Can rock weathering halt global warming?

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The Lockdown has hardly been a subject for celebration, but there have been two aspects that are, to some extent, a comfort: the trickle of road traffic and the absence of convection trails. As a result the air is less polluted and much clearer, and the quietness, even in cities, has been almost palpable. Wildlife seems to have benefitted and far less CO2 has been emitted. Apart from the universal tension of waiting for one of a host of potential Covid-19 symptoms to strike and the fact that the world economy is on the brink of the greatest collapse in a century, it is tempting to hope that somehow business-as-usual will remain this way. B*gger the gabardine rush to work and the Great Annual Exodus to ‘abroad’. The crisis in the fossil fuel industry can continue, as far as I am concerned, But then, of course, I am retired, lucky to have a decent pension and live rurally. Despite the health risks, however, global capital demands that business-as-it-was must return now. A planet left to that hegemonic force has little hope of staving off anthropogenic ecological decline. But is there a way for capital to ‘have its cake and eat it’? Some would argue that there are indeed technological fixes. Among them is sweeping excess of the main greenhouse gas ‘under the carpet’ by burying it. There are three main suggestions: physically extracting CO2 where it is emitted and pumping it underground into porous rocks; using engineered biological processes in the oceans to take carbon into planktonic carbohydrate or carbonate shells and disposing the dead remains in soil or ocean-floor sediments; enhancing and exploiting the natural weathering of rock. The last is the subject of a recent cost-benefit analysis (Beerling, D.J. and 20 others 2020. Potential for large-scale CO2 removal via enhanced rock weathering with croplands. Nature, v. 583, p. 242–248; DOI: 10.1038/s41586-020-2448-9).

Carbon dioxide in the rock cycle (Credit: Skeptical Science, in Wikipedia)

Research into the climatic effects of rock weathering has a long history, for it represents one of the major components of the global carbon cycle, as well as the rock cycle. Natural chemical weathering is estimated to remove about a billion metric tons of atmospheric carbon annually. That is because the main agent of weathering is the slightly acid nature of rainwater, which contains dissolved CO2 in the form of carbonic acid (H2CO3). This weak acid comprises hydrogen ions (H+), which confer acidity, that are released by the dissolution of CO2 in water, together with HCO3–ions (bicarbonate, now termed hydrogen carbonate). During weathering the hydrogen ions break down minerals in rock. This liberates metals that are abundant in the silicate minerals that make up igneous rocks – predominantly Na, Ca, K, and Mg – as their dissolved ions, leaving hydrated aluminium silicates (clay minerals) and iron oxides as the main residues, which are the inorganic basis of soils. The dissolved metals and bicarbonate ions may ultimately reach the oceans. However, calcium and magnesium ions in soil moisture readily combine with bicarbonate ions to precipitate carbonate minerals in the soil itself, a process that locks-in atmospheric carbon. Another important consequence of such sequestration is that it may make the important plant nutrient magnesium – at the heart of chlorophyll – more easily available and it neutralises any soil acidity built-up by continuous agriculture.  But carbon sequestration naturally achieved by weathering amounts to only about a thirtieth of that emitted by the burning of fossil fuels, and we know that is incapable of coping with the build-up of anthropogenic CO2 in the atmosphere: it certainly has not since the start of the Industrial Revolution.

What could chemical weathering do if it was deliberately enhanced?  One of the most common rocks, basalt, is made up of calcium-rich feldspar and magnesium-rich pyroxene and olivine. In finely granulated form this mix is particularly prone to weathering, and the magnesium released would enrich existing soil as well as drawing down CO2. Hence the focus by David Beerling and his British, US and Belgian colleagues on systematic spreading of ground-up basalt on cropland soils, in much the same way as crushed limestone is currently applied to reverse soil acidification. It is almost as cheap as conventional liming, with the additional benefit of fertilising: it would boost to crop yields. The authors estimate that removal of a metric ton of CO2 from the atmosphere by this means would cost between US$ 55 to 190, depending on where it was done. One of their findings is that the three largest emitters of carbon dioxide – China, the US and India – happen to have the greatest potential for carbon sequestration by enhanced weathering. Incidentally, increased fertility also yields more organic waste that itself could be used to increase the actual carbon content of soils, if converted through pyrolysis to ‘biochar’ .

It all sounds promising, almost ‘too good to be true’. The logistics that would be needed and the carbon emissions that the sheer mass of rock to be finely ground and then distributed would entail, for as long as global capital continues to burn fossil fuels, are substantial, as the authors admit. The grinding would have to be far more extreme than the production of igneous-rock road aggregate. Basalt or related rock is commonly used for resurfacing motorways, not especially well known for degrading quickly to a clay-rich mush. It would probably have to be around the grain size achieved by milling to liberate ore minerals in metal mines, or to produce the feedstock for cement manufacture: small particles create a greater surface area for chemical reactions. But there remains the issue of how long this augmented weathering would take to do the job: its efficiency. Experimental weathering to test this great-escape hypothesis is being conducted by a former colleague of mine, using dust from an Irish basalt quarry to coat experimental plots of a variety of soil types. After two months Mg and Ca ions were indeed being released from the dust, and tiny fragments of olivine, feldspar and pyroxene do show signs of dissolution. Whether this stems from rainwater – the main objective – or from organic acids and bacteria in the soils is yet to be determined. No doubt NASA is doing much the same to see if dusts that coat much of Mars can be converted into soils  Beerling et al. acknowledge that the speed of weathering is a major uncertainty. Large-scale field trials seem some way off, and are likely to be plagued by cussedness! Will farmers willingly change their practices so dramatically?

See also: Lehmann, J & Possinger, A. 2020. Removal of atmospheric CO2 by rock weathering holds promise for mitigating climate change. Nature, v. 583, p. 204-205; DOI: 10.1038/d41586-020-01965-7

Note (added 15 July 2020): Follower Walter Pohl has alerted me to an interesting paper on using ultramafic rocks in the same way (Kelemen, P.B. et al. 2020. Engineered carbon mineralization in ultramafic rocks for CO2 removal from air: Review and new insights. Chemical Geology, v.  550, Article 119628; DOI:10.1016/j.chemgeo.2020.119628). Walter’s own blog contains comments on the climatic efficacy of MgCO3 (magnesite) formed when olivine is weathered.

Turmoil in Roman Republic followed Alaskan volcanic eruption

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That activities in the global political-economic system are now dramatically forcing change in natural systems is clear to all but the most obdurate. In turn, those changes increase the likelihood of a negative rebound on humanity from the natural world. In the first case, data from ice cores suggests that an anthropogenic influence on climate may have started with the spread of farming in Neolithic times. Metal pollution of soils had an even earlier start, first locally in Neanderthal hearths whose remains meet the present-day standards for contaminated soil, and more extensively once Bronze Age smelting of copper began. Global spread of anomalously high metal concentrations in atmospheric dusts shows up as ‘spikes’ in lead within Greenland ice cores during the period from 1100 BCE to 800 CE. This would have resulted mainly from ‘booms and busts’ in silver extraction from lead ores and the smelting of lead itself. In turn, that may reflect vagaries in the world economy of those times

Precise dating by counting annual ice layers reveals connections of Pb peaks and troughs with major historic events, beginning with the spread of Phoenician mining and then by Carthaginians and Romans, especially in the Iberian Peninsula. Lead reaches a sustained peak during the acme of the Roman Republic from 400 to 125 BC to collapse during widespread internal conflict during the Crisis of the Republic. That was resolved by the accession of Octavian/Augustus as Emperor in 31 BCE and his establishment of Pax Romana across an expanded empire. Lead levels rose to the highest of Classical Antiquity during the 1st and early 2nd centuries CE. Collapse following the devastating Antonine smallpox pandemic (165 to 193 CE) saw the ice-core records’ reflecting stagnation of coinage activity at low levels for some 400 years, during which the Empire contracted and changed focus from Rome to Constantinople. Only during the Early Medieval period did levels rise slowly to the previous peak.

The Okmok caldera on the Aleutian island of Umnak (Credit: Desert Research Institute, Reno, Nevada USA)

Earth-logs has previously summarised how natural events, mainly volcanic eruptions, had a profound influence in prehistory. The gigantic eruption of Toba in Sumatra (~73 ka ago) may have had a major influence on modern-humans migrating from Africa to Eurasia. The beginning of the end for Roman hegemony in the Eastern Mediterranean was the Plague of Justinian (541–549 CE), during which between 25 to 50 million people died of bubonic plague across the Eastern Empire. This dreadful event followed the onset of famine from Ireland to China, which was preceded by signs of climatic cooling from tree-ring records, and also with a peak of volcanogenic sulfate ions in the Greenland and Antarctic ice caps around 534 CE. Regional weakening of the populace by cold winters and food shortages, also preceded the Black Death of the mid-14th century. In the case of the Plague of Justinian, it seems massive volcanism resulted in global cooling over a protracted period, although the actual volcanoes have yet to be tracked down. Cooling marked the start of a century of further economic turmoil reflected by lead levels in ice cores (see above). Its historical context is the Early Medieval equivalent of world war between the Eastern Roman Empire, the Sassanid Empire of Persia and, eventually, the dramatic appearance on the scene of Islam and the Arabian, Syrian and Iraqi forces that it inspired (see: Holland, T. 2013. In the Shadow of the Sword: The battle for Global Empire and the End of the Ancient World. Abacus, London)

An equally instructive case of massive volcanism underlying social, political and economic turmoil has emerged from the geochemical records in five Greenlandic ice cores and one from the Siberian island of Severnaya Zemlya (McConnell, J.R. and 19 others 2020. Extreme climate after massive eruption of Alaska’s Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom. Proceedings of the National Academy of Sciences, recent article (22 June 2020); DOI: 10.1073/pnas.2002722117). In this case the focus was on ice layers in all six cores that contain sulfate spikes and, more importantly, abundant volcanic dust, specifically shards of igneous glass. Using layer counting, all six show major volcanism in the years 45 to 43 BCE. The Ides (15th) of March 44 BCE famously marked the assassination of Julius Caesar, two years after the Roman Republic’s Senate appointed him Dictator, following four years of civil war. This was in the later stages of the period of economic decline signified by the fall in ice-core levels of Pb (see above). The Roman commentator Servius reported “…after Caesar had been killed in the Senate on the day before, the sun’s light failed from the sixth hour until nightfall.” Other sources report similar daytime dimming, and unusually cold weather and famine in 43 and 42 BCE.

As well as pinning down the date and duration of the volcanic dust layers precisely (to the nearest month using laser scanning of the ice cores’ opacity), Joseph McConnell and the team members from the US, UK, Switzerland, Germany and Denmark also chemically analysed the minute glass shards from one of the Greenlandic ice cores. This has enabled them to identify a single volcano from 6 possible candidates for the eruption responsible for the cold snap: Okmok, an active, 8 km wide caldera in the Aleutian Islands of Alaska. Previous data suggest that its last major eruption was 2050 years ago and blasted out between 10 to 100 km3 of debris, including ash. Okmok is an appropriate candidate for a natural contributor to profound historic change in the Roman hegemony. The authors also use their ice-core data to model Okmok’s potential for climate change: it had a global reach in terms of temperature and precipitation anomalies. Historians may yet find further correlations of Okmok with events in other polities that kept annual records, such as China.

See also: Eruption of Alaska’s Okmok volcano linked to period of extreme cold in ancient Rome (Science Daily, 22 June 2020); Kornei, K. 2020. Ancient Rome was teetering. Then a volcano erupted 6,000 miles away. (New York Times, 22 June 2020)

Did an impact affect hunter gatherers at the start of the Younger Dryas?

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Whether or not the return to a glacial climate between 12.8 and 11.7 thousand years (ka) ago, known as the Younger Dryas (YD), was triggered by some kind of extraterrestrial impact has been a hot and sometimes fractious issue since 2007 (see: Whizz-bang view of Younger Dryas; Earth-logs, July 2007). Before then the most favoured causal mechanism was a shutdown of the Gulf Stream’s Arctic warming influence as a result of some kind of catastrophic flooding of fresh water into the North Atlantic. That would have lowered the density of surface waters, thereby preventing them from sinking to drive the deep circulation that draws surface water from the tropics into high northern latitudes (see: The Younger Dryas flood; May 2010). In 2008 the melt-water flood supporters were sufficiently piqued by the suggestion of a hitherto unsuspected impact event to mount a powerful rejoinder (see: Impact cause for Younger Dryas draws flak; May 2008), casting doubt on the validity of the data that had been presented. It seemed like a repeat of the initial furore over claims for a ‘mountain falling out of the sky’ wiping out the dinosaurs and much else. Yet, like the claims by Alvarez pere et fils for the K-T impact, accumulated weight of evidence published by its protagonists eventually has given the idea of an impact trigger for the YD a measure of respectability. This began with evidence of an impact crater beneath the Greenland icecap (see: Subglacial impact structure in Greenland: trigger for Younger Dryas?; November 2018), then signs of a 12.8 ka fire storm in Chile followed by geochemical evidence from South Carolina, USA for a coinciding impact (see: More on the Younger Dryas causal mechanism; November 2019).

Colour-coded subglacial topography from radar sounding over the Hiawatha Glacier of NW Greenland, showing a possible impact crater (Credit: Kjaer et al. 2018; Fig. 1D)

The YD played havoc with humans who had begun to repopulate northern Europe from their Ice Age refuges in the south and those who had first ventured into the Americas  across the Beringia land bridge between Siberia and Alaska. The climate decline was extremely rapid, spanning a mere decade or so, and many would have been trapped to perish in what again became frigid steppe land. There is now evidence that late-Palaeolithic to Mesolithic hunter gatherers living far south of the reglaciated zone also suffered devastation at the start of the YD (Moore, A.M.T. and 13 others 2020. Evidence of Cosmic Impact at Abu Hureyra, Syria at the Younger Dryas Onset (~12.8 ka): High-temperature melting at > 2,200 °C. Nature Science Reports, v. 10, p. 1-22; doi: 10.1038/s41598-020-60867-w). Abu Hureyra is a tell – a mound settlement – originally on the banks of the Euphrates in northern Syria. It now lies beneath Lake Assad, but was excavated in the early 1970s to reveal a charcoal-littered habitation surface with signs of a settlement and some cultivation. Charcoal from archived samples yielded a precise radiocarbon age of 12825 ± 55 ka, coinciding with the start of the YD. The sediment from the habitation floor also contained signs compatible with ejecta from a high-energy impact: tiny diamonds and glass spherules. Analyses of the glass by the authors suggests that it formed at a temperature up to 2200°C, far greater than that of magma associated with a volcanic eruption or in hearths used by the inhabitants. However, others have analysed the glass and suggest more mundane temperatures that could be explained more simply by accidental burning of thatched huts. That possibility might explain the lack of other impact indicators, such as shocked mineral grains and anomalous geochemistry, particularly the platinum-group metals that were the original ‘smoking gun’ for the K-T boundary event and other major impacts. Incidentally, these crucial indicators have been reported from other YD sites investigated by several members of the team behind this paper. My view is that what seems to be a remarkable coincidence will not settle the matter, but will probably draw the same kind of ‘flak’ as did others on this topic. It is hardly likely that new samples will be collected from the now submerged Abu Hureyra site.

See also: Cometary Debris may have destroyed Paleolithic settlement 12,800 years ago (Science News. 2 July 2020)

Fossil fuel, mercury and the end-Palaeozoic catastrophe

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Siberian flood-basalt flows in the Putorana Plateau, Taymyr Peninsula, Russia. (Credit: Paul Wignall)

The end of the Permian Period (~252 Ma ago) saw the loss of 90% of marine fossil species and 70% of those known from terrestrial sediments: the greatest known extinction in Earth’s history. In their naming of newly discovered life forms, palaeontologists can become quite lyrical. Extinctions, however, really stretch their imagination. They call the Permo-Triassic boundary event ‘The Great Dying’. Why not ‘Permageddon’? Sadly, that was snaffled in the 1980s by an astonishingly short-haired heavy-metal tribute band. Enough bathos … The close of the Palaeozoic left a great many ecological niches to be filled by adaptive radiation during the Triassic and later Mesozoic times. Coinciding with the largest known flood-basalt outpouring – the three million cubic kilometres of Siberian Traps – the P-Tr event seemed to be ‘done and dusted’ after that possible connection was discovered in the mid 1990s. Notwithstanding, the quest for a gigantic, causative impact crater continues (see: Palaeobiology Earth-logs, May, September and October 2004), albeit among a dwindling circle of enthusiasts. The Siberian Traps are suitably vast to snuff the fossil record, for their eruption must have belched all manner of climate-changing gases and dusts into the atmosphere; CO2 to encourage global warming; SO2 and dusts as cooling agents. There is also evidence of a role for geochemical toxicity (see: Nickel, life and the end-Permian extinction, June 2014). The extinctions accompanied not only climate change but also a catastrophic fall in atmospheric oxygen content (see: Homing in on the great end-Permian extinction, April 2003; When rain kick-started evolution, December 2019). Recovery of the biosphere during the early Triassic was exceedingly slow.

Research focussed on the P-Tr boundary eventually uncovered an element of pure chance. Shales in Canada that span the boundary show major, negative δ13C excursions in the carbon-isotope record that coincide with fly ash in the analysed layers. This material is similar in all respects to that emitted from coal-fired power stations (see: Coal and the end-Permian mass extinction, March 2011). The part of Siberia onto which the flood basalts were erupted is rich in Permian coal measures and oil shales that lay close to the surface 252 Ma ago. The coal ash and massive emissions of CO2 may have resulted from their burning by the flood basalt event. Now evidence has emerged that this did indeed happen (Elkins-Tanton, L.T. et al. 2020. Field evidence for coal combustion links the 252 Ma Siberian Traps with global carbon disruption. Geology, v. 48, early publication; DOI: 10.1130/G47365.1).

The US, Canadian and Russian team found large quantities of burnt coal and woody material, and bituminous blobs in 600 m thick volcanic ashes at the base of the Siberian traps themselves. They concluded that the magma chamber from which the flood basalts emerged had incorporated sizeable volumes of the coal measures, leading to their combustion and distillation. This would have released CO2 enriched in light 12C due to isotopic fractionation by biological means, i.e. its δ13C would have been sufficiently negative to affect the carbon locked up in the Canadian P-Tr boundary-layer shales that show the sharp isotopic anomalies. The magnitude of the anomalies suggest that between six to ten thousand billion tons of carbon released as CO2 or methane by interaction of the Siberian Traps with sediments through which their magma passed could have created the global δ13C anomalies. That is about one tenth of the organic carbon originally locked in the Permian coal measures beneath the flood basalts

Another paper whose publication coincided with that by Elkins-Tanton et al. suggests that environmental mercury appears to have followed the same geochemical course as did carbon at the end of the Palaeozoic Era (Dal Corso, J. and 9 others 2020. Permo–Triassic boundary carbon and mercury cycling linked to terrestrial ecosystem collapse. Nature Communications, v. 11, paper 2962; DOI: 10.1038/s41467-020-16725-4). This group, based at Leeds and Oxford Universities, UK and the University of Geosciences in Wuhan, China, base their findings on biogeochemical modelling of the global carbon and mercury cycles at the end of the Permian. Their view is that the coincidence in marine sediments at the P-Tr boundary of a short-lived spike in mercury and an anomaly in its isotopic composition with the depletion in 13C, described earlier, shows an intimate link between mercury and the biological carbon cycle in the oceans at the time. They suggest that this synergy marks ecosystem collapse and derives ‘from a massive oxidation of terrestrial biomass’; i.e. burning of organic material on the land surface. Their modelling hints at huge wildfires in equatorial peatlands but also a role for the Siberian flood-basalt volcanism and the incorporation of coal measures into the Siberian Trap magma chamber.

A protein clue to H. antecessor’s role in human evolution

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Homo_antecessor child
Forensic reconstruction of the remains of a Homo antecessor child from Gran Dolina Cave in northern Spain (credit Élisabeth Daynès, Museo de la Evolución, Burgos, Spain)

The older a fossil, no matter how well preserved it is, the less chance it has to contain enough undegraded DNA for it to be extracted and sequenced using the most advanced techniques. At present the oldest fossil DNA not to have passed its ‘sell-by’date is that of a 560 to 780 thousand year-old horse’s legbone found in Canadian permafrost. For human remains the oldest mtDNA is that of a ~430 ka individual from the Sima de los Huesos in northern Spain (see: Mitochondrial DNA from 400 thousand year old humans; Earth-logs December 2013). But there is another route to establishing genetic relatedness from the amino-acid sequences of proteins recovered from ancient individuals (see: Ancient proteins: keys to early human evolution?). Fossil teeth have proved to be good repositories of ancient protein and are the most commonly found hominin fossils.

A key species for unravelling the origins of the three most recent human groups (ourselves, Neanderthals and Denisovans) is thought to be Homo antecessor who inhabited the Gran Dolina Cave in the Atapuerca Mountains in northern Spain between about 1.2 Ma and 800 ka ago (see: Human evolution: bush or basketwork? Earth-logs, January 2014). Palaeoanthropologists excavated 170 skeletal fragments from six individuals in the most productive layer at Gran Dolina. Incomplete facial bones suggest a ‘modern-like’ face, although the remains as a whole are insufficient to reconstruct the oldest Europeans with sufficient detail to place them in anatomical relation to the younger groups. But there are several teeth. One of them, a permanent molar, has yielded informative proteins (Welker, F. and 26 others 2020. The dental proteome of Homo antecessor. Nature, v. 580, p. 235-238; DOI: 10.1038/s41586-020-2153-8) and has been dated to between 772 to 949 ka.

Amino acids in the dental proteins, sequenced using mass spectrometry, were compared with those of other hominins. Because protein sequences are coded by an animal’s genome they are a ‘proxy’ for DNA. The outcome is that the Gran Dolina proteins are roughly equally related to Denisovans, Neanderthals and ourselves, suggesting that, although the younger three groups are closely related, H. antecessor is an ‘outlier’. Being significantly older, it is likely to be the common ancestor of all three. Another species with close anatomical affinities is H. heidelbergensis (700 to 300 ka) found in Africa as well as in Europe. Its mtDNA (see: Mitochondrial DNA from 400 thousand year old humans; Earth-logs December 2013) matches that of Denisovans better than it does Neanderthals, yet without protein and full-genome analysis all that can be concluded is that it may be an intermediary between H. antecessor and the well known interbreeding triad of more recent times.

We are getting closer to a documented web of interrelationships between humans in general whose time span from 2 Ma ago is now well established. The remaining genetic link to be documented is that to H. erectus, the longest lived and most travelled of all ancient humans. Frido Welker and co-workers also had a shot at the proteomics of one of the first humans known to have migrated from Africa, using an isolated, presumably H. erectus, molar found at the 1.77 Ma site at Dmanisi in the Caucasus foothills of Georgia. Although inconclusive in placing that precociously intrepid group firmly in the human story, the fact that dental proteins were discovered is cause for optimism.

See also: Campbell, M. 2020. Protein analysis of 800,000-year-old human fossil clarifies dispute over ancestors (Technology Networks, 1 April 2020)

What controls the height of mountains?

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‘Everybody knows’ that mountains grow: the question is, ‘How?’ There is a tale that farmers once believed that they grew from pebbles: ‘every year I try to rid my field of stones, but more are back the following year, so they must grow’… Geoscientists know better – or so they think[!] – and for 130 years have referred to ‘orogeny’, a classically-inspired term (from the Ancient Greek óros and geneia – high-ground creation’) adopted by the US geologist Grove Gilbert. It incorporates the concept of crustal thickening that results from lateral forces and horizontal compression. Another term, now rarely used, is ‘epeirogeny’ (coined too by G.K. Gilbert), wherein the continental surface rises or falls in response to underlying gravitational forces. That could include: changing mantle density over a hot, rising plume; detachment or delamination into the mantle of dense lower lithosphere; loading or unloading by ice during glacial cycles. Epeirogeny is bound up with isostasy, the maintenance of gravitational balance of mass in the outermost Earth.

A small part of the High Himalaya (credit: Access-Himalaya)

In 1990, Peter Molnar and Philip England pointed out that the incision of deep valleys into mountain ranges results in stupendous and rapid removal of mass from orogenic belts, which adds a major isostatic force to mountain building (Molnar, P. & England, P. 1990. Late Cenozoic uplift of mountain ranges and global climate change: chicken or egg? Nature, v. 346, p. 29–34; DOI: 10.1038/346029a0). In their model, the remaining peaks are driven higher by isostasy. They, and others, coupled climate change with compressional tectonics in a positive feedback that drives peaks to elevations that they would otherwise never achieve. Molnar and England’s review saw complex interplays contributing to mountain building, accompanying chemical weathering even changing global climate by sequestering atmospheric CO2 into the minerals that it produces. As well as the height of peaks in active zones of crustal shortening and thickening, such as the Himalaya, Molnar and England’s theory explained the aberrant high peaks at the edge of high plateaus that are passively subject to erosion. Examples of the latter are the isolated peaks beyond the eastern edge of the Ethiopian Plateau that locally have the greatest elevation than the flood basalts that form the plateau: unloading around these peaks has caused them to rise isostatically.

Thirty years on, this paradigm is being questioned, at least as regards active orogens (Dielforder, A. et al. 2020. Megathrust shear force controls mountain height at convergent plate margins. Nature, v. 582, p. 225–229; DOI: 10.1038/s41586-020-2340-7). Armin Dielforder and colleagues at the German Research Centre for Geosciences in Potsdam and The University of Münster consider that overall mountain height is sustained by interactions between three forces. 1. They are prevented from falling apart under their own weight or being pushed up further against gravity by lateral tectonic force. 2. Climate controlled erosion limits mountain height by removing material from the highest elevations. 3. Isostasy keeps the mountains ‘afloat’ above the asthenosphere. The authors have attempted to assess and balance all three major forces that determine the overall elevation of mountain belts.

At a convergent plate margin where one plate is shoved beneath another, the megathrust above the subduction zone behaves in a brittle fashion, with associated friction, towards the surface. At depth this transitions to a zone of ductile deformation dominated by viscosity. A major assumption in this work is that stress in the crust below a mountain belt is neutral; i.e. horizontal, tectonic compression is equal to the weight of the mountains themselves and thus to their height. So, the greater the tectonic compressive force the higher the mountain range that it can support. The test is to compare the actual elevation with that predicted from plate-tectonic considerations. For 10 active orogenic belts there is a remarkable correspondence between the model and actuality. the authors conclude that variation over time of mountain height reflects log-term variations in the force balance, in which they find little sign of a climatic/erosional control. But that doesn’t resolve the issue satisfactorily, at least for me.

The study focuses on the mean elevation, and this leaves out the largest mountains; for instance, their maximum mean elevation for the Himalaya is about 5.46 km (in fact for a narrow  NE-SW swath that may not be representative of the whole range). Yet the Himalaya contains 10 of the world’s highest mountains, all over 8 km high and 50 peaks that top 7 km, adjacent to the Tibetan Plateau. The mean elevation of the whole Himalayan range is 6.1 km. Consequently, it seems to me, the range’s maximum mean elevation must be somewhat higher than that reported by Dielforder et al.  The difference suggests that non-tectonic forces do contribute significantly to Himalayan terrain

See also:  Wang, K. 2020. Mountain height may be controlled by tectonic force, rather than erosion. Nature, v. 582, p. 189-190; DOI: 10.1038/d41586-020-01601-4

Geochemistry and the Ediacaran animals

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Hopefully, readers will be fairly familiar with the sudden appearance of the Ediacaran fauna – the earliest abundant, large animals – at the start of the eponymous Period of the Neoproterozoic around 635 Ma. If not, use the Search Earth-logs box in the side bar to find extensive coverage since the start of the 21st century. A June 2019 Earth-logs review of the general geochemical background to the Ediacaran Period can be found here. Ten years ago I covered the possible role of the element phosphorus (P) – the main topic here – in the appearance of metazoans (see: Phosphorus, Snowball Earth and origin of metazoans – November 2010).

One of the major changes in marine sedimentation seen during the Ediacaran was a rapid increase in the deposition on the ocean floor of large bodies of P-rich rock (phosphorite), on which a recent paper focuses (Laakso, T.A. et al. 2020. Ediacaran reorganization of the marine phosphorus cycle. Proceedings of the National Academy of Sciences, v. 117, p. 11961-11967; DOI: 10.1073/pnas.1916738117). It has been estimated that on million-year time scales phosphorites remove only a tiny amount of the phosphorus carried into the oceans by rivers. So, conversely, an increase in deposition of marine P-rich sediment would have little effect on the overall availability of this essential nutrient from the oceans. The Ediacaran boost in phosphorites suggests a connection between them and the arrival of totally new ecosystems: the global P-cycle must somehow have changed. This isn’t the only change in Neoproterozoic biogeochemistry. Thomas Laakso and colleagues note signs of slightly increased ocean oxygenation from changes in sediment trace-element concentrations, a major increase in shallow-water evaporites dominated by calcium sulfate (gypsum) and changes in the relative proportions of different isotopes of sulfur.

Because all marine cycles, both geochemical and those involving life, are interwoven, the authors suggest that changes in the fate of dead organic matter may have created the phosphorus paradox. Phosphorus is the fifth most abundant element in all organisms after carbon, hydrogen, nitrogen and oxygen, followed by sulfur (CHNOPS), P being a major nutrient that limits the sheer bulk of marine life. Perhaps changes to dead organic matter beneath the ocean floor released its phosphorus content, roughly in the manner that composting garden waste releases nutrients back to the soil. Two chemical mechanisms can do this in the deep ocean: a greater supply of sinking organic matter – essentially electron donors – and of oxidants that are electron acceptors. In ocean-floor sediments organic matter can be altered to release phosphorus bonded in organic molecules into pore water and then to the body of the oceans to rise in upwellings to the near surface where photosynthesis operates to create the base of the ecological food chain.

Caption The Gondwana supercontinent that accumulated during the Neoproterozoic to dominate the Earth at the time of the Ediacaran (credit: Fama Clamosa, at Wikimedia Commons)

There is little sign of much increase in deep-ocean oxygen until hundreds of million years after the Ediacaran. It is likely, therefore, that increased availability of oxidant sulfate ions (SO42-) in ocean water and their reduction to sulfides in deep sediment chemically reconstituted the accumulating dead organic matter to release P far more rapidly than before. This is supported by the increase in CaSO4 evaporites in the Ediacaran shallows. So, where did the sulfate come from? Compressional tectonics during the Neoproterozoic Era were at a maximum, particularly in Africa, South America, Australia and Antarctica, as drifting continental fragments derived from the break-up of the earlier Rodinia supercontinent began to collide. This culminated during the Ediacaran around 550 Ma ago with assembly of the Gondwana supercontinent. Huge tracts of it were new mountain belts whose rapid erosion and chemical weathering would have released plenty of sulfate from the breakdown of common sulfide minerals.

So the biological revolution and a more productive biosphere that are reflected in the Ediacaran fauna ultimately may have stemmed from inorganic tectonic changes on a global scale

Is there water in the Earth’s core?

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Understandably, the nature of what lies at the centre of the Earth is as much the subject of speculation as tangible evidence. That there must be something very dense within the planet emerged once the Earth’s bulk density was calculated. Because a high proportion of meteorites are dominated by an alloy of the metals iron and nickel, geoscientists adopted that combination as plausible core material. Study of the arrival times around the globe of seismic waves from earthquakes then revealed the actual size of the Earth’s core. Iron-nickel alloy fitted the bill quite nicely. It also fits geochemical evidence, such as the crust and mantle’s depletion in some trace elements that theoretically have an affinity for iron. The fact that seismology showed also that the outer core was molten and able to flow, together with metals’ high electrical conductivity, gave rise to the current concept of the geomagnetic field being generated by a dynamo effect in the core. However the density of Fe-Ni is not ‘quite right’ because the core is somewhat lighter than predicted for the pure alloy under stupendous pressure: it must contain a substantial amount – up to 13% – of lower density materials.  Silicon, sulfur and oxygen have been suggested as candidates, with evidence from a variety of minor minerals in metallic meteorites.

A recent model for core formation (credit Crystal Y. Shi et al 2013; DOI: 10.1038/NGEO1956 Fig. 5)

The world is currently awash with models that attempt to throw light on the course of the Covid-19 pandemic. Many are based on highly uncertain data, leading to suggestions by some people that they have become tools for political elites and a means of helping ambitious scientists into the limelight: a sort of fuel for hubris. In the midst of this unprecedented turmoil there has appeared a suggestion (from modelling) that the core also contains abundant hydrogen (Li, Y. et al. 2020. The Earth’s core as a reservoir of water. Nature Geoscience, v. 13, published online; DOI: 10.1038/s41561-020-0578-1). Yunguo Li and colleagues, from University College London, the Chinese Academy of Science and the University of Oslo, explore the idea that the dominant hydrogen of the pre-planetary Solar nebula, which accreted to form the Earth, may have joined iron during core formation. This had been predicted from the thermodynamics of chemical reactions between water and iron. The team takes this further through the geochemical theory that elements and compounds tend to enter other materials preferentially. For example, during partial melting of the crust alkali metals (Na, K etc) are more likely to enter the granitic melt than to remain in the solid residue. Li et al. have used thermodynamics to predict the partitioning of hydrogen between iron and silicate melts under the very high temperature and pressure conditions at the boundary between the core and mantle.

Their calculations suggest that hydrogen then behaves in much the same manner as, say gold and platinum: it becomes ‘iron-loving’ or siderophile and prefers the molten core, as would H2O. The amount that gets in depends on the water content of the molten silicate that eventually becomes the mantle. If the water now making up Earth’s ocean was ‘degassed’ from the mantle during core formation then the original silicate melt would have been ‘wetter’ than it is now. The implication of such early degassing is that the core may contain 5 ‘oceans worth’ of water! The alternative scenario for Earth’s becoming a watery world is the later accretion of, for instance, cometary material. In that case, the early core would have been drier. Yet, the continual subduction of hydrated oceanic lithosphere into the deep mantle during billions of years of plate tectonics would steadily have added water to the core, in the form of iron oxides and hydrogen. So, the core might, in either case, contain several ‘oceans’ of the components of water. One line of indirect evidence is the deficiency in Earth’s actual water of the heavier isotope of hydrogen (deuterium) relative to the D/H ratio of chondritic meteorites. Theory suggests that D has slightly more affinity for joining iron than does H. Substantial water in the core does help explain the core’s apparent low density, but that notion requires as much faith as politicians seem to have in ‘following the Science’ during the current pandemic …

Arsenic hazard on a global scale

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I have been following the harrowing story of how arsenic gets into domestic water supplies for 20 years (see: Earth-logs Geohazards for 2002; 2003; 2004; 2005; 2006; 2008; 2009; 2011; 2013; 2017). In my opinion, it is the greatest natural hazard in terms of the numbers at risk of poisoning. In 2006 I wrote about the emergence in Bangladesh of arsenic poisoning on a huge scale during the mid 1990s for a now defunct Open University course. If people depend for drinking water on groundwater from tube wells driven into alluvium they would not know of the risk, unless the water is rigorously analysed for levels of As greater than 10 micrograms per litre (μg l-1), the WHO recommended maximum. The sad fact is that the affected population were advised to switch from surface water supplies, which carry a high risk of biological infection, to well water. That is because during downward percolation from the surface oxidation destroys bacteria and viruses as well as parasites. Opportunities provided by a massive UN-funded drilling programme and local well digging made the choice seemingly obvious. Most people came to prefer well water as gastro-intestinal infections and child mortality fell rapidly.

Arsenic adds no taste, which is why it was once the ‘poison of choice’. How it gets into groundwater is difficult to judge, unless wells are downflow of areas riddled with metal mines. Years of research uncovered an unsuspected mechanism. The most common colorant of mineral grains, and thus sedimentary rocks, is brownish iron hydroxide (goethite), and that is able to adsorb a range of dissolved elements, including arsenic. One would think, therefore, that groundwater should be made safe by such a natural ‘filtering’ process: indeed goethite can be used in decontamination. The problem is that iron hydroxide, which contains Fe-3, is only stable in water with a high capacity for oxidation. Under reducing conditions it breaks down to soluble Fe-2 and water, thereby releasing to solution any other element that it has adsorbed. In alluvium, beds containing organic matter are prone to this ‘reductive dissolution’ of goethite. If weathering upstream has released even seemingly insignificant amounts of arsenic during the build up of alluvium, there is a potential life-threatening problem as arsenic builds up in the goethite coating of sedimentary grains to become ‘locked in’, with the potential to be released in high concentrations if subsurface chemical conditions change. The colour of the alluvial sediments penetrated by wells is a clue. If they are reddish brown, groundwater is safe, if they are greyish and goethite-free then, ‘beware’. But it is rare to examine ‘cuttings’ from a drill site aimed at groundwater, unlike those aimed at ores or oil

Since the tragedy of Bangladesh, which resulted after 5 years or so in obvious signs of arsenicosis – dark wart-line keratoses on hands and feet or black blotches on facial and torso skin – several alluvial basins in large river systems have had their well water tested. But by no means all such basins have been screened in this way, and there are many less-obvious signs of arsenic poisoning. After long exposure to the lower range of dangerous arsenic levels a variety of cancers develop in known areas of arsenic risk. There are also high levels of endemic respiratory problems, cardiovascular disease, reduced intellectual development in children and even diabetes. Geochemical monitoring of all populated and farmed river systems is a huge task that is far beyond the resources of many countries through which they run. One approach to ‘screening’ for hazard or safety is to use geological, hydrological, soil, climate and topographic data. Those from known arsenic-prone basins and those where its levels are shown to be consistently below the 10 μg l-1 danger threshold help to develop a predictive model (Podgorski, J. & Berg, M. 2020. Global threat of arsenic in groundwater. Science, v. 368, p. 845-850; DOI: 10.1126/science.aba1510).

Modelled global probability of arsenic concentration in groundwater exceeding 10 μg l-1. Click to display a larger map in a separate browser tab. (credit: Podgorski & Berg; Fig 2A, with enhanced colour)

Rather than trying to model the full range of arsenic concentrations, Joel Podgorski and Michael Berg of the Swiss Federal Institute of Aquatic Science and Technology focussed on assessing probabilities that arsenic in well water exceeds the WHO recommended maximum safe level of 10 μg l-1. Their global map highlights areas of concern for environmental health. Thankfully, huge (blue) areas are suggested to present low risk, the pale, yellow, orange and red patches signifying areas of increasing concern. No populated continent is hazard-free. What is very clear is that Asia presents the greatest worries. Most of the Asian ‘hot zones’ are spatially close to large mountain ranges and plateaus. In the case of the Indus and Ganges-Brahmaputra plains the sources for excessive arsenic in groundwater implicated by previous geochemical investigations lie in the Himalaya. The factor common to all major hot spots seems to be rapid transport of huge amounts of sediment released by weathering from areas of high topographic relief, rather than local large-scale mining operations. There are hazardous areas related to historic and active mining, such as the Andes of Bolivia, Peru and Chile and the western USA, but they are tiny by comparison with the dominance of natural arsenic mobilisation.

Despite the WHO recommended maximum of 10 μg l-1 of arsenic, many countries base their policy on levels that are five times higher, largely because of the difficulty of analysing for the lower concentration without expensive analytical facilities. Field analyses are often done using simple semi-quantitative tests based on paper impregnated with reagents that show a colour range for different concentrations, which are unreliable for those lower than 100 μg l-1. Thankfully, despite the many risky areas, most of them have population densities less than 1 per km2.

If you are interested in the geological details of the arsenic problems of Bangladesh, the course text that I produced for the Open University (Drury, S. 2006. Water and well-being: arsenic in Bangladesh. The Open University: Milton Keynes, UK. ISBN 0-7492-1435-X), the course itself (S250 Science in Context) was withdrawn some years ago.  It may be possible to arrange a PDF for private use.

See also: Zheng, Y. 2020. Global solutions to a silent poison. Science, v. 368, p. 818-819; DOI: 10.1126/science.abb9746

Update on climate and sea-level change during the Cenozoic

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The Cenozoic Era was a period of fundamental change in the outer part of the Earth system. It culminated in the greatest climatic cooling since the Permian Period, during which upright apes emerged between 6 to 10 Ma ago.  The most decisive part of hominin evolution – the appearance of our own genus Homo – took place in the last 2.5 Ma that saw icecaps plastered over both polar regions and repeated pulses of major climate upheaval that dramatically affected all parts of the continents. Whereas the Mesozoic was dominated by reptiles, most famously the dinosaurs, the Cenozoic is rightly known as the age of mammals and of birds. The flowering plants, especially grasses, also transformed terrestrial ecosystems. The background to what has become ‘our time’ is not only climate change, but massive shifts in sea level and the outlines of the continents. For more than two decades many palaeoclimatologists have focused on the Cenozoic, gathering data using a variety of rapidly advancing technologies from a growing number of sites, in sediments from the continents and the ocean floor. One problem has been correlating all this global data precisely, coming as it does from many incomplete sedimentary sequences dotted around the planet. A great deal of basic information has come from the petroleum industry, which, of course, has continually eyed sedimentary rocks as the source of hydrocarbons through the 20th century. It was seismic reflection surveying that first gave clues to global ups and downs of sea level from onlaps and offlaps of strata that are visible on seismic sections, amplified by sequence stratigraphy. Six geoscientists from Rutgers University in New Jersey, USA have blended oil-industry archives with academic research to produce the first fully calibrated, comprehensive record of the Cenozoic (Miller, K.G. et al. 2020. Cenozoic sea-level and cryospheric evolution from deep-sea geochemical and continental margin records. Science Advances, v. 6, article eaaz1346; DOI: 10.1126/sciadv.aaz1346).

Latest palaeoclimate data for the Cenozoic Era. A oxygen-isotope data from benthic foraminifera (pale blue = polar icecaps, green = ice-free, pink – hothouse); B estimated mean sea-surface temperature from the calcium/magnesium ratio in Pacific Ocean cores; C variation in global mean sea level estimated from A and corrected for changes in the density of seawater due to water temperature (B); D atmospheric CO2 variations estimated using various proxies – see top right box. Click on the image to show a full-size version in a new browser tab. (Credit: Miller et al. 2020; Fig. 1)

Fluctuations in the proportion of 18O (δ18O) in the tests of foraminifera that lived in deep water are the key to global changes in sea level  and point to the influence of glacial ice accumulating on land (see Cooling sets in: Stepping Stones, Chapter 17). This is because glaciers are made from water that has evaporated from the oceans. When this happens, water that incorporates the lighter 16O isotope evaporates more easily and becomes enriched in atmospheric water vapour. When this water falls as snow that accumulates on land to form ice, the oceans are slightly enriched in the heavier 18O: δ18O incorporated into the shelly material of organism dwelling in the deep ocean increases at levels of a few parts per thousand. Conversely, their δ18O decreases when huge ice caps melt (A on the figure). The oxygen isotope records from fossils in ocean floor sediments give a far more precise impression of fluctuating sea level than do seismic sections and sequence stratigraphy of sedimentary rocks that interest the oil industry. But it is an ‘impression’, because other factors affect sea level.

Not only the global volume of ocean water is involved: the volume of the ocean basins changes too. This can occur because of changes in the rate of sea-floor spreading: when that is fast the hot new oceanic lithosphere is less dense and so buoys-up part of the ocean floor to drive sea level upwards. Slow spreading does the converse, as more lithosphere cools and sinks slightly. Another factor is the changing rate of marine sedimentation of material eroded from the continents. That fills ocean basins to some extent, again displacing the water upwards. When sediments are compacted as they become more deeply buried that has an effect too, to increase basin volume and result in sea-level fall. Oil industry geoscientists have attempted to allow for these long-term, slow mechanisms, to give a more accurate sea-level record.

Yet there is another important factor: the density and thus volume of ocean water changes with temperature. The warmer it is the greater the volume of ocean water and the higher is sea level. This is where academic work comes in handy. Two common elements that are dissolved in ocean water are magnesium and calcium. They also occur in the carbonate tests of the same deep-water forams that are used for oxygen-isotope measurements. It turns out that the warmer the water is the more magnesium enters the foram tests, and vice versa: their Mg/Ca ratio is a reliable proxy for mean ocean temperature and can be measured easily, centimetre-by-centimetre through cores. Kenneth Miller and colleagues have used this with the oxygen isotope proxy for land-ice volume to correct the sea-level record.

The Cenozoic ocean temperature record (B on the figure) is, in itself, interesting. It reveals far more large fluctuations than previously thought, especially in the Palaeocene and early Eocene. Yet, overall, the trend is one of steady cooling compared with the sudden shifts in δ18O that mark the onset of the Antarctic ice cap at the Eocene-Oligocene boundary around 34 Ma ago, and the apparent, temporary emergence from ‘ice-house conditions in the Middle Miocene. Also, sea level corrected for ocean temperature effects (C on the figure) suggests that for much of the Cenozoic sea level was lower than expected; i.e. it rarely exceeded 60 m above the current level, which is that expected when no substantial mass of  land ice exists.

The other important compilation made by Miller et al. is that of the CO2 content of the atmosphere estimated using six different proxies. It is a lot more fuzzy than the oceanic records because the proxies are not precise. Nevertheless, it is interesting. The current, partly anthropogenic level of around 400 parts per million (ppm) is not unique. In fact from 55 to 23 Ma it was consistently above this ‘Anthropocene’ level, peaking at twice that level at the end of the Eocene. That’s odd, because it doesn’t tally with the oxygen isotopes that indicate the onset of large scale Antarctic glaciation shortly afterwards. In fact most of the climatic highlights shown by A on the figure are not reflected in the Cenozoic history of the most influential greenhouse gas. In the short term of glacial-interglacial cycles during the late Pleistocene, atmospheric CO2 levels are very closely related to fluctuations of land-ice volume. In the 65 Ma of the Cenozoic such a link is hard to argue for. There are more puzzles than revelations in this otherwise major addition to palaeoclimatology.

More time for modern humans to have mingled with Neanderthals

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When anatomically modern humans (AMH) became established in Europe the days of the Neanderthals were numbered. Yet, genomic evidence is mounting for many instances of interbreeding between the two groups (see Human evolution links). The longer they were in contact the chances of meeting and having sex were likewise increased. So, for how long were the two groups able to make contact? Neanderthals declined and eventually disappeared between 41 and 39 ka, except for a possible refuge for a tiny number in southern Spain until 37 ka and maybe in the northern Urals where there are disputed Mousterian stone tools as young as 34 to 31 ka. Undoubtedly, the appearance of AMH somehow contributed to the demise of our close relatives, but there are many possible reasons why. Until recently, the earliest European entry of AMH had been placed at around 41 ka, based on dating of H. sapiens remains in Romania (but note: a single 210 ka possible AMH skull from Greece). This is now exceeded by data from a Bulgarian cave.

Bacho Kiro cave in Bulgaria (credit: Getty images)

The Bacho Kiro site was first excavated in the 1970s, and revealed stone tools that represent the earliest Upper Palaeolithic culture, known as the Bachokirian. Mitochondrial DNA from excavated bone fragments is clearly of AMH origin (Hublin, J.-J. and 31 others 2020. Initial Upper Palaeolithic Homo sapiens from Bacho Kiro Cave, Bulgaria. Nature, v. 581, online; DOI: 10.1038/s41586-020-2259-z). Dating the Bacho Kiro cave sediments has been difficult, but new analytical and statistical approaches using the radiocarbon (14C) method have yielded ages between 46 to 44 ka and perhaps as far back at 47ka (Fewlass, H. and 20 others 2020. A 14C chronology for the Middle to Upper Palaeolithic transition at Bacho Kiro Cave, Bulgaria. Nature Ecology and Evolution, v. 4, online; DOI: 10.1038/s41559-020-1136-3). This is the earliest unequivocal, direct evidence of our species in Europe and its association with the initial Upper Palaeolithic culture. Among the finds are perforated animal teeth and ivory beads that probably formed pendants, which resemble those found elsewhere in association with late Neanderthals: the Chatelperronian culture that seems to have been shared between AMH and Neanderthals.

The new data add up to 6 thousand years to the period of AMH-Neanderthal co-occupation of Europe, or about 400 generations. Plenty of time to ‘get to know one another’, and perhaps to assimilate genetically

See also: Rincon, P. 2020. Longer overlap for modern humans and Neanderthals. (BBC News 11 May 2020); Metcalfe, T. 2020. A tooth offers evidence modern humans reached Europe earlier than previously thought. (NBC News 11 May 2020)

Changing conditions of metamorphism since the Archaean

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Metamorphic petrologists have known since their branch of geology emerged that the intensity or ‘grade’ of metamorphism varies with position in an orogenic belt. This is easily visualised by the sequence mudstone-shale-slate-phyllite-schist-gneiss that results from a clay-rich starting material as metamorphic grade increases. Very roughly speaking, the sequence reflects burial, heat and pressure, and must have been controlled by temperature increasing with depth and pressure: the geothermal gradient. In turn, that depends on internal heat production, geothermal heat flow and the way in which heat is transferred through the deep crust: by thermal conduction or mechanical convection. A particular rock composition gives rise to different metamorphic mineral assemblages under different temperature and pressure conditions.

George Barrow was the first to recognise this in the Southern Highlands of Scotland as a series of zones marked by different index minerals. For instance, in once clay-rich sediments he recognised a succession of new minerals in the sequence chlorite; biotite; garnet; staurolite; kyanite; sillimanite in rocks of progressively higher metamorphic grade. Barrow found that once basaltic lavas interleaved with the sediments displayed zones with different characteristic minerals. Other metamorphic terrains, however, revealed different index minerals. Experimental mineralogy eventually showed that Barrow’s zones and others reflected a wide range of chemical reactions between minerals that reach equilibrium over different combinations of pressure and temperature. This enabled geologists to distinguish between metamorphism that had occurred under conditions of high-pressure and low-temperature, low-P and high-T and intermediate conditions (see diagram). This suggested that metamorphic rocks can form in areas with different heat flow and geothermal gradients. Geochemical means of assessing the actual temperatures and pressures at which particular rocks had reached mineralogical equilibrium, known as ‘thermobarometry’, now enable such variations to be assessed quantitatively.

The latest division in pressure-temperature space of different styles of metamorphism (colours) and the main mineral equilibria (dashed lines) that define them

It has long been suspected that the average T/P conditions revealed by metamorphic rocks have varied over geological time, as well as from place to place at any one time. A recent paper has analysed thermobarometric data from the earliest Archaean to recent times (Brown, M. et al. 2020, Evolution of geodynamics since the Archean: Significant change at the dawn of the Phanerozoic: Geology, v. 48, p. 488–492; DOI: 10.1130/G47417.1) They conclude that from the Archaean to the start of the Neoproterozoic the average P/T ratio was more than twice as high as it was in the following billion years. At about 2 Ga they suggests a relatively sudden decrease that correlates with what they regard as the first major assembly of continental crust: the Columbia (Nuna) supercontinent. The Mesoproterozoic Era, occupied by the disassembly of Columbia and the eventual creation of the Rodinia supercontinent, retained a high mean T/P. That began to decline with the break-up of Rodinia and a succession of tectonic cycles of ocean opening and closing during the Neoproterozoic and the Phanerozoic. This phase of truly modern plate textonics saw first the assembly of Gondwana and then the all-encompassing Pangaea, followed by its break up as we witness today. There are other correlations with the T/P variations, but they need not detain us.

The raw metamorphic data (564 points spanning 3.5 Ga) are by no means evenly spaced in time, and four dense clusters of points show a very wide spread of T/P – up to 2 orders of magnitude. Yet the authors have used locally weighted scat­terplot smoothing (LOWESS) to reduce this to a smoothed curve with a zone of uncertainty that is a great deal narrower than the actual spread of data. Frankly, I do not believe the impression of systematic change that this approach has produced, though I am not a statistician. To a lesser extent than me, it seems that neither does Peter Cawood, who comments on the paper in the same issue of Geology: more clearly than do the authors themselves.

Peter Cawood’s ‘take’ on the relationship between tectonic development and other important variables in the Earth-system with the estimate by Brown et al. of the mean metamorphic T/P (‘thermobaric’) variation through Earth history

Cawood’s view is that it was all due to a steady fall in mantle temperature and related broad changes in tectonic processes. But metamorphic rocks form in only the outermost 100 km of the Earth. The post-800 Ma examples include a much greater proportion of those formed under high- and ultrahigh pressures – blueschists and various kinds of eclogite – than do the earlier metamorphic belts. This weights the post-800 Ma record to lower mean T/P. Such rocks form in subduction zones and their high density might seem to doom them to complete resorption into the deep mantle. Yet large chunks now end up embedded in continents, interleaved with less extreme materials. Cawood suggests, as do others, that cooling of the mantle has enabled deeper break-off of subducted slabs to meet their end at the core-mantle boundary. The retained low T/P lithosphere since 800 Ma may have been sliced into the continents by increased underthrusting during continent-continent collisions that dominate the more modern orogenic-metamorphic belts.

See also:  Cawood, P.A. 2020 Earth Matters: A tempo to our planet’s evolution: Geology, v. 48, p. 525–526; DOI: 10.1130/focus052020.1

Genetic material from a baby dinosaur

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A clutch of Massospondylus carinatus eggs from the Jurassic of South Africa (credit: Brett Eloff)

Recently, a lot of publicity focussed on stunning CT scans of embryos preserved in fossilised eggs of a Jurassic sauropodomorph dinosaur, which were obtained using very high energy X-rays generated by a synchrotron in France (Chapelle, K.E.J. et al. 2020. Conserved in-ovo cranial ossification sequences of extant saurians allow estimation of embryonic dinosaur developmental stages. Nature Scientific Reports, v. 10, article 4224; doi: 10.1038/s41598-020-60292-z). The images suggest that the embryos’ skulls developed in much the same way as do those of living reptiles. Within a week there emerged an even more compelling dinosaurian scoop: a fossil nestling of a duck-billed dinosaur (hadrosaur) from the Upper Cretaceous of Montana is reported to have yielded evidence for a broad spectrum of cellular materials (Bailleul, A.M. et al. 2020. Evidence of proteins, chromosomes and chemical markers of DNA in exceptionally preserved dinosaur cartilage. National Science Review, v. 7, advance publication NWZ206; DOI: 10.1093/nsr/nwz206).

Alida Bailleul, who works at the Chinese Academy of Sciences in Beijing, and fellow molecular palaeontologists from Canada, the US and Sweden, examined material from the nestling’s skull that was suspected to contain traces of cartilage. Their methods involved microscopic studies of thin sections together with staining and fluorochemical analysis of cellular material extracted by dissolving away bone tissue in acid. The same methodologies were also applied to similar material from modern emu chicks as a means of validating the results from the fossil. Staining used the same chemical that previously had revealed blood proteins in a specimen of Tyrannosaurus rex (see: Blood of the dinosaurs  in Palaeobiology, January 2011). The fluorescence approach dosed the dinosaur cartilage with antibodies against bird collagen, and revealed an immune reaction (green fluorescence) in both fossil material and that from the baby emus.

The researchers also isolated cartilage cells (chondrocytes) from the dinosaur preparations. Two stains (PI and DAPI, for short) that show up DNA were applied, giving positive responses. The PI (propidium iodide) stain is useful as it does not respond to DNA in living material, bit only to that in dead cells, thereby helping to rule out contamination with modern material. Apparently, the double-staining experiments support the presence of double-stranded material that involves at least six base pairs (of ACTG amino acids). This does not prove the existence of dinosaur DNA, but does demonstrate that the hadrosaur’s cell nuclei are preserved.

Does that suggest that the hunt is on for a dinosaur genome, with all its connotations? OK, a complete genome has been extracted from a frozen Siberian mammoth a few tens of thousand years old, which encourages ‘re-wilding’ aficionados, but that animal preserved intact cells of many kinds. A 70 Ma old dinosaur fossil, however exquisitely preserved, is mostly ‘rock’, in that preservation is through mineralisation of bone and tissue, and even cells … Moreover, it is possible that what the team found may even be material from post-mortem bacterial colonisation of any age younger than 70 Ma.

See also: De Lazaro, E. 2020. Scientists Use X-rays to Peer inside Fossilized Dinosaur Eggs Sci News, 10 April 2020; Black, R. 2020. Possible dinosaur DNA has been found. Scientific American, 17 April 2020

Earliest direct evidence of plate motions

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There are two ways that we recognise the movement of tectonic plates. Since the latter half of the Mesozoic Era, following break up of the Pangaea supercontinent, it bests manifests itself in the magnetic ‘stripes’ on the ocean floor. They result from alternating polarisation of the geomagnetic field as new oceanic lithosphere is generated at constructive plate boundaries to drive sea-floor spreading. The oldest remaining stripes date back to the early Jurassic. For earlier times geologists have to turn to the continental crust.  Lavas and some sedimentary rocks undergo magnetisation at the time of their formation and retained that imprint. Such remanent, palaeomagnetism reveals the original latitude at which it was imprinted, together with the subsequent rotation of a drifting continent relative to an assumed N to S axis joining the opposed magnetic poles. The apparent ‘wandering’ of the pole through time when successive ancient pole positions of different ages are plotted in relation to the present position of a continent is a good guide to its history of drifting as a result of plate tectonics. Comparing the polar-wander paths of two continents allows the time when they were formerly united to be estimated. So palaeomagnetic pole data makes it possible to reconstruct not just Pangaea but a whole series of earlier supercontinents, ancient magnetic data being supplemented by other geological evidence such as reconnecting the trends on different continents of ancient mountain belts.

Apparent polar wander paths for two continents for a period when they were united then split and were separated by sea-floor spreading, eventually to collide and reunite

The further back in time the fewer palaeomagnetic pole positions have been estimated, and the more uncertain are the apparent polar wander paths and the more complex each continent’s accumulated geological history. One of the reasons for such uncertainty is that episodes of metamorphism can reset a rock’s remanent magnetisation, hundreds of million years after it originally formed. Thus, the harder it becomes to be certain about early supercontinents that have been suggested, of which there are quite a few. The earliest that has been proposed is Vaalbara, albeit on grounds of geological similarity, that supposedly united the Kaapvaal and Pilbara Cratons of southern Africa and Western Australia, respectively. Its duration is suggested to have been between 3.6 to 2.8 Ga (billion years ago). The oldest supercontinents with sound palaeomagnetic records date from the end of the Archaean Eon (2.5 Ga). It is the lack or uncertainty of earlier palaeomagnetic evidence that makes the start of plate tectonics the subject of so much debate.

However, geophysicists continually strive to improve the detection of ancient magnetisation, and advances have been made recently to unravel original magnetisation signals from those that have been superimposed later. The fruits of these developments are borne out by a study of a sequence of mafic lavas from the Pilbara Craton that formed about 3.2 Ga ago (Brenner, A.R. et al. 2020. Paleomagnetic evidence for modern-like plate motion velocities at 3.2 Ga. Science Advances, v. 6, article eaaz8670; DOI: 10.1126/sciadv.aaz8670). Alec Brenner and colleagues from several US universities measured palaeomagnetism in more than 200 diamond drill cores from two localities in this sequence and combined their data with others from the Pilbara to cover a roughly 600 Ma period between 3.35 to 2.77 Ga. The palaeopoles form a polar wander path that spans roughly 50 degrees of palaeolatitude. From this they have been able to estimate, in considerable detail, the rate at which the Pilbara Craton had moved in Mesoarchaean. In the first 170 Ma the average horizontal motion was about 2.5 cm per year, falling rapidly to 0.4 cm per year over the following 410 Ma. The earlier speed is comparable with the average of modern plate motions. Data from the later period suggests relative stagnation. Motions over the entire ~600 Ma could be due to episodic operation of plate tectonics on the global scale, or a local slowing in the rate of plate growth.

Pterosaur corner

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I recall an anecdote related by David Attenborough about a celebrity reception that he once attended one evening after he had been filming for a sequence on the aerodynamics of pterodactyls. A venerable and obviously well connected lady engaged him in conversation, and asked him what he had been doing recently. “Actually, today I was flying a pterodactyl”. To which the old lady retorted, “Yes, they are so graceful, aren’t they”. They do have a large following, perhaps second only to dinosaurs, and three interesting items came to my attention in the last couple of weeks.

One of the known pterosaur groups is the Tapejaridae, comprising small to medium-sized pterosaurs with wingspans up to 4 m. They are quite spectacular in appearance, having large crests relative to their overall size. Their fossils have turned up in Cretaceous sediments in South America, Europe and China, and a new find in Morocco (Afrotapejara zouhrii) extends their range to Africa (Martill, D.M. et al. 2020. A new tapejarid (Pterosauria, Azhdarchoidea) from the mid-Cretaceous Kem Kem beds of Takmout, southern Morocco. Cretaceous Research. V. 112: onlin, 104424; DOI: 10.1016/j.cretres.2020.104424). See also: De Lazaro, E. 2020. New species of pterosaur discovered in Morocco (Sci News, 6 April)

Also reported in Cretaceous Research are three new species of toothed, fish-eating pterosaurs of the ornithocheirid group. They too come from the Cretacous Kem Kem beds of Morocco, and again adding Africa to the range of the genera to which they belong. Even the largest flying animals known to science have emerged from the same strata. These are the azhdarchid pterosaurs, the largest of which had a wing span of more than 9 metres and stood at the height of a giraffe when on the ground.

See: Anderson, N, 2020. New pterosaur fossils unearthed in Morocco (Sci News, 26 March)

Being so widely spread, these pterosaur group’s mode of flight must have been extremely efficient, perhaps even matching that of today’s albatrosses, which use turbulence over ocean waves to glide effortlessly, indeed the epitome of graceful travel. How they achieved such vast ranges is partly due to their extremely light-weight bones that were paper thin but strong because they contained vesicles filled with gas, much like the expanded polystyrene used in model pterosaurs of the kind flown by ‘Whispering Dave’ as Sir David Attenborough is fondly known. Their bone structures are similar, in this respect, to those of modern birds.

launch of Hatzegopteryx
Reconstruction of the giant pterosaur Hatzegopteryx launching into the air, just after the forelimbs have left the ground (credit: Mark Witton)

So, how did these graceful beasts fly? Like those of bats, pterosaurs’ wings were membranes, but rather than being supported by five elongated digits, as in bats, those of pterosaurs extended from their bodies to a single elongated ‘finger’ or digit: hence their old name pterodactyl, translated from the Greek as ‘wing finger’. For a long while, it was believed that pterosaurs had to live on high ground, even cliffs, in order to launch themselves in the manner of a hang glider. Reconstructions of their gait on the ground generally look extremely ungainly: they walked on their ‘wrists’ and the other three, small ‘fingers’ of their forelimbs.. How they probably launched themselves emerges from a detailed paper linking natural flight modes of birds, bats and pterosaurs to conceivable developments in aeronautics inspired by them (Martin-Silverstone, E. et al. 2020. Volant fossil vertebrates: potential for bioinspired flight technology. Trends in Ecology and Evolution, v. 35, in press 9 April 2020; DOI: 10.1016/j.tree.2020.03.005). The authors point to the great strength of the membrane structure itself, conferred by its three-layered structure, and to the aerodynamic properties of the wing. They conclude that, whereas pterosaurs were probably incapable of high-speed flight, they were extremely efficient at low speeds, ideal for soaring and for low-speed landing that would not endanger their fragile bodies. Simply by springing into the air using all four limbs they could attain sustained flight, although the largest of them were close to the limit. The necessary muscles actually made up about 40% of their body mass. See a reconstruction of the launch of the largest pterosaur, Quetzalcoatlus from the Late Cretaceous of North America

See also: Fossil Flyers Hold Secrets to Better Flight Technologies (Sci News, 18 April)

How did monkeys get to South America?

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This is one of the great mysteries of palaeontology. There are plenty of monkey species in South and Central America and in Mexico. They are members of five families, collectively known as platyrrhine (‘flat-nosed’) primates, all having wide-spaced nostrils compared with the primates of the ‘Old World’. They are the catarrhines (‘hook-nosed). There are other differences, such as the unique prehensile tails of many ‘New World’ monkeys. The two monkey groups are genetically related, but their last common ancestor is estimated, using the ‘molecular clock’ approach, to have lived at least 31 Ma ago, in the Oligocene. The earliest platyrrhine primates of the Americas date to around the Eocene-Oligocene boundary (34 Ma). Interestingly, they are predated by the earliest rodent remains by only a few million years (41 Ma). Both primates and rodents had been inhabiting other continents long before this, so it is certain that, somehow, members of the two groups must have migrated to become isolated in the Americas. The problem lies with palaeogeography. By the late-Eocene the Americas were completely separated from Eurasia and Africa by the actively spreading Atlantic Ocean, then between 1500 to 2000 km wide. Complete isolation of the Americas dates from around 60 Ma ago, when the northernmost part of the North Atlantic began to open. The South Atlantic had become a wide ocean long before that, beginning in the far south during the early Cretaceous Period (138 Ma), with the mid-Atlantic Ridge steadily propagating northwards thereafter.

35 Ma
World palaeogeography at the Eocene-Oligocene boundary. The site of a recent fossil primate discovery in eastern Peru is marked by the yellow dot.

Since 60 Ma years ago it would have been impossible for the ancestors of ‘New World’ rodents and primates simply to have walked there. In any case the earliest known primate fossils from China are just 55 Ma old. Island hopping across the far northern, narrowest part of the North Atlantic during the Eocene may have been possible, although many islands there could have been subject to intense volcanic activity, as is Iceland today. The only alternative is a sea trip across the mighty Atlantic. Unless, that is, there is a hitherto undiscovered land bridge. The Walvis-Rio Grande Rise – a hotspot track – that spans the South Atlantic Ocean floor from Namibia to São Paulo in Brazil, has been the subject of some speculation since it is dotted with sea mounts and in places has micro-continental fragments. But it is too deep to have emerged as a result of falls in sea level. To suggest that the > 1500 km migration to the Americas of ancestral platyrrhine primates, or rodents for that matter, involved their being carried on drifting vegetation rafts obviously invites scepticism. For starters, why only two groups of animals? Or, could that imply a one-off event carrying only ancestral rodents and monkeys? It would need to be a special kind of raft: large enough to provide security against storm waves; immune to waterlogging, and carrying substantial food. On the plus side, there are powerful east-to-west currents in the equatorial Atlantic and trade winds going in the same direction, thanks to the Coriolis effect and ultimately Earth’s rotation. Islands as ‘way-points’ or temporary refuges are less convincing, for they would have to be heavily vegetated themselves to provide onward rafts. Apparently, in the absence of anything more plausible, Sherlock Holmes’s principle points to trans-Atlantic rafting.

This issue recently became ‘live’ again, with a fossil discovery in Peru, in an upper Amazon river bank close to at the Andean watershed but around 4000 km from the east coast of South America (Seiffert, E.R.  et al. 2020. A parapithecid stem anthropoid of African origin in the Paleogene of South America. Science, v. 368, p. 194-197; DOI: 10.1126/science.aba1135). The site had previously yielded both playrrhine monkey and rodent remains. To these have been added teeth with distinct similarities to those of fossils previously known only from Egypt, Libya and Tanzania: parapithecid anthropoids whose teeth are sufficiently different from those of platyrrhines to warrant a separate suborder, which includes baboons and primates. This is the only trace of parapithecids in South America and it may be assumed that, although they were possibly fellow-travellers with New World monkey ancestors, they were unable to compete and became extinct.

However, there is another possibility. Albeit with a sparse record of fossils resembling primates, North America does have at least one. George Gaylord Simpson (1902-1984), once the doyen of US palaeontologists, found a marmoset-like fossil in the early-Eocene of Wyoming, which he named Teilhardinia after the French Jesuit philosopher and palaeontologist Teihard de Chardin. It is about 56 Ma old and the size of a mouse. So was this diminutive the pioneer New World primate that crossed the northern North Atlantic? If so it would have had an equally perilous journey to reach South America, because the Isthmus of Panama was also open sea until around 4.5 Ma ago. With Teilhardinia, the plot thickens for there are several known species: in the US T. brandti from Wyoming and T. magnoliana from Mississippi; in Asia and Europe T. asiatica and T. belgica respectively. An embarrassment of riches that may well ignite: it has been suggested that North American Teilhardinia may have been the first of all primates and spread across the Eocene forests of North America, Europe and Asia. That hypothesis sort of implies that the entry of monkeys into South America may well have started with the tiny continent hopper who passed on its proclivities to its descendants in Africa

See also: Godinot, M. 2020. Rafting on a wide and wild ocean. Science, v. 368, p. 136-137; DOI: 10.1126/science.abb4107; Ancient teeth from Peru hint now-extinct monkeys crossed Atlantic from Africa. Science Daily, 9 April 2020. Oldest-known ancestor of modern primates may have come from North America, not Asia. Science Daily, 29 November 2018

Human evolution links

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Time and energy permit me to summarise only one or two research developments each week. Yet there is a continual flow of other publications in fields which interest me, and hopefully most readers of Earth-logs. I come across them during my weekly search for suitable inspiration, so have decided occasionally to provide links to informative summaries in other blogs.

Last week, Science Daily reported on a paper in the journal Genetics that evaluates new genetic evidence that interbreeding between anatomically modern humans (AMH) occurred more often than previously suggested, when the two groups were in contact in Eurasia (New research adds to growing evidence that our ancestors interbred with Neanderthals at multiple times in history. Science Daily 1 April 2020). Other Neanderthals also left signs that around 40 ka ago they wove cordage from woody cellulose (in Scientific Reports): they were clearly as technologically adept as contemporary AMH (40,000 year old evidence that Neanderthal’s wove string. Science Daily 9 April 2020).

The early-April issue of Science also published dating of a key site in South Africa to show that around 2 Ma ago the earliest known Homo erectus co-inhabited the surrounding area with Australopithecus naledi and the earliest known Paranthropus. One of the highlights is that this rules out A. naledi as a direct human ancestor, as previously claimed by some. (When three species of human ancestor walked the Earth. Science Daily 2 April 2020).

In its last March issue Science carried a paper suggesting that Neanderthals in Portugal were avid consumers of seafood (Neanderthals ate mussels, fish, and seals too. Science Daily 26 March 2020).

Alternative explanation for interglacial climate instabilities; and a warning

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For the past two and a half million years there has been no such thing as a stable climate on our home world. The major fluctuations that have given rise to glacial and interglacial episodes and the times that separate them are most familiar, as is their connection with the periodicity of gravitational effects on the Earth’s orbital and rotational behaviour. There are mysteries, such as the dominance of a ~100 ka cyclicity with the least effect on solar heating since a million years ago and the shift that took place then from dominant ~40 ka cycles that preceded it. But over shorter time scales there are more irregular climatic perturbations that can not be attributed to gravity variations in the Inner Solar System. In the run-up to maximal glacial conditions in the Northern Hemisphere are changes in the isotopic records that reveal increases and decreases in the mass of ice on continental masses. Known as Dansgaard-Oeschger events they occurred on a (very) roughly 10 ka basis and lasted between 1000 and 2000 years. They resulted in rapid temperature changes spanning up to 15°C over the Greenland ice cap and have been explained by changes in surface- and deep-water circulation within the North Atlantic. Effectively, the Gulf Stream and the thermohaline circulation that drives it were periodically shut down and turned on. Even more irregular in occurrence are sudden global coolings in the midst of general warming into interglacial episodes. The most spectacular of these was the Younger Dryas cooling to almost full-glacial conditions between 12.8 and 11.5 ka, at a time when the Earth had achieved a mean surface temperature almost as high as that which has prevailed over the last 11,000 years.  There have been lesser cold ‘snaps’ during the Holocene, and in every one of the earlier interglacials for which there are data. Their occurrence seems unpredictable, even chaotic.

In 2006 the Younger Dryas was explained as the result of massive amounts of freshwater flooding into the Arctic Ocean from huge, ice-dammed lakes in North America. Decreased density of the high-latitude surface water resulted in its failure to sink and thus drive thermohaline circulation (see The Younger Dryas and the Flood June 2006). This hypothesis has subsequently been applied to other such sudden climatic events, such as the cooling episode around 8.2 ka during the Holocene. A recent study set out to test this notion from ocean-floor records of the last half-million years (Galaasen, E.V. and 9 others 2020. Interglacial instability of North Atlantic Deep Water ventilation. Science, v. 367, p. 1485-1489; DOI: 10.1126/science.aay6381). The data are from a seafloor sediment core in a trough south of Greenland, where cold, salty and dense bottom water flows southward from the Arctic to drag warmer surface water northwards in the Gulf Stream to replace it. That warm surface water has a high salinity because of evaporation in the tropics, so once it cools it sinks, thereby maintaining thermohaline circulation.

Modelled circulation rate of Atlantic circulation during 10 ka of the last interglacial before the Holocene (credit: Thomas Stocker, 2020 Science)

Eirik Galaasen of the University of Bergen and colleagues from several countries flanking the North Atlantic found large, abrupt changes in the mass flow of water through the trough – based on studies of carbon isotopes in bottom-living foraminifera – during each of the four interglacials that preceded the current one. The higher the δ13C in the forams the more vigorous the deep flow, whereas low values suggest weak flow or stagnation, due to waning of thermohaline circulation. Transition between the two states is rapid and each state lingered for several centuries. While the Holocene records only one such perturbation of note, that at 8.2 ka, previous interglacials reveal dozens of them. One possibility is that the thermohaline circulation system of the North Atlantic behaved in a chaotic fashion during previous interglacial episodes, producing similarly erratic shifts in climate. Seemingly, the Holocene bucks the trend, which may have added an element of luck to the establishment of human agricultural economies throughout that Epoch. All the signs are that current, anthropogenic global warming will slow down the water circulation in the North Atlantic. Might that set-off what seems to have been the norm of chaotic interglacial climate shifts for the best part of that half-million years? Hard to tell, without more studies …

See also: Stocker, T.F. 2020. Surprises for climate stability. Science, v. 367, p. 1425-1426; DOI: 10.1126/science.abb3569; How stable is deep ocean circulation in warmer climate? (Science Daily)

Early days of the dog

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Wolves and dogs are interfertile and the mating of a domestic dog with a wolf results in fertile offspring, unlike the case with hybrids of horse and donkey, lion with tiger etc. This suggests that both canids are so closely related that domestication of wolves led to the entire range of dog breeds shown at Crufts every year. The question is, “When did humans first domesticate wolves”? Provided the instinctive ‘rules’ of wolves are followed by a human a wolf pup can become a pet, if it is taken from its mother between 14 and 21 days after birth. But, not only are they expensive to feed on raw meat, they may well attack a stranger as they would in the wild go for a wolf from another pack. They are often loyal and playful towards whoever raised them, but are strictly ‘one-person’ animals, and difficult to train because they easily become bored. Taming wolf puppies and deliberate selection is one route to domestication and the first dogs, another being ‘self-domestication’ when wolves become dependent on humans for a share in food.

pet wolf
Raven the wolf greets a visitor to the Mission: Wolf sanctuary in Colorado USA (credit: Wikipedia)

Comparison of wolf (Canis lupus) and domestic dog (Canis familiaris) genomes suggest an age of divergence for the two populations may have occurred between 20 to 60 thousand years ago. Indeed the DNA of wolf remains from Siberia showed it to belong to a wolf population whose descendants contributed to domestication of sledge dogs, such as Greenlandic huskies and Alaskan malemutes. Yet this approach is difficult and the results uncertain. Discovery of canid skulls associated with the remains of humans and mammoths at a 28.5 ka old site in the Czech Republic seems to have resolved both a minimum age for domestication and how it was achieved (Prassack, K.A. et al. 2020.  Dental microwear as a behavioral proxy for distinguishing between canids at the Upper Paleolithic (Gravettian) site of Předmostí, Czech Republic. Journal of Archaeological Science, v. 115, published online; DOI: 10.1016/j.jas.2020.105092).paleolithic dog

The Předmostí canids show two skull shapes: one with long jaws like wolves, the other with shorter, more dog-like jaws. Kari Prassack of the US National Park Service and colleagues from the USA, the Czech Republic and Belgium, turned to dental micro-wear patterns to resolve differences between the two groups as regards diet. Teeth from the more wolf-like group showed wear patterns consistent with a diet dominated by raw flesh, whereas the short-jawed canids ate mainly hard, brittle foods, probably bones. A truly remarkable find at the site was a near-complete canid skull of the short-jawed type, with a bone between its front teeth. Could this be a sign of a carefully buried pet ‘proto-dog’?

Earlier studies of the Předmostí canids included isotopic analyses of their bones, and those of associated humans. Interestingly, the more wolf-like group and the humans had diets dominated by mammoth flesh. The possible proto-dogs had focused on reindeer and other prey, as had the lions whose bones also occur at the site. This further complicates interpretation. Did both wolves and proto-dogs accompany the humans, the first being fed with mammoth meat that they helped bring down, while the second were fed scraps from smaller, more commonly killed prey? Perhaps the early dogs developed over a long period as scavengers on the kills of lions, and then became associates of humans. Yet neither canid would find a mammoth easy prey, even hunting in packs. So did the ice-age hunters have two companion animals, perhaps one to help in hunting mammoth, the other for more day-to-day hunting, which became more domesticated and even kept as pets? As the authors conclude; more data are needed.

See also: Dog domestication during ice age (Science Daily)

A lowly worm from the Ediacaran?

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Humans are more or less symmetrical, our left and right sides closely resembling each other. That is not so comprehensive for our innards, except for testes and ovaries, kidneys, lungs, arteries and veins, lymph and nervous systems. We have front- and rear ends, top and bottom, input and output orifices. All that we share with almost all other animals from mammals to worms, particularly at the earliest, embryonic stage of development. We are bilaterians, whereas sponges, ctenophores, placozoans and cnidarians are not – having either no symmetry at all, or just a bottom and a top – and are in a minority.  Fossil collections from Cambrian times also reveal bilaterians in the majority, at least insofar as preservation allows us to tell. Before 541 Ma ago, in the Precambrian, there are few signs of such symmetry and faunas are dominated by the flaccid, bag like creatures that form much of the Ediacaran Fauna, although there are traces of creatures that could move and graze, and had a rudimentary sense of direction (see: Burrowers: knowing front from back, July 2012 and Something large moved 2 billion years ago). Unsurprisingly, palaeobiologists would like to know when ‘our lot’ arose. One route is via comparative genetics among living animals, using DNA differences and the ‘molecular clock’ approach to estimate the age of evolutionary separation between ‘us’ and ‘them’. But the spread of estimated ages is so broad as to render them almost meaningless. And the better constrained ages of very old trace fossils rely on accepting an assumption that they were, indeed, formed by bilaterians. Yet ingenuity may have revealed an actual early bilaterian from such traces.lowly worm

Palaeobiologists from the US and Australia have scoured the famous Ediacara Hills of South Australia for traces of burrowing and signs of the animal that did it (Evans, S.D. et al. 2020. Discovery of the oldest bilaterian from the Ediacaran of South Australia. Proceedings of the National Academy of Sciences, v. 117, online; DOI: 10.1073/pnas.2001045117). One Ediacaran trace fossil, known as Helminthoidichnites is preserved as horizontal trails on the tops and bottoms of thin, discontinuous sand bodies. Luckily, these are sometimes accompanied by elongate ovoids, like large grains of rice. From numerous laser scans of these suspected burrowers, and the traces that they left the authors have reconstructed them as stubby, possibly segmented, worm-like animals that they have called Ikaria wariootia, which may have grazed on algal mats. This name is derived from the local Adnyamathanha people’s word (Ikara  or ‘meeting place’) for the locality, a prominent landmark, near Warioota Creek. The age of the sedimentary sequence is between 551 to 560 Ma, and perhaps a little earlier. They could be the earliest-known bilaterians, but the sandy nature of the rocks in which they occur precludes preservation of the necessary detail to be absolutely sure: that would require silt- or. clay-sized granularity

See also: Fossil worm shows us our evolutionary beginnings (BBC, Science and Environment)

Dinosaur corner

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Many adjectives have been applied to dinosaurs: terrifying; lumbering; long-dead; fierce; huge; nimble, carnivorous; herbivorous and so on. But exquisite and tiny do not immediately spring to mind. The mineral amber – strictly speaking a mineraloid because it isn’t crystalline – having formed from resins exuded by trees, preserves materials, including animals, that became trapped in the resin. The shores of the Baltic Sea used to be the main source of this semi-precious gemstone, but it has been overtaken by high-quality supplies from Kachin State in Myanmar. Most specimens contain small invertebrates, including spiders and insects, in varying states of preservation. Once in a while truly spectacular amber pebbles turn up. In early March 2020 the world’s media splashed a unique find: a miniature dinosaur (Xing, L. et al. 2020. Hummingbird-sized dinosaur from the Cretaceous period of Myanmar. Nature v. 579, p. 245–249; DOI: 10.1038/s41586-020-2068-4).

Amber pebble from Myanmar containing a tiny vertebrate’s skull (credit: Lida Xing, China University of Geosciences)

The amber specimen, from Middle Cretaceous (99 Ma) sediments, contains a perfectly preserved skull less than 2 cm long. At first glance it appears to be that of a tiny bird. The authors used micro-CT scanning to reconstruct the entire skull in 3-D. Although superficially resembling that of a bird, with eye sockets ringed by scleral ossicles that modern birds also have. These suggest that the animal was active during the daytime. Its beak-like jaws have many small teeth, as do many ancient fossil birds but not modern ones. These features led to its name: Oculudentavis khaungraaeI, translated as ‘eye-tooth bird’. So, is it a bird? A number of features shown by the skull suggest that, strictly speaking, it is not. Anatomically, it is a dinosaur, possibly descended from earlier types, such as the Jurassic winged and feathered dinosaur Archaeopterix, which evolved to early, true birds with which Oculudentavis coexisted during the Cretaceous Period. Having teeth, it was probably carnivorous and preyed on invertebrates: it may have been fatally attracted to tree resin in which insects had been trapped.

Micro-CT image of Oculudentavis khaungraaeI skull (top); artist’s impression of it in life (bottom) (credits: Xing, L. et al. 2020; Jingmai O’Connor, China University of Geosciences)

Even if it was a bird , it is smaller than the smallest living example, the bee hummingbird (Mellisuga helenae) and, weighing an estimated 2 grams,  Oculudentavis is about one-sixth the size of the smallest known fossil bird. As a dinosaur, it is two orders of magnitude smaller than the most diminutive example of those found as fossils, the chicken-sized Compsognathus. Rather than being just an oddity, Oculudentavis demonstrates that extreme miniaturisation among avian dinosaurs held out evolutionary advantages.

Watch a video about the discovery and analysis of the tiny dinosaur

See also: Benson, R.B.J. 2020. Tiny bird fossil might be the world’s smallest dinosaur. Nature, v. 579, p. 199-200; DOI: 10.1038/d41586-020-00576-6.

Artist’s rendering of a Middle Jurassic coastal plain in what is now the Isle of Skye across which a mixed dinosaur megafauna is migrating (credit: De Polo et al. 2020; Fig. 24; artist Jon Hoad)

And now for the lumbering and sometimes scary kinds of dinosaur. Since discovery of Middle Jurassic sauropod and theropod trackways with up to 0.5 m wide footprints at Brothers’ Point on the Trotternish Peninsula of Skye, the Inner Hebridean island has become a magnet for those wishing to commune with big beasts. Now the same team from the University of Edinburgh report more from the same locality (De Polo, P.E. and 9 others 2020. Novel track morphotypes from new tracksites indicate increased Middle Jurassic dinosaur diversity on the Isle of Skye, Scotland. PLoS ONE, v. 15, article e0229640; DOI: 10.1371/journal.pone.0229640). One set, referred to as Deltapodus was probably made by a species of stegosaur: the one with vertical plates on its back and a tail armed with large spikes, animated caricatures of which figure in inane YouTube clips, especially beating off Tyrannosaurs. The new locality preserves 50 dinosaur tracks that suggest a rich community of species. The most prominent suggest bipedal ornithopod herbivores and small, possible carnivorous theropods, both with three-toed feet, large quadripedal sauropods whose prints resemble those of elephants, as well as those with larger back feet than front attributed to stegosaurs. The sediment sequence displaying the tracks contains structures typical of deposition on a wide coastal plain.

Earliest plate tectonics tied down?

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Papers that ponder the question of when plate tectonics first powered the engine of internal geological processes are sure to get read: tectonics lies at the heart of Earth science. Opinion has swung back and forth from ‘sometime in the Proterozoic’ to ‘since the very birth of the Earth’, which is no surprise. There are simply no rocks that formed during the Hadean Eon of any greater extent than 20 km2. Those occur in the 4.2 billion year (Ga) old Nuvvuagittuq greenstone belt on Hudson Bay, which have been grossly mangled by later events. But there are grains of the sturdy mineral zircon ZrSiO4)  that occur in much younger sedimentary rocks, famously from the Jack Hills of Western Australia, whose ages range back to 4.4 Ga, based on uranium-lead radiometric dating. You can buy zircons from Jack Hills on eBay as a result of a cottage industry that sprang up following news of their great antiquity: that is, if you do a lot of mineral separation from the dust and rock chips that are on offer, and they are very small. Given a laser-fuelled SHRIMP mass spectrometer and a lot of other preparation kit, you could date them. Having gone to that expense, you might as well analyse them chemically using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to check out their trace-element contents. Geochemist Simon Turner of Macquarie University in Sydney, Australia, and colleagues from Curtin University in Western Australia and Geowissenschaftliches Zentrum Göttingen in Germany, have done all this for 32 newly extracted Jack Hills zircons, whose ages range from 4.3 to 3.3 Ga (Turner, S. et al. 2020. An andesitic source for Jack Hills zircon supports onset of plate tectonics in the Hadean. Nature Communications, v. 11, article 1241; DOI: 10.1038/s41467-020-14857-1). Then they applied sophisticated geochemical modelling to tease out what kinds of Hadean rock once hosted these grains that were eventually eroded out and transported to come to rest in a much younger sedimentary rock.

Artist’s impression of the old-style hellish Hadean (Credit : Dan Durday, Southwest Research Institute)

Zircons only form duuring the crystallisation of igneous magmas, at around 700°C, the original magma having formed under somewhat hotter conditions – up to 1200°C for mafic compositions. In the course of their crystallising, minerals take in not only the elements of which they are mainly composed, zirconium, silicon and oxygen in the case of zircon , but many other elements that the magma contains in low concentrations. The relative proportions of these trace elements that are partitioned from the magma into the growing mineral grains are more or less constant and unique to that mineral, depending on the particular composition of the magma itself. Using the proportions of these trace elements in the mineral gives a clue to the original bulk composition of the parent magma. The Jack Hills zircons  mainly  reflect an origin in magmas of andesitic composition, intermediate in composition between high-silica granites and basalts that have lower silica contents. Andesitic magmas only form today by partial melting of more mafic rocks under the influence of water-rich fluid driven upwards from subducting oceanic lithosphere. The proportions of trace elements in the zircons could only have formed in this way, according to the authors.

Interestingly, the 4.2 Ga Nuvvuagittuq greenstone belt contains metamorphosed mafic andesites, though any zircons in them have yet to be analysed in the manner used by Turner et al., although they were used to date those late-Hadean rocks. The deep post-Archaean continental crust, broadly speaking, has an andesitic composition, strongly suggesting its generation above subduction zones. Yet that portion of Archaean age is not andesitic on average, but a mixture of three geochemically different rocks. It is referred to as TTG crust from those three rock types (trondhjemite, tonalite and granodiorite). That TTG nature of the most ancient continental crust has encouraged most geochemists to reject the idea of magmatic activity controlled by plate tectonics during the Archaean and, by extension, during the preceding Hadean. What is truly remarkable is that if mafic andesites – such as those implied by the Jack Hills zircons and found in the Nuvvuagittuq greenstone belt – partially melted under high pressures that formed garnet in them, they would have yielded magmas of TTG composition. This, it seems, puts plate tectonics in the frame for the whole of Earth’s evolution since it stabilised several million years after the catastrophic collision that flung off the Moon and completely melted the outer layers of our planet. Up to now, controversy about what kind of planet-wide processes operated then have swung this way and that, often into quite strange scenarios. Turner and colleagues may have opened a new, hopefully more unified, episode of geochemical studies that revisit the early Earth . It could complement the work described in An Early Archaean Waterworld published on Earth-logs earlier in March 2020.

Further back in the Eurasian human story

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About 800 to 950 thousand years (ka) ago the earliest human colonisers of northern Europe, both adults and children, left footprints and stone tools in sedimentary strata laid down by a river system that then drained central England and Wales. The fossil flora and fauna at the Happisburgh (pronounced ‘Haze-burra’) site in Norfolk suggest a climate that was somewhat warmer in summers than at present, with winter temperatures about 3°C lower than now: similar to the climate in today’s southern Norway. At that time the European landmass extended unbroken to the western UK, so any hunter-gatherers could easily follow migrating herds and take advantage of seasonal vegetation resources. These people don’t have a name because they left no body fossils. A group known from their fossils as Homo antecessor had occupied Spain, southern France and Italy in slightly earlier times (back to 1200 ka). Since the discovery of their unique mix of modern and primitive traits, they have been regarded as possible intermediaries between H. erectus and H. heidelbergensis – once supposed to be the predecessor of Neanderthals and possibly anatomically modern humans (AMH). Since the emergence about 10 years ago of ancient genomics as the prime tool in examining human ancestry the picture has been shown to be considerably more complex. Not only had AMH interbred with Neanderthals and Denisovans, those two groups were demonstrably interfertile too, and a complex web of such relationships had been pieced together by 2016. But there has been a new development.

700 ka Homo erectus from Java: a possible Eurasian ‘super-archaic’ human (credit: Gibbons 2020)

Population geneticists at the University of Utah, USA, have devised sophisticated means of making more of the detailed ATCG nucleotide sequences in ancient human DNA, despite there being very few full genomes of Neanderthals and Denisovans (Rogers, A.R. et al. 2020. Neanderthal-Denisovan ancestors interbred with a distantly related hominin. Science Advances, v. 6, article eaay5483; DOI: 10.1126/sciadv.aay5483). In Earth-logs you may already have come across the idea of the ancestral ‘ghosts’ that are represented by unusual sections of genomes from living West African people. Those sections seem likely to have resulted from interbreeding with an unknown archaic population – i.e. neither Neanderthal nor Denisovan. It now seems that both Neanderthal and Denisovan genomes also show traces of such introgression with ‘ghost’ populations during much earlier times. The ancestors of both these groups separated from the lineage that led to AMH perhaps 750 ka ago. Rogers et al. refer to the earliest as ‘neandersovans’ and consider that they split into the two groups after they entered Eurasia, at some time before 600 ka – perhaps around 740 ka. This division may well have occurred as a result of a population of ‘neandersovans’ having spread over the vastness of Eurasia and growing genetic isolation. The reanalysis of both sets of genomes show evidence of a ‘neandersovan’ population crash before the split. Thereafter, the early Neanderthal population may have risen to around 16 thousand then slowly declined to ~3400 individuals.

A ‘state-of-play’ view of human interbreeding in Eurasia since 2 Ma ago (credit: Gibbons 2020)

However, the ‘neandersovans’ did not enter a new continent devoid of hominins, for as long ago as 1.9 Ma archaic H. erectus had arrived from Africa.  Both Neanderthal and Denisovan genomes record the presence of sections of ‘super-archaic’ DNA, which reflect early  interbreeding with earlier Eurasian populations. Indeed, Denisovans seem to have repeated their ancestors’ sexual exploits, once they became a genetically distinct group.  From the ‘ghost’ DNA fragments Rogers et al. conclude that the ‘super-archaics’ separated from other humans about two million years ago. They were descended from the first ‘Out-of-Africa’ wave of humans, represented by the fossils humans from Dmanisi in Georgia (see First out of Africa, November 2003 and An iconic early human skull,  October 2013 in Earth-logs Human evolution and migrations). A measure of the potential of novel means of analysing available ancient human DNA is the authors’ ability even to estimate the approximate population size of the interbreeding ‘super-archaic’ group at 20 to 50 thousand. Long thought to be impossible, it now seems possible to penetrate back to the very earliest human genetics, and the more DNA that can be teased out of other Neanderthal and Denisovan fossils the more we will know of our origins.

See also: Gibbons, A. 2020. Strange bedfellows for human ancestors. Science, v. 367, p. 838–839; doi:10.1126/science.367.6480.838

An Early Archaean Waterworld

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In Earth-logs you may have come across the uses of oxygen isotopes, mainly in connection with their variations in the fossils of marine organisms and in ice cores. The relative proportion of the ‘heavy’ 18O isotope to the ‘light’ 16O, expressed by δ18O, is a measure of the degree of fractionation between these isotopes under different temperature conditions when water evaporates. What happens is that H216O, in which the lighter isotope is bound up, slightly more easily evaporates thus enriching the remaining liquid water in H218O. As a result the greater the temperature of surface water and the more of evaporates, the higher is its δ18O value. Shells that benthonic (surface-dwelling) organism secrete are made mainly of the mineral calcite (CaCO3). Their formation involves extracting dissolved calcium ions and CO2 plus an extra oxygen from the water itself, as calcite’s formula suggests. So plankton shells fossilised  in ocean-floor sediments carry the δ18O and thus a temperature signal of surface water at the place and time in which they lived. Yet this signal is contaminated with another signal: that of the amount of water evaporated from the ocean surface (with lowered  δ18O) that has ended up falling as snow and then becoming trapped in continental ice sheets. The two can be separated using the δ18O found in shells of bottom-dwelling (benthonic) organisms, because deep ocean water maintains a similar low temperature at all time (about 2°C). Benthonic δ18O is the main guide to the changing volume of continental ice throughout the last 30 million year or so. This ingenious approach, developed about 50 years ago, has become the key to understanding past climate changes as reflected in records of ice volume and ocean surface temperature. Yet these two factors are not the only ones at work on marine oxygen isotopes.

Artistic impression of the Early Archaean Earth dominated by oceans (Credit: Sci-news.com)

When rainwater flows across the land, clays in the soil formed by weathering of crystalline rocks preferentially extract 18O and thus leave their own δ18O mark in ocean water. This has little, if any, effect on the use of δ18O to track past climate change, simply because the extent of the continents hasn’t changed much over the last 2 billion years or so. Likewise, the geological record over that period clearly indicates that rain, wet soil and water flowing across the land have all continued somewhere or other, irrespective of climate. However, one of the thorny issues in Earth science concerns changes of the area of continents in the very long term. They are suspected but difficult to tie down. Benjamin Johnson of the University of Colorado and Boswell Wing of Iowa State University, USA, have closely examined oxygen isotopes in 3.24 billion-year old rocks from a relic of Palaeoarchaean ocean crust from the Pilbara district of Western Australia that shows pervasive evidence of alteration by hot circulating ocean water (Johnson, B.W. & Wing, B.A. 2020. Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean. Nature Geoscience, v. 13, p. 243-248; DOI: 10.1038/s41561-020-0538-9). Interestingly, apart from the composition of the lavas, the altered rocks look just the same as much more recent examples of such ophiolites.

The study used many samples taken from the base to the top of the ophiolite along some 20 traverses across its outcrop. Overall the isotopic analyses suggested that the circulating water responsible for the hydrothermal alteration 3.2 Ga ago was much more enriched in 18O than is modern ocean water. The authors’ favoured explanation is that much less continental crust was exposed above sea level during the Palaeoarchaean Era than in later times and so far less clay was around on land. That does not necessarily imply that less continental crust existed at that time compared with the Archaean during the following 700 Ma , merely that the continental ‘freeboard’ was so low that only a few islands emerged above the waves. By the end of the Archaean 2.5 Ga ago the authors estimate that oceanic δ18O had decreased to approximately modern levels. This they attribute to a steady increase in weathering of the emerging continental landmasses and the extraction of 18O into new, clay-rich soils as the continents emerged above sea level. How this scenario of a ‘drowned’ world developed is not discussed. One possibility is that the average depth of the oceans then was considerably less than it was in later times: i.e. sea level stood higher because the volume available to contain ocean water was less. One possible explanation for that and the subsequent change in oxygen isotopes might be a transition during the later Archaean Eon into modern-style plate tectonics. The resulting steep subduction forms deep trench systems able to ‘hold’ more water. Prior to that faster production of oceanic crust resulted in what are now the ocean abyssal plains being buoyed up by warmer young crust that extended beneath them. Today they average around 4000 m deep, thanks to the increased density of cooled crust, and account for a large proportion of the volume of modern ocean basins.

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}