfort blanket’, along with water vapour, another greenhouse gas.  Their combined effects have remained crudely balanced so that neither inescapable frigidity nor surface temperatures high enough to boil-off the oceans have ever occurred in the last 4.5 billion years. Earth has remained like the wee bear’s porridge in the Goldilocks story! Even so, global climate has fluctuated again and again from that akin to a steamy greenhouse, through long periods of moderation to extensive glacial conditions, including three that extended from pole-to-pole – ‘Snowball’ Earths –  during in the Precambrian. During the Phanerozoic the Earth has entered three long periods of generally low global temperatures, in the Ordovician, the Carboniferous and during the last 2.5 Ma  that allowed polar ice caps and sea-ice to extend a third of the way to the Equator. These were forced back and forth repeatedly by cyclical influences apparently triggered by astronomically controlled changes to Earth’s orbital and rotational parameters – the Milankovich Effect. Anthropogenic emissions of greenhouse gases in vast and increasing amounts now threaten to disrupt natural climate variation, effectively overthrowing the gravitational influences of distant giant planets that have controlled climate changes that shaped our own evolution since the genus Homo first emerged.

Bubbles of air trapped in cores through the ice sheets of Antarctica and Greenland record decreased volumes of land ice as CO2 content increased and the opposite during glacial episodes. Somehow in step with the astronomical forcing the Earth released greenhouse gas to warm the climate and drew it down to bring on cooling. Since all life forms are built from carbon-rich compounds and some extract it from the environment to build carbonate hard parts, climate and life on land and in the oceans are interlinked. In fact life and death are involved, because once dead organisms and their hard parts are buried before being oxidised in sediments on land, as in peat and ultimately coal, and on the ocean floors as limestones or carbonaceous mudstones, atmospheric carbon is sequestered. Exposed to acid water containing dissolved CO2 from the atmosphere or to oxygen, respectively, the two forms of carbon in solid form are released as greenhouse gas once more. Both take place when sedimentary deposits are exhumed as a result of erosion and tectonics. Another factor is the abundance of available nutrients, themselves released and distributed by erosion and agents of transportation. At present surface waters of the most distant parts of the oceans contains plenty of such nutrients, except for a vital one, dissolved iron. So they are wet ‘deserts’. It seems that during the much dustier times of glacial episodes iron in fine form reached far out into the world’s oceans so that phytoplankton at the base of the food chain ‘bloomed ‘and so did planktonic animals. Dead organisms ‘rained’ to the ocean floor so drawing down CO2 from the atmosphere and decreasing the greenhouse effect. The surface parts of the carbon and rock cycles are extremely complex and climatologists have yet to come to grips with modelling its future climates convincingly. Yet the carbon cycle and much deeper parts of the rock cycle are interwoven too.

Carbon in sedimentary rock can be heated by burial, and some can be subducted to great depths at destructive plate margins together. The same applies to in ocean-floor basalts that have been permeated by circulating sea water through hydrothermal circulation to form carbonates in the altered volcanic rock. In both cases carbon stored for hundreds of million years can be released by metamorphism in orogenic belts at zones of continental collision and deep below island arcs. Carbon from mantle depths that has never ‘seen the light of day’ is also added to the atmosphere when magmas form below oceanic constructive margins, hot spots and subduction zones, and where magmas flood the continental surface. Consequently, plate tectonics and deep mantle convection have surely played a long-term role in the evolution of our planet’s climate system. Geoscientists based in Australia and the UK have used geochemical data to reconstruct the stores of carbon in oceanic plates and thermodynamic modelling to track what may have happened to it and the climate through the last 250 Ma (Müller, R.D. et al. 2022. Evolution of Earth’s tectonic carbon conveyor belt. Nature, v. 605, p. 629-639; DOI: 10.1038/s41586-022-04420-x). Their review is an important step in understanding what underpins climate on a geological time scale, onto which much shorter-term surface influences are superimposed.

The amount of carbon being outgassed as CO2 each year along plate boundaries in the early Jurassic (185 Ma) shown in dark purple (low) to yellow (high). Also shown in shades of blue is the accumulation of carbon stored in each square metre of the ocean plates. Plate motions are shown as grey arrows (credit: Müller, R.D. et al. Clip from video in Supplementary Information)

At mid-ocean ridges basaltic magma wells up from mantle depths and loses much of its content of dissolved CO2. The annual outgassing at ridges, which depends on the global rate of plate formation, has varied from 13 to 30 million tonnes of carbon  (MtC yr-1) since the start of the Mesozoic Era 250 Ma ago. Similarly, there is greenhouse-gas escape from volcanic arcs above subduction zones, estimated to have ranged from 0 to 18 MtC yr-1. As an oceanic plate moves away from its source various processes sequester CO2 into the oceanic crust and upper mantle through accumulation of deep-sea sediments and hydrothermal alteration of basaltic crust and peridotite mantle (ranging from 30 to 311 MtC yr-1). Of this influx of carbon into oceanic plates between 36 to 103 MtC yr-1 has gone down subduction zones in descending slabs. Between 0 to 49 MtC yr-1 of that has been outgassed by arc volcanic activity or absorbed into the overriding plate. The rest continues down into the deep mantle, perhaps to form diamonds. Overall, when the rate at which oceanic plates grow is rapid and plate motion speeds up, outgassing should be high. When plate growth slows, so does the rate of CO2 release. Variations in plate growth can be estimated from the magnetic reversal stripes above the ocean floors.  The authors have released an animation of the break-up of Pangaea (well worth watching at full screen – you can skip the ad at the start), with the rate of carbon emission at ridges and volcanic arcs being colour-coded. Also shown is the storage of carbon within oceanic plats plates as time passes.

Length of mid-ocean ridges (orange) and subduction zones (blue) through the last 250 Ma (top). The areas of oceanic crust produced at ridges and consumed by subduction (bottom) (credit: Müller, R.D. et al., Figs 1a, 1c)

Before Pangaea began to break up at the end of the Triassic (200 Ma) the total length of mid-ocean ridges was at a minimum of about 40 thousand km. Through the Jurassic it never exceeded 50,000 km, but rose to a maximum of 80,000 km during the Cretaceous then declined slowly to the current length of 60,000 km. Throughout the last 250 Ma the length of subduction zones stayed roughly the same at about 65 thousand km – not always in the same places – although the overall rate of subduction changed in line with the rate of oceanic plate growth  (the volume that is added must be balanced roughly by the amount that returns to the mantle).  Between the end of the Jurassic and the mid-Cretaceous crustal production and destruction doubled, shown by the bottom plot in the figure above. The very fast  movement of plates and an increase in the global length of ridges during Jurassic to mid-Cretaceous times led to a dramatic increase in CO2 outgassing from ridges so that its content in the atmosphere rose as high as 1200 ppm – more than four times that before the Industrial Revolution. That level resulted in global ‘hothouse’ conditions during the Cretaceous. Another factor behind the Cretaceous climate was a decrease in the global complement of mountains. That led to decreases in erosion and the weathering of silicates by acid rain, thus reducing natural sequestration of carbon.

During the Cenozoic (after 65 Ma) declining ridge outgassing was actually outpaced by that associated with subduction, according to the modelling. That is strange, for by around 35 Ma glaciation had begun  on Antarctica as the Earth was cooling, which implies a major, unexpected sink for excess CO2. The most likely way this might have arisen is through increased erosion and silicate weathering on the exposed continents that consumed CO2 faster than tectonics was releasing the gas. The length of continental arcs shows no sign of a major increase during the Cenozoic, which might have accelerated that kind of sequestration, but a variety of proxies for signs of weathering definitely suggests that there was an upsurge. Also there was increased storage of carbon on the deep ocean floor, shown by the video. Increased calcium released by weathering to enter ocean water in solution would allow more planktonic organisms to secrete calcite (CaCO3) skeletons that would then fall to the ocean floor when they died.

There may be more to be discovered in this hugely complex interplay between tectonics and climate. For instance, when the bottom waters of the oceans are oxygenated by deep currents of cold dense seawater sinking from polar regions, carbon in tissues of sunken dead organism is oxidised to release CO2. If bottom waters are anoxic, this organic carbon is preserved in sediments. The authors mention this as something to be considered in their future work on  the ‘tectonic carbon conveyor belt’.

When Greenland was a warm place

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On 14-15 August 2021 it rained for the first time since records began at the highest point on the Greenland ice cap. Summit Camp at 3.216 m is run by the US National Science Foundation, which set it up in 1989, and is famous for climate data gleaned from two deep ice cores there. This odd event came at a time when surface melting of the ice cap covered 870 thousand km2: over half of its total 1.7 million km2 extent: a sure sign of global warming. The average maximum temperature in August at Summit is -14°C, but since the mid 20th century the Arctic has been warming at about twice the global rate. Under naturally fluctuating climatic conditions during the Pleistocene, associated with glacial-interglacial cycles, Greenland may have been ice-free for extended periods, perhaps as long as a quarter of a million years around 1.1 Ma ago. If 75% of the up to 3 km thick ice on Greenland melted that would add 5 to 6 m to global sea level, perhaps as early as 2100 if current rates of climate warming persist.

The edge of the ice cap in NE Greenland (credit: Wikipedia)

The worst scenario is runaway warming on the scale of that which took place 56 Ma ago during the Palaeocene-Eocene Thermal Maximum (PETM) when global mean temperature rose by between 5 to 8°C at a rate comparable with what the planet is experiencing now as a result of growth in the world economy. In fact, the CO2 released during the PETM emerged at a rate that was only about tenth of modern anthropogenic emissions  Sediments that span the Palaeocene-Eocene boundary occur in NE Greenland, a study of which was recently published by scientists from Denmark, Greenland, the UK, Australia and Poland (Hovikoski, J. and 13 others 2021. Paleocene-Eocene volcanic segmentation of the Norwegian-Greenland seaway reorganized high-latitude ocean circulation. Communications Earth & Environment, v. 2, article 172; DOI: 10.1038/s43247-021-00249-w). The greenhouse world of NE Greenland that lay between 70 and 80°N then, as it still does, was an area alternating between shallow marine and terrestrial conditions, the latter characterised by coastal plain and floodplain sediments deposited in estuaries, deltas and lakes. They include coals derived from lush, wooded swamps, inhabited by hippo-like ungulates, primates and reptiles.

At that time the opening of the northern part of the North Atlantic had barely begun, with little chance for an equivalent of the Gulf Stream to have had a warming influence on the Arctic. Shortly after the PETM volcanism began in earnest, to form the flood basalts of the North Atlantic Igneous Province. Each lava flow is capped by red soil or bole: further evidence for a warm, humid climate and rapid chemical weathering. As well as lava build-up, tectonic forces resulted in uplift, effectively opening migration routes for animals and land plants to colonise the benign – for such high latitudes – conditions and perhaps escape the far hotter conditions further south.

The situation now is much different, with the potential for even more rapid melting of the Greenland ice cap to flood freshwater into the North Atlantic, as is currently beginning. Diluting surface seawater reduces its density and thus its tendency to sink, which is the main driving force that pulls warmer water towards high-latitudes in the form of the Gulf Stream. Slowing and even shutting down its influence may have an effect that contradicts the general tendency for global warming – a cooling trend at mid- to high latitudes with chaotic effects on atmospheric pressure systems, the jet stream and weather in general.

See also: Barham, M. et al. 2021. When Greenland was green: rapid global warming 55 million years ago shows us what the future may hold. The Conversation, 23 August 2021.

Soluble iron and global climate

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The environment that humans inhabit is better described as the Earth System, for a good reason. Every part of our planet, the living and the seemingly inert, from the core to the outermost atmosphere, is and always has been interacting with all the others in one way or another. Earth-logs aims to express that, as does my recently revised and now free book Stepping Stones. The vagaries of the Earth’s climate present good examples, the most obvious being the role of chemistry in the form of atmospheric greenhouse gases, especially carbon dioxide, and their interaction with other parts of the Earth System.

Carbon and oxygen atoms that make up CO2 are also present in dissolved form in rain, freshwater and the oceans as the dissolved gas itself, carbonic acid (H2CO­3­) and the soluble bicarbonate ion HCO3–, in proportions that depend on water temperature and acidity (pH). Those forms make the oceans an extremely large ‘sink’ for carbon; i.e. CO2 in dissolved form is removed from the atmospheric greenhouse effect. In the short term, there is a rough balance because water bodies also emit CO2, particularly when they heat up.

Phytoplankton bloom in the Channel off SW England (Landsat image)

Carbon dioxide enters more resilient forms through the marine part of the biosphere, at the base of which is photosynthesising phytoplankton. Photosynthesisers ‘sequester’ CO2 from the oceans as various carbohydrates in their soft tissue. Some of them use bicarbonate ions to form calcium carbonate in shells or tests. Once the organisms die both their soft and hard parts may end up buried in ocean-floor sediments: a longer-term sink. How much carbon is buried in these two forms depends on whether bacteria break down the soft tissues by oxidation and on the acidity of water that tends to dissolve the carbonate. Both processes ultimately yield dissolved CO2 that returns to the atmosphere.

Even the simplest phytoplankton cannot live on carbon dioxide and water alone: they need nutrients. The most familiar to any gardener are nitrogen, phosphorus and potassium. These are mainly supplied in runoff from the continents; although marine upwellings supply large amounts where deep ocean water is forced to the surface. Large tracts in the central parts of the oceans are, in effect, marine deserts whose biological productivity is very low. Surprisingly this is not because of severe shortages of N, P and K. This is because a key nutrient, albeit a minor one, is missing; dissolved iron that phytoplankton and ocean fertility in general depend on. This was discovered in the 1970s by US oceanographer John Martin. Just how important iron is to fertility of the oceans and to global climate emerged from studies of ice cores from the Antarctic ice sheet. Air bubbles in the myriad annual layers reveal that their CO2 content falls with each change in oxygen isotopes related to the periodic build up of polar ice caps during cold periods. The greenhouse effect diminished as a result during each stadial, for the simple reason that up to a third of all atmospheric carbon dioxide – about 200 billion tonnes – was withdrawn. The clearest of these are at the last glacial maximum and during the rapid build up glacial ice between 70 and 60 thousand years ago; a time of low sea level when a major ‘out-of-Africa’ human migration took place. A possible candidate for achieving this could have been massively increased ocean fertility and the burial of dead phytoplankton and their shells.

Analyses of Antarctic ice cores record fluctuations in atmospheric CO2 trapped in bubbles during the last ice age (top) and how iron-rich dust deposition onto the ice increased hugely during two major cold periods (bottom) – the last glacial maximum (35 to 18 ka) and between 70 and 60 ka. (Credit, Stoll; Fig. 1)

During stadials the ice cores also reveal that a great deal more dust found its way from the continents to the polar ice sheets. Analysing the dusty layers showed that to have included lots of iron. Falling into the cold ocean-surface waters around the polar regions would have added this crucial nutrient to a medium already rich in CO2 – the colder water is the more gas it will dissolve. These distant oceans bloomed with phytoplankton, speeding up the sequestration of carbon into ocean-floor sediments. Iron may have triggered a biological pump of gargantuan proportions that amplified ice-age cooling. Today the remotest parts of the world’s oceans are starved of iron so the pump only functions in a few places where iron is supplied by rivers or upwellings of deep ocean water

The marine biosphere is clearly a very important active component in the Earth’s climate subsystem. Climate’s continually changing interactions with the rest of the Earth System make climate change hugely complex. It is difficult to predict but growing understanding of its past behaviour is helpful. The late John Martin’s hypothesis of the effects on climate of changing iron concentrations in surface ocean water has a corollary: the stronger the biological pump the more oxygen in deep water must be used up in bacterial decay of descending organic matter. Indeed it was as recent estimates of the degree of oxygenation in ocean-sediment layers correlate with changes in climate that they also reveal.

So, would deliberate iron-fertilisation of polar oceans help draw down greenhouse warming? When several small patches of the Southern Ocean were injected with a few tonnes of dissolved iron they did indeed respond with phytoplankton blooms. However, it is impossible to tell if that had any effect on the atmosphere. ‘Going for broke’ with a massive fertilisation of this kind has been proposed, but this ventures dp into the political swamp that currently surrounds global warming and the wider environment. It is becoming possible to model such a strategy by using the data from the experiments and from ice cores, and early results seem to confirm the role of iron and the biological pump in CO2 sequestration by suggesting that half the known draw-down during ice ages can be explained in this way.

Based on a review by: Heather Stoll in February 2020. (30 years of the iron hypothesis of ice ages. Nature, v. 578, p. 370-371; DOI: 10.1038/d41586-020-00393-x}

Chaos and the Palaeocene-Eocene thermal maximum

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The transition from the Palaeocene to Eocene Epochs (56 Ma) was marked by an abrupt increase in global mean temperature of about 5 to 8°C within about 10 to 20 thousand years. That is comparable to a rate of warming similar to that currently induced by human activities. The evidence comes from the oxygen isotopes and magnesium/calcium ratios in the tests of both surface- and bottom dwelling foraminifera. The event is matched by a similarly profound excursion in the δ13C of carbon-rich strata of that age, whose extreme negative value marks the release of a huge mass of previously buried organic carbon to the atmosphere. The Epoch-boundary coincides with the beginning of rapid diversification among mammals and plants that had survived the end-Cretaceous mass extinction some 10 Ma beforehand. The most likely cause was the release of methane, a more potent greenhouse gas than CO2, from gas hydrate buried just beneath the surface of sea-floor sediments on continental shelves. An estimated mass of 1.5 trillion tonnes of released methane has been suggested. Methane rapidly oxidizes to CO2 in the atmosphere, which dissolves to make rainwater slightly acid so that the oceans also become more acid; a likely cause for the mass extinction of foraminifera species at the boundary.

Since the discovery of the Palaeocene-Eocene Thermal Maximum (PETM) in the late-1990s a range of possible causes have been suggested. Releasing methane suddenly from sea-floor gas hydrates needs some kind of trigger, such as a steady increase in the temperature of ocean-bottom water to above the critical level for gas-hydrate stability. The late-Palaeocene witnessed slow global warming by between 3 to 5°C over 4 to 5 Ma. There are several hypotheses for this precursor warming, such as a direct CO2 release from the mantle by volcanic activity for which there are several candidates in the geological record of the Palaeocene. Such surface warming would have had to be transferred to the sea floor on continental shelves to destabilise gas hydrates, which implicates a change in oceanic current patterns. An extraterrestrial cause has also been considered (see Impact linked to the Palaeocene-Eocene boundary event, Earth-logs October 2016). Sediment cores from the North Atlantic off the eastern seaboard of the US have revealed impact debris including glass spherules and shocked mineral grains at the same level as the PETM, together with iridium in terrestrial sediments onshore of the same age: there are no such global signatures). But apart from two small craters in Texas and Jordan (12 and 5 km across, respectively) of roughly the same age, no impact event of the necessary magnitude for truly global influence is known. However, there may have been an altogether different triggering mechanism.

Since the confirmation of the Milanković-Croll hypothesis to explain the cyclical shifts in climate during the Pleistocene Epoch in terms of changes in Earth’s orbital characteristics induced by varying gravitational forces in the solar system, the findings have been used as an alternative means of dating other stratigraphic events that show cyclicity. In essence, the varying forces at work are inherently chaotic in a formally mathematical sense. Although Milanković cycles sometimes pop-up when ancient, repetitive stratigraphic sequences are analysed, consistently using the method as a tool to calibrate the geological record to an astronomical timescale breaks down for sediments older than about 50 Ma. Calculations disagree markedly beyond that time. Richard Zeebe and Lucas Lourens of the Universities of Hawaii and Utrecht tried an opposite approach, using the known geological records from deep-sea cores to calibrate the astronomical predictions and, in turn, used the solution to take the astronomical time scale further back than 50 Ma (Zeebe, R.E. & Lourens, L.J. 2019. Solar System chaos and the Paleocene–Eocene boundary age constrained by geology and astronomy. Science, v. 365, p. 926-929; DOI: 10.1126/science.aax0612). They reached back about 8 Ma, so putting the PETM in focus. As well as refining its age (56.01 ± 0.05 Ma) they showed that the PETM coincided with a 405 ka maximum in Earth’s orbital eccentricity lasting around 170 ka: a possible orbital trigger for the spike in temperature and δ13C together, with evidence for a period of chaos in the Solar System about 50 Ma ago. But, what did that chaos actually do, other than mess up orbital dating? To me it seems to suggest something narsty happening to the behaviour of the Giant Planets that are the Lords of the astronomical dance…

See also: Grabowski, M. 2019. Deep-sea sediments reveal solar system chaos: an advance in dating geologic archives. SOEST News

Odds and ends about Milankovitch and climate change

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It is some 40 years since the last explosive development in understanding the way the world works. In 1976 verification of Milutin Milanković’s astronomical theory to explain cyclical climate change as expressed by surface processes has had a similar impact as the underpinning of internal processes by the emergence of plate tectonics in the preceding decade. Signals that match the regularity of changes in the Earth’s orbital eccentricity and the tilt and precession of its axis of rotation, with periods of roughly 96 and 413 ka, 41 ka, 21 and 26 ka respectively, were found in climate change proxies in deep-sea sediment cores (oxygen isotope sequences from benthonic foraminifera) spanning the last 2.6 Ma. The findings seemed as close to proof as one might wish, albeit with anomalies. The most notable of these was that although Milanković’s prediction of a dominant 41 ka effect of changing axial tilt, the strongest astronomical forcing, had characterised cooling and warming cycles in the early Pleistocene, since about a million years ago a ~100 ka periodicity took over – that of the weakest forcing from changing orbital obliquity. Analysis of sedimentary cycles from different episodes in earlier geological history, as during Carboniferous to Permian global frigidity, seemed to confirm that gravitational fluctuations stemming from the orbits of other planets, Jupiter and Saturn especially, had been a continual background to climate change.

All manner of explanations have been offered to explain why tiny, regular and predictable changes in Earth’s astronomical behaviour produce profound changes in the highly energetic and chaotic climate system. Much attention has centred on the mathematically based concept of stochastic resonance. That is a phenomenon where weak signals may be induced to show themselves if they are mixed with a random signal – ‘white noise’ spanning a great range of frequencies. The two resonate at the hidden frequencies thereby strengthening the weak, non-random signal. Noise is already present in the climate system because of the random and highly complex nature of the components of climate itself and the surface processes that it induces.

The latest development along these lines suggests that something quite simple may be at the root of inner complexities in the climatic history of the Pleistocene Epoch: the larger an ice sheet becomes and the longer it lasts the easier it is to cause it to melt away (Tzedakis, P.C. et al. 2017. A simple rule to determine which insolation cycles lead to interglacials. Nature, v. 542, p. 427-432; doi:10.1038/nature21364). The gist of the approach taken in the investigation lies in analysing the degree to which the onsets of major ice-cap melting match astronomically predicted peaks in summer insolation north of 65° N. It also subdivides O-isotope signals of periods of sea level rise into full interglacials, interstadials during periods of climate decline and a few cases of extended interglacials. Through time it is clear that there has been an  increase in the number of interstadials that interrupt cooling between interglacials. Plotting the time of peaks in predicted summer warming closest to major glacial melting events against their insolation energy is revealing.

Before 1.5 Ma the peak energy of summer insolation in the Northern Hemisphere exceeded a threshold leading to full interglacials rather than interstadials more often than it did during the period following 1 Ma. Although Milanković’s 41 ka periodicity remained recognisable throughout, from about 1.5 Ma ago more and more of the energy peaks resulted in only the partial ice melting of interstadial events. The energy threshold for the full deglaciation of interglacials seems to have increased between 1.5 to 1.0 Ma and then settled to a ‘steady state’. The balance between glacial growth and melting increasingly ‘skipped’ 41 ka peaks in insolation so that ice caps grew bigger with time. Deglaciation then required additional forcing. But considering the far larger extent of ice sheets, the tiny additional insolation due to shifts in  orbital eccentricity every ~100 ka surprisingly tipped truly savage ice ages into warm interglacials.

Resolving this paradox may lie with three simple, purely terrestrial factors associated with great ice caps: thicker and more extensive ice becomes warmer at its base and more prone to flow; climate above and around large ice caps becomes progressively colder and drier, so reducing their growth rate; the more sea level falls as land ice builds up, the more the vertical structure and flow of ocean water change. The first of these factors leads to periodic destabilisation when ice sheets surge outwards and increase the rate of iceberg calving into the surrounding oceans. Such ‘iceberg armadas’ characterised the last Ice Age to result in sudden irregularly spaced changes in ocean dynamics and global climate to return to metastable ice coverage, as did earlier ones of similar magnitude. The second factor results in dust lingering at the surface of ice caps that reduced the ability of ice to reflect solar radiation back to space, which enhances summer melting. The third and perhaps most profound factor reduces the formation of ocean bottom water into which dissolved carbon dioxide has accumulated from thermohaline sinking of surface water. This leads to more CO2 in the atmosphere and a growing greenhouse effect. Comforting as finding simplicity within huge complexity might seem, that orbital eccentricity’s weak effect on climatic warming – an order of magnitude less than any other astronomical forcing – can tip climate from one extreme to the other should be a grave warning: climate is chaotic and responds unpredictably to small changes …

Ancient CO2 estimates worry climatologists

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Concerns about impending, indeed actual, anthropogenic climate change brought on by rapidly rising levels of the greenhouse gas carbon dioxide have spurred efforts to quantify climates of the distant past. Beyond the CO2 record of the last 800 ka established from air bubbles trapped in glacial ice palaeoclimate researchers have had to depend on a range of proxies for the greenhouse effect. Those based on models linking plate tectonic and volcanic CO2 emissions with geological records of the burial of organic matter, weathering and limestone accumulation are imprecise in the extreme, although they hint at considerable variation during the Phanerozoic. Other proxies give a better idea of the past abundance of the main greenhouse gas, one using the curious openings or stomata in leaves that allow gases to pass to and fro between plant cells and the atmosphere. Well preserved fossil leaves show stomata nicely back to about 400 Ma ago when plants first colonised the land.


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Stomata on a rice leaf (credit: Getty images)

Stomata draw in CO2 so that it can be combined with water during photosynthesis to form carbohydrate. So the number of stomata per unit area of a leaf surface is expected to increase with lowering of atmospheric CO2 and vice versa. This has been observed in plants grown in different air compositions. By comparing stomatal density in fossilised leaves of modern plants back to 800 ka allows the change to be calibrated against the ice-core record. Extending this method through the Cenozoic, the Mesozoic and into the Upper Palaeozoic faces the problems of using fossils of long-extinct plant leaves. This is compounded by plants’ exhalation of gases to the atmosphere – some CO2 together with other products of photosynthesis, oxygen and water vapour. Increasing stomatal density when carbon dioxide is at low concentration risks dehydration. How extinct plant groups coped with this problem is, unsurprisingly, unknown. So past estimates of the composition of the air become increasingly reliant on informed guesswork rather than proper calibration. The outcome is that results from the distant past tend to show very large ranges of CO2 values at any particular time.

An improvement was suggested some years back by Peter Franks of the University of Sydney with Australian, US and British co-workers (Franks, P.J. et al. 2014. New constraints on atmospheric CO2 concentration for the Phanerozoic. Geophysical Research Letters, v. 41, p. 4685-4694; doi:10.1002/2014GL060457). Their method included a means of assessing the back and forth exchange of leaf gases with the atmosphere from measurements of the carbon isotopes in preserved organic carbon in the fossil leaves, and combined this with stomatal density and the actual shape of stomata. Not only did this narrow the range of variation in atmospheric CO2 results for times past, but the mean values were dramatically lessened. Rather than values ranging up to 2000 to 3000 parts per million (~ 10 times the pre-industrial value) in the Devonian and the late-Triassic and early-Jurassic, the gas-exchange method does not rise above 1000 ppm in the Phanerozoic.

The upshot of these findings strongly suggests that the Earth’s climate sensitivity to atmospheric CO2 (the amount of global climatic warming for a doubling of pre-industrial CO2 concentration) may be greater than previously thought; around 4° rather than the currently accepted 3°C. If this proves to be correct it forebodes a much higher global temperature than present estimates by the Intergovernmental Panel on Climate Change (IPCC) for various emission scenarios through the 21st century.

See also: Hand, E. 2017. Fossil leaves bear witness to ancient carbon dioxide levels. Science, v. 355, p. 14-15; DOI: 10.1126/science.355.6320.14.

Kelly, H. 2017. How did plants evolve stomata.

Out of Africa: a little less blurred?

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DNA from the mitochondria of humans who live on all the habitable continents shows such a small variability that all of us must have had a common maternal ancestor, and she lived in Africa about 160 ka ago. Since this was first suggested by Rebecca Cann, Mark Stoneking and Allan Wilson of the University of California, Berkeley in 1987 there has been a stream of data and publications – subsequently using Y-chromosome DNA and even whole genomes – that both confirm an African origin for Homo sapiens and illuminate it. Analyses of the small differences in global human genetics also chart the routes and – using a ‘molecular clock’ technique – the timings of geographic and population branchings during migration out of Africa. As more and better quality data emerges so the patterns change and become more intricate: an illustration of the view that ‘the past is always a work in progress’. The journal Nature published four papers online in the week ending 25 September 2016 that demonstrate the ‘state of the art’.

Three of these papers add almost 800 new, high-quality genomes to the 1000 Genomes Project that saw completion in 2015. The new data cover 270 populations from around the world including those of regions that have previously been understudied for a variety of reasons: Africa, Australia and Papua-New Guinea. All three genomic contributions are critically summarized by a Nature News and Views article (Tucci, S & Akey, J.L. 2016. A map of human wanderlust. http://dx.doi.org/10.1038/nature19472). The fourth paper pieces together accurately dated fossil and archaeological findings with data on climate and sea-level changes derived mainly from isotopic analyses of marine sediments and samples from polar ice sheets (Timmermann, A & Friedrich, T. 2016. Late Pleistocene climate drivers of early human migration. Nature, doi:10.1038/nature19365). Axel Timmermann and Tobias Friedrich of the University of Hawaii have attempted to simulate the overall dispersal of humans during the last 125 ka according to how they adapted to environmental conditions; mainly the changing vegetation cover as aridity varied geographically, together with the opening of potential routes out of Africa via the Straits of Bab el Mandab and through what is now termed the Middle East or Levant. They present their results as a remarkable series of global maps that suggest both the geographic spread of human migrants and how population density may have changed geographically through the last glacial cycle. Added to this are maps of the times of arrival of human populations across the world, according to a variety of migration scenarios. Note: the figure below estimates when AMH may have arrived in different areas and the population densities that environmental conditions at different times could have supported had they done so. Europe is shown as being possibly settled at around 70-75 ka, and perhaps having moderately high densities for AMH populations. Yet no physical evidence of European AMH is known before about 40 ka. Anatomically modern humans could have been in Europe before that time but failed to diffuse towards it, or were either repelled by or assimilated completely into its earlier Neanderthal population: perhaps the most controversial aspect of the paper.

timmermann
Estimated arrival time since the last continuous settlement of anatomically modern human migrants from Africa (top); estimated population densities around 60 thousand years ago. (Credit: Axel Timmermann University of Hawaii)

The role of climate change and even major volcanic activity – the 74 ka explosion of Toba in Indonesia – in both allowing or forcing an exodus from African homelands and channelling the human ‘line of march’ across Eurasia has been speculated on repeatedly. Now Timmermann and Friedrich have added a sophisticated case for episodic waves of migration across Arabia and the Levant at 106-94, 89-73, 59-47 and 45-29 ka. These implicate the role of Milankovich’s 21 ka cycle of Earth’s axial precession in opening windows of opportunity for both the exodus and movement through Eurasia; effectively like opening and closing valves for the flow of human movement. The paper is critically summarised by a Nature News and Views article (de Menocal, P.B. & Stringer, C. 2016. Climate and peopling of the world. Nature, doi:10.1038/nature19471.

This multiple-dispersal model for the spread of anatomically modern humans (AMH) finds some support from one of the genome papers (Pangani, L. and 98 others 2016. Genomic analyses inform on migration events during the peopling of Eurasia. Nature (online). http://dx.doi.org/10.1038/nature19792). A genetic signature in present-day Papuans suggests that at least 2% of their genome originates from an early and largely extinct expansion of AMH from Africa about 120 ka ago, compared with a split of all mainland Eurasians from African at around 75 ka. It appears from Pangani and co-workers’ analyses that later dispersals out of Africa contributed only a small amount of ancestry to Papuan individuals. The other two genome analyses (Mallick, S. and 79 others 2016. The Simons Genome Diversity Project: 300 genomes from 142 diverse populations. Nature (online) http://dx.doi.org/10.1038/nature18964; Malaspinas, A.-S. and 74 others 2016. A genomic history of Aboriginal Australia. Nature (online). http://dx.doi.org/10.1038/nature18299) suggest a slightly different scenario, that all present-day non-Africans branched from a single ancestral population. In the case of Malaspinas et al. an immediate separation of two waves of AMH migrants led to settlement of Australasia in one case and to the rest of Mainland Eurasia. Yet their data suggest that Australasians diverged into Papuan and Australian population between 25-40 ka ago. Now that is a surprise, because during the lead-up to the last glacial maximum at around 20 ka, sea level dropped to levels that unified the exposed surfaces of Papua and Australia, making it possible to walk from one to the other. These authors appeal to a vast hypersaline lake in the emergent plains, which may have deterred crossing the land bridge. Mallick et al. see an early separation between migrants from Africa who separately populated the west and east of Eurasia, with possible separation of Papuans and Australians from the second group.  These authors also show that the rate at which Eurasians accumulated mutations was about 5% faster than happened among Africans. Interestingly, Mallick et al. addressed the vexed issue of the origin of the spurt in cultural, particularly artistic, creativity after 50 ka that characterizes Eurasian archaeology. Although their results do not rule out genetic changes outside Africa linked to cultural change, they commented as follows:

‘… however, genetics is not a creative force, and instead responds to selection pressures imposed by novel environmental conditions or lifestyles. Thus, our results provide evidence against a model in which one or a few mutations were responsible for the rapid developments in human behaviour in the last 50,000 years. Instead, changes in lifestyles due to cultural innovation or exposure to new environments are likely to have been driving forces behind the rapid transformations in human behaviour …’.

Variations in interpretation among the four papers undoubtedly stem from the very different analytical approaches to climate and genomic data sets, and variations within the individual sets of DNA samples. So it will probably be some time before theoretical studies of the drivers of migration and work on global human genomics and cultural development find themselves unified. And we await with interest the pooling of results from all the different genetics labs and agreement on a common data-mining approach.

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Focus on glaciation…and avoid physics envy

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About 1.3 billion years ago two small black holes, each weighing in at about 30 solar masses, ran into one another and fused. At that time Earthly life forms had neither mouths nor anuses, nor even a nervous system, and they were not much bigger than a sand grain. The distant collision involved  rapid acceleration of considerable masses. A century ago Albert Einstein predicted that the movement of any matter in the universe should perturb space-time in a wave-like form that travels at the same speed as light. Well, he was right for, at 9:50:45 universal time on 14 September 2015, four exquisitely engineered mirrors deployed in the two set-ups of a Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana and Washington states in the US minutely shuddered, first in the Deep South and 0.007 seconds later in the Pacific Northwest. The signal lasted 0.25 seconds and, when rendered as sound, comprised a sort of chirrup starting at 35 Hz and rising to 250 Hz before an abrupt end. Five months later, and silent internationally shared theoretical verification, the story was released to the back slapping, stamping and pawing the air that we have come to expect from clever, ambitious and persuasive people who have spent a great deal of our money and have something to show for it. So now we know that the universe is probably throbbing – albeit very, very, very quietly – with gravitational waves generated by every single motion that has taken place in the whole of ‘recorded’ history since the Big Bang. Indeed, it is claimed, LIGO-like machines may one day detect the big wave itself if, that is, it hasn’t already passed through the solar system. Recall, 13.7 billion years ago the Big Bang didn’t take much longer than this comparatively mundane collision at 1.3 Ga . Physicists are going to have a lot to ponder on now they have a lever to get yet greater funds. To put all this in perspective, the detected chirrup had been traveling for 1.3 Ga, and so too must the actual place in the universe where it took place: I guess we will never know where it is now or what damage or otherwise may have been visited upon planetary systems in its vicinity, if indeed it had even the slightest recognisable geological or ecological consequence.

So, onto the mundane world of glaciology and climate change.

Tibet is the third greatest repository of glacial ice on the surface of the Earth’s continents. It is the focus of one of the planet’s greatest climatic system, the South Asian. While much of the Plateau hasn’t borne glaciers continuously throughout even the last glacial cycle, it is becoming clear that its western margin has remained cold enough to retain ice throughout an even longer period. In the Kunlun mountains is a 200 km2 ice cap known as the Guliya. At the start of detailed glacial stratigraphic ventures in 1990s, focused mainly on Greenland and Antarctica, analysis of a core from the Guliya ice cap yielded dates extending back to 130 ka, before the start if the last interglacial. This section lies above ice that at the time could not be dated reliably other than to show that it may be older than about 750 ka. This stemmed from its lack of the radioactive 36Cl formed, similarly to 14C, by cosmic-ray interactions with stable 35Cl in atmospheric salt aerosols: such cosmogenic chlorine can be used for radiometric dating of ice younger than 750 ka.

A News Feature in the 29 January issue of Science (Qiu, J. 2016. Tibet’s primeval ice. Science, v. 351, p. 436-439) focused on the preliminary results of an expedition, led by Yao Tandong of the Institute of Tibetan Plateau Research, Beijing and Lonnie Thompson of Ohio State University, Columbus, to drill a further five ice cores at Guliya in September 2015, one of which penetrated ove 300 m of glacial ice. It is now possible to date ice layers back to a million years using argon isotopes. Combined with stable isotope and other measurements through the cores, the dating should provide a huge amount of new information on the evolution of the monsoon, which is currently understood only vaguely. Such information would sharpen models of how the monsoon system works and even hint at how it might change during a period of anthropogenic warming. An estimated 1.4 billion people – a fifth of humanity – who live in the Indian subcontinent, China and SE Asia depend for their food-production on the monsoon.

With less humanitarian urgency but equally fascinating is the discovery that, as well as sea-ice, the central Arctic Ocean once hosted vast ice shelves during the last-but-one glacial episode (Jakobsson, M. and 24 others 2016. Evidence for an ice shelf covering the central Arctic Ocean during the penultimate glaciations. Nature Communications, v. 7, doi:10.1038/ncomms10365. Clues emerged from multibeam sonar bathymetry that created detailed images of topography on the floor of the Arctic Ocean. These revealed sets of parallel ridges on the shallowest parts of the polar basin, thought to have formed when moving ice shelves grounded. The depths of the grooved areas indicate ice thicknesses up to and exceeding 1 km. The grooves look very similar to the large-scale lineaments that formed on the surface of the Canadian Shield when the Laurentide ice sheet ground its way from zones of glacial accumulation. Grounding of an ice shelf would have resulted in its thickening in the upflow direction as a result of plastic deformation of the ice, tending to lock the flow and direct ice escape over the deeper parts of the Arctic basin.

Antarctic Ice Shelf
Antarctic Ice Shelf (credit: Wikipedia)

Back-tracking the grooves defines the ice shelf’s source regions in the northern Canadian islands, north Scandinavia and the lowlands of eastern Siberia as well as regional flow patterns and the extent of floating continental ice. The last is a major surprise: at over 4 million km2 it was four times larger than all modern Antarctic ice shelves. The ice moved to ‘escape’ to the North Atlantic Ocean through the Fram Strait between East Greenland and Svalbard (Spitzbergen). Dating sediment stratigraphy in the grooved areas using magnetic and fossil data shows that the ice shelves existed between 160 and 140 ka during the penultimate glacial maximum. For such a mass of glacial ice to be expelled into the Arctic Ocean implies that a great deal more snow fell on its fringes then than during the last glacial maximum. Another possibility is that the huge mass of floating ice regulated the salinity and density of the upper Atlantic in a different way from the periodic iceberg ‘armadas’ that characterized the last glacial epoch and help account for a whole number of sudden warming and cooling events.

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Domack, E. 2016. A great Arctic ice shelf. Nature, v. 530, p. 163-164.

The core’s influence on geology: how does it do it?

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Although no one can be sure about the details of processes in the Earth’s core what is accepted by all is that changes in core dynamics cause the geomagnetic field to change in strength and polarity, probably through some kind of physical interaction between core and deep mantle at the core-mantle boundary (CMB). Throughout the last 73 Ma and especially during the Cenozoic Era geomagnetism has been more fickle than at any time since a more or less continuous record began to be preserved in the Jurassic to Recent magnetic ‘stripes’ of the world ocean floor. Moreover, they came in bursts: 5 in a million years at around 72 Ma; 10 in 4 Ma centred on 54 Ma; 17 over 3 Ma around 42 Ma; 13 in 3 Ma at ~24 Ma; 51 over a period of 12 Ma centring on 15 Ma. During the Late Jurassic and Early Cretaceous the core was similarly ‘busy’, the two time spans of frequent reversals being preceded by quiet ‘superchrons’ dominated by the same normal polarity as we have today i.e. magnetic north being roughly around the north geographic pole.

The Cenozoic history of magnetic reversals - black periods were when geomagnetic field polarity was normal and white when reversed. (credit: Wikipedia)
The Cenozoic history of magnetic reversals – black periods were when geomagnetic field polarity was normal and white when reversed. (credit: Wikipedia)

Until recently geomagnetic ‘flips’ between the two superchrons were regarded as random , perhaps suggesting chaotic behaviour at the CMB. But such a view depends on the statistical method used. A novel approach to calculating reversal frequency through time, however, shows peak-trough pairs recurring 5 times through the Cenozoic Era, approximately 13 Ma apart: maybe the chaos is illusory (Chane, J. et al. 2015. The 13 million year Cenozoic pulse of the Earth. Earth and Planetary Science Letters, v. 431, p. 256-263). So, here is a kind of yardstick to see if there may be any connection between core processes and those at the surface, which Chen of the Fujian Normal University, Fushou China and Canadian and Chinese colleagues compared with the very detailed Cenozoic oxygen-isotope (δ18O) record preserved by foraminifera in ocean-floor sediments, which is a well established proxy for changes in climate. Removing the broad trend of cooling through the Cenozoic resulted in a plot of more intricate climatic shifts that matches the geomagnetism record in both shape and timing of peak-trough pairs. It also turns out, or so the authors claim, that both measures correlate with changes in the rate of Cenozoic subduction of oceanic lithosphere (a measure of plate tectonic activity), albeit negative – peaks in magnetism and climate connecting with slowing in the pace of tectonics.

The analyses involved some complicated maths, but taken at face value the correlations beg the questions why and how? Long-term climate change contains an astronomical signal, encapsulated in the Milankovich hypothesis which has been tested again and again with little room for refutation. So is this all to do with gravitational influences in the Solar System. More exotic still is the possibility of 13 Ma cyclicity linking the Milankovich mechanism with the vaster scale of the Sun’s orbit oscillating through the disc of the Milky Way galaxy and theoretical hints of a mysterious role for dark matter in or near the galaxy. Or, is it a relationship in which climate and the magnetic field are modulated by plate tectonics through varying volcanic emissions of greenhouse gases and the deep effect of subduction on processes at the CMB respectively? To me that seems more plausible, but it is still as exceedingly complex as the maths used to reveal the correlations.

Pleistocene megafaunal extinctions – were humans to blame?

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Australia and the Americas had an extremely diverse fauna of large beasts (giant wombats and kangeroos in Australia; elephants, bears, big cats, camelids, ground sloths etc in the Americas) until the last glaciation and the warming period that led into the Holocene interglacial. The majority of these megafauna species vanished suddenly during that recent period. To a lesser extent something similar happened in Eurasia, but nothing significant in Africa. Because the last glacial cycle also saw migration of efficient human hunter-gatherers to every other continent except Antarctica, many ecologists, palaeontologists and anthropologists saw a direct link between human predation and the mass extinction (see Earth-Pages of April 2012. Earlier humans had indeed spread far and wide in Eurasia before, and the crude hypothesis that the last arrivals in Australasia and the Americas devoured all the meatiest prey in three continents had some traction as a result: predation in Eurasia and Africa by earlier hominids would have made surviving prey congenitally wary of bipeds with spears. In Australia and the Americas the megafauna species would have been naive and confident in their sheer bulk, numbers, speed and, in some cases, ferocity. Other possibilities emerged, such as the introduction of viruses to which faunas had no immunity or as a result of climate change, but none of the three possibilities has gained incontrovertible proof. But the most popular, human connection has had severe knocks in the last couple of years. A fourth, that the extinctions stemmed from a comet impact proved to have little traction.

English: s were driven to extinction by and hu...
Megafauna in a late-Pleistocene landscape including woolly mammoths and rhinoceroses, horses, and cave lions with a carcass. (credit: Wikipedia)

Since the amazing success of analysing the bulk DNA debris in sea water – environmental DNA or eDNA – to look at the local diversity of marine animals, the analytical and computing techniques that made it possible have been turned to ancient terrestrial materials: soils, permafrost and glacial ice. One of the first attempts revealed mammoth and pre-Columbian horse DNA surviving in Alaskan permafrost, thanks to the herds’ copious urination and dung spreading. Several articles in the 24 July 2015 issue of Science review ancient DNA advances, including eDNA from soils that chart changes in both fauna and flora over the last glacial cycle (Pennisi, E. 2015. Lost worlds found. Science, v. 349, p. 367-369). Combined with a variety of means of dating the material that yield the ancient eDNA, an interesting picture is emerging. The soil and permafrost samples potentially express ancient ecosystems in far more detail than would fossil animals or pollens, many of which are too similar to look at the species level and in any case are dominated by the most abundant plants rather than showing those critical in the food chain.

Nunavut tundra
Plants of the Arctic tundra in Nunavut, Canada (Photo credit: Wikipedia)

The first major success in palaeoecology of this kind came with a 50-author paper using eDNA ‘bar-coding’ of permafrost from 242 sites in Siberia and Alaska IWillerslev, E. and 49 others 2014. Fifty thousand years of Arctic vegetation and megafaunal diet. Nature, v. 506, p. 47-51. doi:10.1038/nature12921). Dividing the samples into 3 time spans – 50-25, 25-15 (last glacial maximum) and younger than 15 ka – the team found these major stages in the last glacial cycle mapped an ecological change from a dry tundra dominated by abundant herbaceous plants (forbs including abundant anemones and forget-me-not), to a markedly depleted Arctic steppe ecosystem then moist tundra with woody plants and grasses dominating. They also analysed the eDNA of dung and gut contents from ice-age megafauna, such as mammoths, bison and woolly rhinos, where these were found, which showed that forbs were the mainstay of their diet. Using bones of large mammals 6 member of the team also established the timing of extinctions in the last 56 ka (Cooper, A. et al. 2015. Abrupt warming events drove Late Pleistocene Holarctic megafaunal turnover. Science, DOI: 10.1126/science.aac4315), showing 31 regional extinction pulses linked to the rapid ups and downs of climate during Dansgaard-Oeschger cycles in the run-up to the last glacial maximum. By the end of the last glacial maximum, the megafauna were highly stressed by purely climatic and ecological factors. Human predation probably finished them off.

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Evidence for North Atlantic current shut-down ~120 ka ago

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Gulf stream map
Warming surface currents of the North Atlantic (credit: Wikipedia)

A stupendous amount of heat is shifted by ocean-surface currents, so they have a major influence over regional climates. But they are just part of ocean circulation systems, the other being the movement of water in the deep ocean basins. One driver of this world-encompassing system is water density; a function of its temperature and salinity. Cold saline water forming at the surface tends to sink, the volume that does being replaced by surface flow towards the site of sinking: effectively, cold downwellings ‘drag’ major surface currents along. This is especially striking in the North Atlantic where sinking cold brines are focused in narrow zones between Canada and Greenland and between Greenland and Iceland. From there the cold water flows southwards towards the South Atlantic at depths between 1 and 5 km. The northward compensating surface flow, largely from tropical seas of the Caribbean, is the Gulf Stream/North Atlantic Current whose warming influence on climate of western and north-western Europe extends into the Arctic Ocean.

Circulation in the Atlantic Ocean. the orange and red water masses are those of the Gulf stream and North Atlantic Deep Water (credit: Science, Figure 1 in Galaasen et al. 2014)
Circulation in the Atlantic Ocean. the orange and red water masses are those of the Gulf stream and North Atlantic Deep Water (credit: Science, Figure 1 in Galaasen et al. 2014)

 

Since the discovery of this top-to-bottom ‘conveyor system’ of ocean circulation oceanographers and climatologists have suspected that sudden climate shifts around the North Atlantic, such as the millennial Dansgaard-Oeschger events recorded in the Greenland ice cores, may have been forced by circulation changes. The return to almost full glacial conditions during the Younger Dryas, while global climate was warming towards the interglacial conditions of the Holocene and present day, has been attributed to huge volumes of meltwater from the North American ice sheet entering the North Atlantic. By reducing surface salinity and density the deluge slowed or shut down the ‘conveyor’ for over a thousand years, thereby drastically cooling regional climate. Such drastic and potentially devastating events for humans in the region seem not to have occurred during the 11.5 thousand years since the end of the Younger Dryas. Yet their suspected cause, increased freshwater influx into the North Atlantic, continues with melting of the Greenland ice cap and reduction of the permanent sea-ice cover of the Arctic Ocean, particularly accelerated by global warming.

 

The Holocene interglacial has not yet come to completion, so checking what could happen in the North Atlantic region depends on studying previous interglacials, especially the previous one – the Eemian – from 130 to 114 ka. Unfortunately the high-resolution climate records from Greenland ice cores do not extend that far back. On top of that, more lengthy sea-floor sediment cores rarely have the time resolution to show detailed records, unless, that is, sediment accumulated quickly on the deep sea bed. One place that seems to have happened is just south of Greenland. Cores from there have been re-examined with an eye to charting the change in deep water temperature from unusually thick sediment sequences spanning the Eemian interglacial (Galaasen, E.V. and 7 others 2014. Rapid reductions in North Atlantic Deep Water during the peak of the last interglacial period. Science, v. 343, 1129-1132).

 

The approach taken by the consortium of scientiosts from Norway, the US, France and Britain was to analyse the carbon-isotope composition of the shells of foraminifers that lived in the very cold water of the ocean floor during the Eemian. The ratio of 13C to 12C, expressed as δ13C, fluctuates according to the isotopic composition of the water in which the forams lived. What show up in the 130-114 ka period are several major but short-lived falls in δ13C from the general level of what would then have been North Atlantic Deep Water (NADW). It seems that five times during the Eemian the flow of NADW slowed and perhaps stopped for periods of the order of a few hundred years. If so, then the warming influence of the Gulf Stream and North Atlantic Current would inevitably have waned through the same intervals. Confirmation of that comes from records of surface dwelling forams. This revelation should come as a warning: if purely natural shifts in currents and climate were able to perturb what had been assumed previously to be stable conditions during the last interglacial, what might anthropogenic warming do in the next century?

 

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Did ice-age climate changes across Europe happen at the same time?

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Although the frigid conditions at the last glacial maximum, around 19 to 20 thousand years ago, gradually relinquished their grip through slow global warming, this amelioration came to sudden stop around 12 800 years before the present. Northern hemisphere ice-core and other climate records show that there was a return to glacial conditions over a period of a few decades at most, to launch what is known as the Younger Dryas stadial that lasted over a thousand years until about 11 500 years ago, with the onset of the warm, climatically more stable Holocene that launched the transformation of the human way of life. The start of the Younger Dryas had dramatic effects throughout the northern hemisphere, the cold conditions emerging suddenly from an immense oceanographic change; a weakening or the halt of the North Atlantic thermohaline circulation in which cold, very salty surface waters at the fringe of the Arctic Ocean sink to drag warmer water to high latitudes. In short, the Gulf Stream slowed or stopped its warming influence at high northern latitudes.  Current thoughts centre on a freshening of surface sea water following the collapse of the North American ice sheet to gush meltwater and icebergs into the North Atlantic to buoy-up surface waters.

Iceage time 18kyr
Major climate shifts in Europe since 18 ka (credit: Wikipedia)

Most of the data about this climatic shock can only be dated accurately to within a few centuries: it is clear that the initial cooling was very rapid, on the scale of a few years, as was the warming that closed the Younger Dryas and marked the start of the Holocene, but the ‘when’ is known only to within a few hundred years. To resolve the start and stop ages needs records that include several indicators: clear signs of the beginning and end of the episode, an accurate means of dating them and confirmation from other sites, which presupposes a cast-iron means of correlating the records over large distances. The most reliable markers for correlation are volcanic ashes that can be dated radiometrically and which drift on the wind to be deposited over very large areas. If sedimentary sequences that accumulated continuously preserve such ashes, contain clear signs of climatic change and clearly record the passage of time in great detail, there is a chance of resolving climatic events very accurately; but they are no common.  A British-German team have located and analysed two such promising sites (Land, C.S. et al. 2013. Volcanic ash reveals time transgressive abrupt climate change during the Younger Dryas. Geology, v. 41, p. 1251-1254). One of them is from the bed of a lake that formed by a single volcanic eruption (Meerfelder Maar) in the Eifel region of western Germany. Quiet sediment accumulation has occurred there continuously to form very narrow, alternating dark and light layers, the variegation being due to sedimentation under ice in winter and open water in summer respectively. Twelve thousand of these annual varves provide a means of dating potentially with a precision of ± 1 year, but calibration to absolute time is necessary. The maar sediments contain three ash layers, two of which are from small local eruptions; the older having an age of 12 900 years before 2000 AD, the other being 11 000 years old, showing that the entire Younger Dryas is spanned by the Meerfelder Maar sediments. The third was dated by varve counting, showing the eruption had taken place 12 140 years ago. That age coincides closely with that of major eruption in Iceland.

Panorama Weinfelder Maar oder Totenmaar, Eifel
A typical volcanic maar in Eifel Region of Germany (credit: Wikipedia)

One prominent climatic feature of the Younger Dryas of Europe is a shift around halfway through: it started with the fiercest cold and then ameliorated. This change shows up in the Meerfelder Maar record as a reduction in mean varve thickness and an increase in the titanium content of the clays, the latter taking place in about a year (12 250 years ago) some 100 years before the Icelandic ash was deposited. The same kind of change occurs in records from lakes as far north as the Arctic Circle. One of the core records from Kråkenes in Northern Norway also contains the tell-tale Icelandic ash (as do ice cores from Greenland), but in its case it occurs 20 years before the abrupt climate shift. This clearly shows that major climate changes at the end of the last ice age occur at different times from place to place. The authors ascribe the 120 year difference between the two records to the times when prevailing, warm westerly winds began to affect central and northern Europe, linked to a gradual northward migration of the polar front. The data from both lakes also suggest that the Younger Dryas ended about 20 years earlier in Norway than in Germany, although Lane et al. do not comment..

Hitherto, correlation between climate records has been based on an assumption that major climate changes were at the same time, so that climate proxies such those discussed here have been ‘wiggle-matched’. Quite probably a lot of subtleties have thereby been missed.

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Yes, it was hot during the Permian

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For those of us living in what was the heart of Pangaea – Europe and North America – more than 250 Ma ago this item’s title might seem like the ultimate truism. However, despite our vision of desert dune sands and evaporating inland seas, glaciation blanketed much of the Gondwana part of the supercontinent until the Middle Permian then lying athwart the South Pole. That would go a long way to accounting for extreme dryness at low to mid-latitudes, especially in the deep interior of Pangaea, but just how hot might tropical climates have been? The deglaciation of Gondwana was abrupt and has been touted as an analogue for a possible anthropogenic closure to the Cenozoic glacial epoch that began around 34 Ma in Antarctica and has periodically gripped land at northern latitudes as low as 40°N for the last 2.5 Ma. Since the present distribution of continents is totally different from the unique pole-to-pole shape of Pangaea, that is probably a view that is not widely held by palaeoclimatologists. Nonetheless, getting hard data on Permian conditions has an intrinsic interest for most geoscientists.

The bottom of Death Valley, USA
Playa lake in Death Valley, USA (credit: Wikipedia)

One of the best ways of measuring past temperatures, whether surficial or deep within the crust, almost directly is based on fluids trapped within minerals formed at the time of interest. In Permian strata there is no shortage of suitable material in the form of evaporite minerals, especially common salt or halite.  A distinctive chevron-like texture develops in halite that forms at the water-atmosphere interface in playa lakes that dry out every year. When thin sections of samples that contain fluid inclusions are slowly heated the air bubbles trapped in salt during crystallisation gradually homogenise with the other trapped fluids. Based on samples that have formed at the present day under a range of air temperatures, the temperature of homogenisation indicates the prevailing air temperature accurately. So well, in fact, that it is possible to assess diurnal temperature variations in suitable halite crystals.

Results have been obtained from Middle Permian halites in Kansas, USA (Zambito, J.J. & Benison, K.C. 2013. Extremely high temperatures and paleoclimate trends recorded in Permian ephemeral lake halite. Geology, v. 41, p. 587-590). In part of the section studied air temperatures reached 73°C, compared with a modern maximum of 57°C recorded in halites from the playas of Death Valley. Moreover, they exhibit changes of more than 30°C during daily cycles. But that kind of weather is common in other hot dry areas today, such as the Dasht-e Lut in eastern Iran. Also, the full data show crystallisation at lower temperatures (maxima of 30-40°C) in part of the sequence. What is noteworthy is that these data are the first quantitative indicators of weather before the last 2.5 Ma. Since evaporites extend back into the Precambrian, the method will undoubtedly extend accuracy and precision to paleoclimate  where only proxies and a modicum of guesswork were previously available.