Wildfires and the formation of sugar-loaf hills

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One iconic feature of Rio de Janeiro is Corcovado Mountain, topped by the huge Cristo Redentor (Christ the Redeemer) statue. Another is the Sugar Loaf (Pão de Açúcar) that broods over Botafogo Bay. Each is an inselberg: a loan word from the German for ‘island mountain’. Elsewhere they are known as kopjes (southern Africa), monadnocks (North America) or bornhardts after the German explorer who first described them. But, being on the coast, the Brazilian examples are not typical. Most rise up spectacularly from almost featureless plains, a well-known case being Uluru (Ayers Rock) almost at the centre of Australia. Arid and semi-arid plains of Africa and the Indian subcontinent are liberally dotted with them. So scenically dominant and spectacularly stark, inselbergs are often revered by local people, and have been so for millennia. The only thing that I remember from a desperately boring, but compulsory, first-year course on geomorphology in 1965 is their connection with the ‘cosmogonic egg’: a mythological motif that spans Eurasia, Australia and Africa, signifying that from which the universe hatched. It is perhaps no coincidence that hills in England that suddenly rise from flat land, such as the Wrekin in Shropshire and Malvern Hill in Worcestershire, still host the sport of rolling hard-boiled eggs to celebrate the pagan festival of Eostre (now Easter) that marks the spring rebirth of the land.    

Vista of Rio de Janeiro and its inselbergs (Credit: Leonardo Ferreira Mendes, Creative Commons)

How inselbergs and their surrounding plains formed has long been a hot topic in tropical geomorphology. One theory is that they are especially resistant rocks around which eroding rivers meandered during the formation of peneplains, a variant being that they were surrounded by lines of weakness, such as faults or major joint systems. Another is that they formed by erosion into a deeply but irregularly weathered surface. Then there is L.C. Kings theory of escarpment retreat and, of course, a mixture of processes in different stages, or a unique origin for each inselberg. In effect, there has been no final, widely agreed explanation. But that that may be about to change.

A common element to most inselbergs is their very steep and sometimes vertical flanks. Some even display overhangs at their base. Such potential shelters encouraged local people to camp there and, in response to the awe inspired by the sheer majesty of the looming inselberg, to use them for sacred rites and decoration. That is especially true of Australia, so it is fitting that what may be a breakthrough in understanding inselberg formation should have arisen there. (Buckman, S. et al. 2021. Fire-induced rock spalling as a mechanism of weathering responsible for flared slope and inselberg development. Nature Communications, v. 12, article 2150; DOI: 10.1038/s41467-021-22451-2). Breaking rock by deliberate use of fire has been done for millennia. For instance, Hannibal is said to have used fire to break down huge fallen boulders that blocked passage for his war elephants as his army advanced on Rome. Fire setting is still used by villagers in South India to spall large flakes of rock from outcrops. It is done with such skill that thin slabs up to 3-4 metres across can be lifted, and then split into thin posts for fencing or training vines: an essential alternative to wooden posts that termites would otherwise devour in a matter of months.

Solomon Buckman and colleagues from the University of Wollongong, Australia were drawn to a new hypothesis for inselberg formation by observations around low rock faces and boulders after the 2019-20 “Black Summer” wildfires in eastern Australia. Where burned trees had fallen against rock faces up to hundreds of kg of spalled flakes lay at the base of each face, which also bore freshly formed scars: clear signs of fire action. Thermal expansion and contraction of rock caused by air temperatures of hundreds of degrees close to wildfires is clearly a powerful means of rapid erosion. If the rock is damp – most likely at the base of a rockface as all rainfall on the outcrop drains in its direction – the mechanism is enhanced: Hannibal’s engineers poured vinegar onto the boulders heated by fire, to great effect. Buckman et al. estimate the rate of lateral erosion by fire at slope bases in Australia to be around ten thousand times faster than those operating on horizontal rock surfaces, which are not exposed to fire as no vegetation grows on them. Over time, slopes steepen aided by the formation of flared surfaces at the base. If spalled debris is carried away quickly the developing inselberg evolves to its classical sugarloaf shape. In more arid conditions the debris builds around the outcrop to steadily smother inselberg development, leaving tors and kopjes. The paper came to press remarkably quickly relative to the authors’ field work and analyses. This is a work-in-progress to be followed up by cosmogenic-isotope and other means of surface dating of the tops and flanks of suitably accessible inselbergs and simiar features such as Western Australia’s famous Wave Rock (a flared escarpment).

Wave Rock in the interior of Western Australia is 15 m high and 100 m long and revered by the local Ballardong people as a creation of the Rainbow Serpent

Climate change has shifted Earth’s poles

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The shifting position of the Tropic of Cancer in Mexico due to nutation from 2005 to 2010 (Credit: Roberto González, Wikimedia Commons)

First suggested by Isaac Newton and confirmed from observations by Seth Chandler in 1891, the Earth’s axis of rotation and thus its geographic poles wander in much the same manner as does the axis of a gyroscope, through a process known as nutation. The best-known movement of the poles – Chandler wobble – results in a change of about 9 metres in the poles’ positions every 433 days, which describes a rough circle around the mean position of each pole. Every 18.6 years the orbital behaviour of the Moon results in a substantially larger shift, illustrated by a shift in the position of the circles of latitude, as above. Essentially, nutation results from the combined effects of gravitational forces imposed by other bodies. The axial precession cycle of 26 thousand years that is part of the Milankovich effect on long-term climate forcing is a result of nutation. But the Earth’s own gravitational field changes too, as mass within and upon it shifts from place to place. So mantle convection and plate tectonics inevitably change Earth’s mode of rotation, as do changes in the Earth’s molten iron core.

The most sensitive instrument devoted to measuring changes in Earth’s gravity is the tandem of two satellites known as the Gravity Recovery and Climate Experiment or GRACE. Among much else, GRACE has revealed the rate of withdrawal of groundwater from aquifers in Northern India and areas of mass deficit over the Canadian Shield that resulted from melting of its vast ice sheet since 18 ka ago (see: Ice age mass deficit over Canada deduced from gravity data, July 2007). Further GRACE data have now confirmed that more recent melting of polar glaciers due to global warming underlie an unusual reversal and acceleration of polar wandering since the 1990s (Deng, S. et al. 2021. Polar drift in the 1990s explained by terrestrial water storage changes. Geophysical Research Letters, v. 48, online article e2020GL092114; DOI: 10.1029/2020GL092114). In 1995 polar drift changed from southwards to eastwards, and increased by 17 times from its mean speed from 1981 to 1995. That tallies with an increase in the flow of glacial meltwater from polar regions and also with changes in the mass balance of surface and subsurface water at lower latitudes, especially in India, the USA and China where groundwater pumping for irrigation is on a massive scale.

Clearly, human activity is not only changing climate, but also our planet’s astronomical behaviour. That connection, in itself, is enough to set alarm bells ringing, even though the axial shift’s main tangible effect is to change the length of the day by a few milliseconds. Polar wandering has been documented for the last 176 years. Conceivably, data on shifts in past direction and speed may allow climatic changes throughout the industrial revolution to be assessed independently of meteorological data and on a whole-planet basis.

Ses also: Climate has shifted the axis of the Earth (EurekaAlert, 22 April 2021)

Multitudes of Tyrannosaurus rex in Cretaceous North America

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Full-frontal skull of ‘Sue’, the best-preserved and among the largest specimens of T. rex (Credit: Scott Robert Anselmo, Wikimedia Commons)

Long-term followers of Earth-logs and its predecessor Earth-pages News will have observed my general detachment from the dinosaur hullabaloo, which just runs and runs. That is, except for real hold-the-front-page items. One popped up in the 16 April 2021 issue of Science (Marshall, C.R. et al. 2021. Absolute abundance and preservation rate of Tyrannosaurus rex. Science, v. 372, p. 284-287; DOI:10.1126/science.abc8300). For over two million years in the Late Cretaceous, just before all dinosaurs – except for birds – literally bit the dust, the authors estimated a lot of the dinosaurian poster-childTyrannosaurus rex lurking in North America. I write ‘lurking’ because ‘tyrant lizard the king’ when fully grown was so big that if it ran and fell over, it would have been unable to get up! Tangible evidence from trackways suggests that it ambled from place to place. The leg bones of a 7-tonner would probably have shattered at speeds above 18 km per hour, and accelerating to the speed of a human jogger would, anyhow, have exhausted its energy reserves, But it was agile enough to be an ambush predator; it could even pirouette! And it could crush bones so well that it was able to consume prey entirely. It has been suggested that T. rex may have been a scavenger, at least in old age. Whatever, how is it possible to estimate numbers of any extinct species, let alone dinosaurs?

The stumbling block to getting a result that is better than guesswork is the fossil record of a species. First, it is incomplete, secondly the chance of finding a fossil varies from area to area, depending on all kinds of factors. These include the degree of exposure of sedimentary rock formed by the environment in which they thrived, as well as the vagaries of preservation due to post-mortem scavenging, erosion and water transport. In life the population density of a particularspecies varies between different ecosystems and from species to species. For instance, more lions can thrive in open rangeland than in wooded environments, whereas the opposite holds for tigers: probably because of different hunting strategies. Many factors such as these conspire to thwart realistic estimates of ancient populations. Studies of living species, however, suggest that population density of an animal species is inversely related to the average body mass of individuals. Take British herbivores: there are many more rabbits than there are deer. On the grasslands of East Africa hyenas and wild dogs outnumber lions. This mass-population relationship (Damuth’s Law) outlined by US ecologist John Damuth also depends on where a species exists in the food chain (its trophic level) as well as its physiology. Yet for living species, populations of flesh-eating mammals of similar mass show a 150-fold variation; a scatter that results from their different habits and habitats and also their energy requirements. Because they are warm-blooded (endothermic), small carnivorous mammals need a greater energy intake than do similar sized, cold-blooded reptiles, which need to eat far less. But not all living reptiles are ectothermic, especially the bigger ones. The Komodo dragon is mesothermic, midway between the two, and uses about a fifth of the energy needed by a similar-sized mammal carnivore. Population densities of dragons in the Lesser Sunda Islands are more than twice those of physiologically comparable mammalian predators.

A number of features suggest that the metabolism of carnivorous dinosaurs lay midway between those of large predatory mammals and big lizards like the Komodo dragon. This is the basic assumption for the analysis by Charles Marshall and colleagues. They did not focus on the biggest T. rex specimens, but on the average, estimated body mass of adults. There are numerous smaller specimens of the beast, but clearly some of these would have been sexually immature. It has been estimated that adulthood would have been achieved by around 15 years. The size data seem to show that achieving sexual maturity was accompanied by a 4 to 5 year growth spurt from the 2 to 3 tonnes of the largest juveniles to reach >7 t in the largest known adults which may have lived into their early 30s. The authors used this range to estimate a mean adult mass of 5.2 t. Taking this parameter and much more intricate factors into account, using intricate Monte Carlo simulations Marshall et al. came up with an estimate of 20 thousand T. rex adults across North America at any one time: but with an uncertainty of between 1,300 to 328,000. Spread over the 2.3 million km2 area of Late Cretaceous North America that lay above sea level their best-estimated population density would have been about 1 individual for every 100 square kilometres. An area the size of California could have had about 3800 adult Tyrannosaurus rex, while there may well have been two in Washington DC. Lest one’s imagination gets overly excited, were tigers and lions living wild today in North America under similar ecological conditions there would have been 12 and 28 respectively in the US capital. Yet those two adult Washingtonian T. rexs would have been unable to catch anything capable of a sustained jog, without keeling over. The juveniles weighing in at up to 3 tonnes would probably have been the real top predators; the smaller, the swifter and thus most fierce. Which leaves me to wonder, “Did the early teenagers catch the prey for their massive parents to chow-down on?”

See also: How many T. rexes were there? Billions. (ScienceDaily 15 April 2021)

Relationships between modern humans and Neanderthals

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Before 40 thousand years (ka) ago Europe was co-occupied by Neanderthals and anatomically modern humans (AMH) for between five to seven thousand years; about 350 generations – as long as the time since farming began in Neolithic Britain to the present day. Populations of both groups were probably low given their dependence on hunting and foraging during a period significantly colder than it is now. Crude estimates suggest between 3,000 to 12,000 individuals in each group; equivalent to the attendance at a single English Football League 2 match on a Covid-free winter Saturday afternoon. Moving around Europe south of say 55°N, their potential range would have been around 5 million square kilometres, which very roughly suggests that population density would be one person for every 200 km2. That they would have moved around in bands of, say, 10 to 25 might seem to suggest that encounters were very infrequent. Yet a hybrid Neanderthal-Denisovan female found in Siberia yielded DNA that suggested a family connection with Croatia, 5,000 km away (see: Neanderthal Mum meets Denisovan Dad, August 2018); early humans moved far and wide.

The likely appearances of Neanderthals and anatomically modern humans when they first met between 50 and 40 thousand years ago. (Credit: Jason Ford, New York University)

A sparsely populated land can be wandered through with little fear other than those of predators, sparse resources or harsh climate and lack of shelter. But it still seems incredible for there to have been regular meetings with other bands. But that view leaves out knowledge of good places to camp, hunt and forage that assure shelter, water, game and so forth, and how to get to them – a central part of hunter-gatherers’ livelihoods. There would have been a limited number of such refuges, considerably increasing chances of meeting. Whatever the physiognomic differences between AMH and Neaderthals, and they weren’t very striking, meeting up of bands of both human groups at a comfortable campsite would be cause for relief, celebration, exchanges of knowledge and perhaps individuals of one group to partner members of the other.

As well as that from Neanderthals, ancient DNA from very early European AMH remains has increasingly been teased out. The latest comes from three individuals from Bacho Kiro Cave in Bulgaria dated to between 45.9 to 42.6 ka; among the earliest known, fully modern Europeans. One had a Neanderthal ancestor less than six generations removed (perhaps even a great-great grandparent 60 years beforehand). Because of the slight elapsed time, the liaison was probably in Europe, rather than in the Middle East as previously suggested for insertion of Neanderthal genes into European ancestry. The genetic roots of the other two families stemmed back seven to ten generations – roughly 100 to 150 years (Hajdinjak, M. and 31 others 2021. Initial Upper Palaeolithic humans in Europe had recent Neanderthal ancestry. Nature, v. 592, p. 253–257; DOI: 10.1038/s41586-021-03335-3). The interpretation of these close relationships stems from the high proportion of Neanderthal DNA (3 to 4 %) in the three genomes. The segments are unusually lengthy, which is a major clue to the short time since the original coupling; inherited segments tend to shorten in successive generations. The groups to which these AMH individuals belonged did not contribute to later Eurasian populations, but link to living East Asians and Native Americans. They seem to have vanished from Europe long before modern times. The same day saw publication of a fourth instance of high Neanderthal genetic content (~3 %) in an early European’s genome, extracted from a ~45 ka female AMH from Zlatý kůň (Golden Horse) Cave in Czechia (Prüfer, K. and 11 others 2021. A genome sequence from a modern human skull over 45,000 years old from Zlatý kůň in Czechia. Nature Ecology & Evolution  DOI: 10.1038/s41559-021-01443-x). In her case, too, the Neanderthal DNA segments are unusually lengthy, but indicate 70 to 80 generations (~2,000 to 3,000 years) had elapsed. Her DNA also suggests that she was dark-skinned and had brown hair and brown eyes. Overall her genetics, too, do not have counterparts in later European AMH. The population to which she belonged may have migrated westwards from the Middle East, where one of her ancestors had mated with a Neanderthal, perhaps as long as 50 ka ago. But that does not rule out her group having been in Europe at that time. A later modern human, dated at 42 to 37 ka, is a young man from the Petştera cu Oase cave in Romania, whose forbears mixed with Neanderthals. His genome contains 6.4% of Neanderthal DNA, suggesting that his Neanderthal ancestor lived a mere 4 to 6 generations earlier, most likely in Europe, and was perhaps one of the last of that group.

The data suggest that once modern humans came into contact with their predecessors in the Middle East and Europe, mixture with Neanderthals was ‘the rule rather than the exception’. Yet their lack of direct relationship to later Europeans implies that AMH colonisation of Europe occurred in successive waves of people, not all of whom survived. As Palaeolithic specialist Chris Stringer of the Natural History Museum in London cautions, of these multiple waves of incomers ‘Some groups mixed with Neanderthals, and some didn’t. Some are related to later humans and some are not’. Even five thousand years after ‘first contact’, relations of modern humans with Neanderthals remained ‘cordial’, to say the least, including with the last few before their extinction.

See also: Gibbons, A. 2021. More than 45,000 years ago, modern humans ventured into Neanderthal territory. Here’s what happened next. Science, v. 372, News article; DOI: 10.1126/science.abi8830. Callaway, E. 2021. Oldest DNA from a Homo sapiens reveals surprisingly recent Neanderthal ancestry. Nature, v. 592, News article; DOI: 10.1038/d41586-021-00916-0. Genomes of the earliest Europeans (Science Daily, 7 April 2021). Bower, B. 2021 Europe’s oldest known humans mated with Neandertals surprisingly often (ScienceNews, 7 April 2021)

When did supercontinents start forming?

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Plate tectonics is easily thought of as being dominated by continental drift, the phenomenon that Alfred Wegener recognised just over a century ago. So it is at present, the major continents being separated by spreading oceans. Yet, being placed on a near-spherical planet, continents also move closer to others; eventually to collide and weld together. Part of Wegener’s concept was that modern continents formed from the breakup of a single large one that he called Pangaea; a supercontinent. The current drifting apart began in earnest around the end of the Triassic Period (~200 Ma), after 200 Ma  of Pangaea’s dominance of the planet along with a single large ocean (Panthalassa) covering 70% of the Earth’s surface. Wegener was able to fit Pangaea together partly on the basis of evidence from the continents’ earlier geological history. In particular the refit joined up zones of intense deformation from continent to continent. Although he did not dwell on their origin, subsequent research has shown these zones were the lines of earlier collisions between older continental blocks, once subduction had removed the intervening oceanic lithosphere; Pangaea had formed from an earlier round of continental drift. Even older collision zones within the pre-Pangaea continental blocks suggested the former existence of previous supercontinents.

Aided by the development of means to divine the position of the magnetic poles relative to differently aged blocks on the continents, Wegener’s basic methods of refitting have resulted in the concept of supercontinent cycles of formation and break-up. It turns out that supercontinents did not form by all earlier continental clanging together at one time. The most likely scenario is that large precursors or ‘megacontinents’ (Eurasia is the current example) formed first, to which lesser entities eventually accreted  A summary of the latest ideas on such global tectonic cycles appeared in the November 2020 issue of Geology (Wang, c. et al. 2020. The role of megacontinents in the supercontinent cycle. Geology, v. 49  p. 402-406; DOI: 10.1130/G47988.1). Chong Wang of the Chinese Academy of Sciences and colleagues from Finland and Canada identify three such cycles of megacontinent formation and the accretion around them of the all-inclusive supercontinents of Columbia, Rodinia and Pangaea since about 1750 Ma (Mesoproterozoic). They also suggestion that a future supercontinent (Amasia) is destined to agglomerate around Eurasia.

Known megacontinents in relation to suggested supercontinents since the Mesoproterozoic (credit: Wang et al.; Fig 2)

The further back in time, the more cryptic are ancient continent-continent collision zone or sutures largely because they have been re-deformed long after they formed. In some cases younger events that involved heating have reset their radiometric ages. The oldest evidence of crustal deformation lies in cratons, where the most productive source of evidence for clumping of older continental masses is the use of palaeomagnetic pole positions. This is not feasible for the dominant metamorphic rocks of old suture zones, but palaeomagnetic measurements from old rocks that have been neither deformed nor metamorphosed offer the possibility of teasing out ancient supercontinents. Commonly cratons show signs of having been affected by brittle extensional deformation, most obviously as swarms of vertical sheets or dykes of often basaltic igneous rocks. Dykes can be dated readily and do yield reliable palaeomagnetic pole positions. Some cratons have multiple dyke swarms. For example the Archaean Yilgarn  Craton of Western Australia, founded on metamorphic and plutonic igneous crust that formed by tectonic accretion between 3.8 to 2.7 Ga, has five of them spanning 1.4 billion years from late-Archaean (2.6 Ga) to Mesoproterozoic (1.2 Ga). Throughout that immense span of time the Yilgarn remained as a single continental block. Also, structural trends end abrubtly at the craton margins, suggesting that it was once part of a larger ‘supercraton’ subsequently pulled apart by extensional tectonics.  The eleven known cratons show roughly the same features.

On the strength of new, high quality pole positions from dykes of about the same ages (2.62 and 2.41 Ga) cutting the Yilgarn and Zimbabwe cratons, geoscientists from Australia, China, Germany, Russia and Finland, based at Curtin University in Western Australia, have attempted to analyse all existing Archaean and Palaeoproterozoic pole positions (Liu, Y. et al. 2021. Archean geodynamics: Ephemeral supercontinents or long-lived supercratons. Geology, v. 49  ; DOI: 10.1130/G48575.1). The Zimbabwe and Yilgarn cratons, though now very far apart, were part of the same supercraton from at least 2.6 Ga ago. Good cases can be made for several other such large entities, but attempting fit them all together as supercontinents by modelling is unconvincing. The modelled fit for the 2.6 Ga datum is very unlike that for 2.4 Ga; in the intervening 200 Ma all the component cratons ould have had to shuffle around dramatically, without the whole supercontinent edifice breaking apart. However, using the data to fit cratons together at two supercratons does seem to work, for the two assemblies remain in the same configurations for both the 2.6 and 2.4 Ga data.

Interestingly, all cratonic components of one of the supercratons show geological evidence of the major 2.4 Ga glaciation, whereas those of the other show no such climatic indicator. Yet the entity with glacial evidence was positioned at low latitudes around 2.4 Ga, the ice-free one spanning mid latitudes. This may imply that the Earth’s axial tilt was far higher than at present. The persistence of two similar sized continental masses for at least 200 Ma around the end of the Archaean Eon also hints at a different style of tectonics from that with which geologists are familiar. Only palaeomagnetic data from the pre 2.6 Ga Archaean can throw light on that possibility. That requires older, very lightly or unmetamorphosed rocks to provide palaeopole positions. Only two cratons, the Pilbara of Western Australia and the Kaapvaal of South Africa, are suitable. The first yielded the oldest-known pole dated at 3.2 Ga, the oldest from the second is 2.7 Ga. A range of evidence suggests that Pilbara and Kaapvaal cratons were united during at least the late Archaean.

The only answer to the question posed by this item’s title is ‘There probably wasn’t a single supercontinent at the end of the Archaean, but maybe two megacontinents or supercratons’. Lumps of continental lithosphere would move and – given time – collide once more than one lump existed, however the Earth’s tectonics operated …

Snippet: Early human collection of useless objects

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The Ga-Mohana rock shelter in North Cape Province, South Africa (Credit: Jayne Wilkins, University of the Witwatersrand)

We all, especially as kids, have collected visually interesting objects for no particular reason other than they ‘caught our eye’: at the beach; from ploughed fields; river gravel, or at the side of a path. They end up in sheds, attics and mantel shelves. In an online News and Views article at the Nature website Pamela Willoughby discusses the significance of a paper on an archaeological site in the southern Kalahari Desert, North Cape Province South Africa (Willoughby, P.R. 2021. Early humans far from the South African coast collected unusual objects. Nature, v. 323, online News and Views; DOI: 10.1038/d41586-021-00795-5). Jayne Wilkins and co-workers from South Africa, Australia, Canada, Austria and the UK have investigated a rock shelter, with floor deposits going back over 100 thousand years. The researchers have, in a sense, continued the long human habit of seeking objets trouvée by using trowels and sieves to excavate the shelter’s floor sediments. They found a collection of cleavage fragments of white calcite and abundant shards of ostrich shell. Ga-Mohana Hill is still a place that locals consider to have spiritual significance. The authors consider the original collectors to have had no other motive than aesthetic pleasure and perhaps ritual, and that this signifies perhaps the earliest truly modern human behaviour. Yet, in 1925 a cave on the other side of South Africa, in Limpopo Province, yielded a striking example of a possible ‘collector’s piece’ from much earlier times. It is associated with remains of australopithecines and has been dated to around 3 Ma ago (see: Earliest sign of a sense of aesthetics, November 2020).

Source: Wilkins, J. et al.2021. Innovative Homo sapiens behaviours 105,000 years ago in a wetter Kalahari. Nature, v. 323 DOI: 10.1038/s41586-021-03419-0

Arctic warmer than now half a million years ago

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Just over a month since evidence emerged that the Arctic Ocean was probably filled with fresh water from 150 to 131 and 70 to 62 thousand years ago (When the Arctic Ocean was filled with fresh water, February 2021), another study has shaken ‘received wisdom’ about Arctic conditions. This time it is about the climate in polar regions, and comes not from an ice core but speleothem or calcium carbonate flowstone that was precipitated on a cave wall in north-eastern Greenland. The existence of caves at about 80°N between 350 to 670 m above sea level in a very cold, arid area is a surprise in itself, for they require flowing water to form. The speleothem is up to 12 cm thick, but none is growing under modern, relatively warm conditions, cave air being below freezing all year. For speleothem to form to such an extent suggests a long period when air temperature was above 0°C. So was it precipitated before glacial conditions were established in pre-Pleistocene times?

Limestone caves in the arid Grottedal region of north-eastern Greenland (Credit: Moseley et al. 2021; Fig 2D)

A standard means of discovering the age of cave deposits, such as speleothem or stalagmites, is uranium-series dating (see: Irish stalagmite reveals high-frequency climate changes, December 2001). In this case the sheet of flowstone turned out to have been deposited between 588 to 537 thousand years ago; a 50 ka ‘window’ into conditions that prevailed during the middle part of 100 ka climatic cycling – about 6 glacial-interglacial stages before present. (Moseley, G.E. et al. 2021. Speleothem record of mild and wet mid-Pleistocene climate in northeast Greenland. Science Advances, v. 7, online article  eabe1260; DOI: 10.1126/sciadv.abe1260). Roughly half the layer formed during an interglacial, the rest under glacial conditions that followed. Detailed oxygen-isotope studies revealed that air temperatures during which calcium carbonate was precipitated were at least 3.5°C above those prevailing in the area at present; warm enough to melt local permafrost and to increase the summer extent of ice-free conditions in the Arctic Ocean, thereby encouraging greater rainfall. These warm and wet conditions correlate with increased solar heating over the North Atlantic region at that time, as suggested by modelling based on Milankovich astronomical forcing.

Unfortunately, the climate record derived from cores through the Greenland ice sheet only reaches back to about 120 ka, during the last interglacial period. So it is not possible to match the speleothem results to an alternative data set. Yet, thanks to the rediscovery of dirt cored from the very base of the deepest part of the ice sheet (beneath Camp Century) in a freezer in Denmark – it was discarded as interest focused on the record preserved in the ice itself – there is now evidence for complete melting of the ice sheet at some time in the past. The dirt contains abundant fossil plants. Analysing radioactive isotopes of aluminium and beryllium that formed in associated quartz grains as a result of cosmic ray bombardment when the area was ice-free suggests two periods of complete melting followed by glaciation , the second  being within the last million years.

The onshore Arctic climate is clearly more unstable than previously believed.

See also:  Geologists Find Million-Year-Old Plant Fossils Deep Beneath Greenland Ice Sheet. Sci News, 16 March 2021.

Where is Mars’s water?

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A delta at the edge of Jazero Crater on Mars; definite evidence that water once flowed into the crater. Colours show different minerals in the delta sediments (credit: Brown University)

Early in the exploration of Mars using orbiting imaging systems it was easy to be sceptical about evidence for water being present at or near the surface of the Red Planet. Resolution was poor and some claims seemed to be wishful thinking or a sort of astronautical agitprop. For instance, gullies on steep slopes appeared so sharp that they must be forming continually, otherwise Mars’s periodic huge dust storms would have muted them. Some scientists claimed that they were signs of flowing water and even presented pictures from different overpasses that showed changes in them, such as darkening and small shifts in microtopography, which may have resulted from flowing water. Because Mars has a mean surface temperature of about -50°C that seems unlikely; at such extremes in Antarctica spit at the ground and it lands as ice. Nonetheless a bit of special pleading that deeply buried ice in Martian sediments might melt because of pressure gave the idea some traction.

A far more plausible explanation for the active gulley formation is that loose fine sediment can flow in the manner of a liquid, as it does in sand dunes on Earth (see: First signs of liquid water on Mars? June 2000). Yet as remotely sensed image coverage expanded and its resolution improved (currently about 50 cm) masses of evidence for drainage networks, signs of catastrophic floods and even glaciers (The glaciers of Mars, July 2003) emerged. Huge areas of the planet bore witness to a period in its past history – 4.1 to 3.8 billion years (Ga) ago – when it was a warm and wet planet. It has even been suggested that the flat, low-elevation northern hemisphere was the bed of a former ocean, covering about a third of Mars to a depth of about a kilometre. Now the planet has a hyperarid surface and a very thin atmosphere dominated by CO2, a little nitrogen and argon but almost no water vapour (~0.03%). Its poles are covered by ice caps whose extents fluctuate seasonally. They each have a core of permanent water ice, and seasonally expand and contract due to formation and sublimation of dry ice made of solid CO2. So what happened to Mars’s once abundant water?

One long-held theory is that water and most of Mars’s original atmosphere escaped to space. A suggested mechanism is the photo-dissociation of water to hydrogen and oxygen. Mars’s gravity cannot prevent hydrogen escape, which would leave an excess of atmospheric oxygen. One thing in abundance on the Martian surface is oxygen combined in iron oxides (Fe2O3); hence its red coloration. This hematite may have formed during chemical weathering of surface rocks and sediments during the wet phase, which released Fe2+ ions that were immediately oxidised by the hyper-oxygenated atmosphere that resulted from photo-dissociation. But there is another plausible explanation …

The lake-bed sediments of Gale Crater on Mars from NASA’s Curiosity rover (credit: NASA/JPL, California Institute of Technology)

The much publicised successful landing of NASA’s Perseverance rover on 18 February 2021 was aimed at the small Jezero Crater, near the Martian equator. This contains an indisputable delta of a large drainage system that must once have filled the crater with a circular lake; a good place to seek out signs of early life, for which Perseverance is impressively equipped. Shortly afterwards there appeared a Research Article in Science (Scheller, E.L. et al. 2021. Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust. Science, Online research article eabc7717; DOI: 10.1126/science.abc7717) that examines the fate of the planet’s water. The authors estimate that by 3.0 Ga Mars’s surface had reached its current dry state. They model three processes – supply of water by volcanic degassing and its loss by atmospheric escape and chemical weathering of the Martian surface. The modelling was constrained by the ratio of deuterium (2H) to hydrogen inferred from meteorites believed to come from Mars and estimates by orbiting spacecraft of the current escape of hydrogen from the atmosphere. The latter is too slow to explain the huge loss of water between 4 and 3 Ga and subsequently. Addition of water from Mars’s mantle by volcanoes, even from the gigantic Olympus Mons, was far slower than on Earth because continuous plate tectonics was never achieved on Mars. Chemical weathering of the surface during Mars’s warm-wet phase formed abundant hydrated minerals as well as the hematite that gives the planet its characteristic hue. Water transport before 3 Ga moved clays and hydroxides etc to sedimentary basins, where they have remained undisturbed. On Earth, tectonics recycles sediments and their content of hydrated minerals into the mantle, eventually to regurgitate their water content through volcanism. On Mars, weathering and deposition has irreversibly locked-up between 30 and 99% of Mars’s original endowment of water in its ancient sedimentary crust.

That seems to be a ‘bit of a downer’ for ambitious prospects of terraforming Mars and making it a human escape destination. There are, however, some locations where water may be available in sufficient quantities to support some kind of permanent presence of small colonies, in the form of buried layers of ice, similar to permafrost (see: Ice cliffs on Mars, January 2018)

See also:  Carr, M.H. 2012. The fluvial history of Mars. Philosophical Transaction of the Royal Society (A), v. 370, p. 2193-2215; DOI: 10.1098/rsta.2011.0500.

The DNA of some old mammoths

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The only positive outcome of the thawing of permafrost is that it exposes remains of ancient animals in a virtually intact state, most famously those of the woolly mammoth (Mammuthus primigenius). But not so well-preserved that anyone could be induced to feast on its thawed-out meat. Tales of select groups being served mammoth at banquets are almost certainly apocryphal, but several have tasted one, and found that the meat smelled rotten and tasted awful. Mammoth bones, being so large, are regularly found and most museums in the Northern Hemisphere display their enormous teeth. DNA from three species of these extinct elephants has been sequenced – North American and European woolly mammoths and the North American Columbian mammoth that thrived on the more temperate central plains. But they lived about 12 to 100 thousand years ago. Now genetic data are available from three molar teeth found in permafrost in the Chukochya river basin in northern Siberia. (van der Valk, T. and 21 others 2021. Million-year-old DNA sheds light on the genomic history of mammoths. Nature v.591, p. 265–269; DOI: 10.1038/s41586-021-03224-9).

Wooly mammoth tooth offered for sale at Christie’s in 2015, which fetched £2750 (Credit: Christie’s on-line archives)

The mammoth molars have been dated at 0.68, 1.0 and 1.2 Ma (conservative estimates), far older than a horse dated between 560 and 780 ka that yielded DNA several years back. The sheer mass of the teeth and the fact that they had been preserved in frozen soil shielded genetic material from complete breakdown, but it was nonetheless heavily degraded to fragments no more than 50 base pairs long. This presented a major challenge to the team of palaeogeneticists’ reconstruction of the three mammoths’ genomes. Comparing the genomes with those of far younger woolly mammoths and their closest living relatives, Indian elephants, reveals that the ancient beasts were cold-adapted and probably had woolly coats. Two of the genomes suggest direct ancestry to both later woolly mammoths, whereas the third – the oldest – can  be linked to the enormous Columbian mammoth (M. columbi) that lived on mid-American grasslands during the Late Pleistocene. During glacial maxima when sea levels were ~100 m lower than at present Siberian faunas could easily have migrated into and colonised the Americas, using the Beringia land bridge across the Bering Strait. An early migration by the oldest Siberian mammoth could have given rise to the Columbian mammoth, later crossings to the American woollies. In fact it seems that genetic strands from the two younger Siberian mammoths also entered the DNA of M. columbi at some stage in its evolution.

Interesting as these revelations are about Arctic ice-age megafaunas, finding human remains that predate a few 10’s of ka in permafrost is unlikely. Modern humans and  Neanderthals are known to have migrated through Arctic Siberia, and perhaps Denisovans did too. Some individuals may have been unfortunate enough to have fallen into boggy ground that froze to form permafrost. However, there is no evidence for older human species having moved north of about 40°N since the first Africans entered 1.8 Ma ago. In any case, without the protection of massive bones, human DNA would probably have degraded more quickly than did that of these old mammoths.

See also: Roca, A.L. 2021. Million-year-old DNA provides a glimpse of mammoth evolution. Nature, v. 591, p. 208-209; DOI: 10.1038/d41586-021-00348-w; Black, R. 2021. Oldest DNA sequenced yet comes from million-year-old mammoths (Smithsonian Magazine, 17 February, 2021)

News from the Chicxulub drilling project

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Artist’s impression of an asteroid slamming into the shallow sea off the present Yucatán Peninsula about 65 Ma ago (Credit: Donald E. Davis of NASA)

Aimed at resolving the impact versus volcanism debate about the causes of the K-Pg mass extinction, the International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) began drilling into the focus of the Chicxulub impact structure off the Yucatán Peninsula, Mexico in 2016. The project recovered 830 m of rock core, of which  about 140 cm contained the boundary between tsunami deposits and the post-impact marine limestones of Danian Age (basal Palaeogene); as close as one can get to the moment when the asteroid hit the sea floor. That an impact close to the start of the Danian had taken place was first discovered from abnormally high concentrations of the platinum-group metal iridium (Ir), shocked mineral grains and glass spherules, among other anomalous materials, in 350 marine and terrestrial sections across the globe. If the Chicxulub crater contained similar features to these ‘smoking guns’ then the link might seem to be done and dusted. A report on the crucial few centimetres from the Chicxulub drill core shows this to be the case (Goderis, S. and 32 others 2021. Globally distributed iridium layer preserved within the Chicxulub impact structure. Science Advances, v. 9, article eabe3647; DOI: 10.1126/sciadv.abe3647).

Yet the boundary layer at Chicxulub could not have been emplaced at the instant of impact. The gigantic power involved would have flung debris outwards, including seawater as well as the rocks that were once at considerable depth below the seabed. Much in the manner of a stone falling into a pond molten crust would have rebounded from the initial strike to form an axial peak and a ringed basin. Likewise huge tsunamis would have rolled away from the impact, then to return and fill the new basin, perhaps several times. Some of the ejected debris would have reached low orbit in the form of pulverised rock and asteroid to remain there for a while before completely falling back to Earth. The core includes about 130 m of once partly molten debris (suevite) above more-or-less intact granitic basement. Only the top 3.5 m show signs of having been deposited in water; fine-grained, well-sorted and laminated suevite containing clasts of once molten material and even late-Cretaceous foraminifera tests, formed probably by the refilling of the impact basin during the backflow of tusunamis. A mere 3 cm of silt and clay just below marine limestones has yielded the characteristic high Ir and nickel concentrations. This Ir-rich layer also contains the earliest Palaeocene foraminifera.

Grains in the Ir-rich layer were the last to settle, the main question being ‘How long after the impact took place did that happen?’ Being very fine they are estimated to have fallen-out from suspension and circulation in the atmosphere over a period of up to a few decades. Coarser material below them would have taken no longer than a few weeks to years. Yet these estimates are based mainly on Stokes’ law governing particles of different sizes falling through a viscous fluid. Taking an empirical view based on actual rates of clay sedimentation in the ocean (~5 mm per thousand years) the Ir-rich layer may have been deposited over 6000 years. That is hardly the ‘instant of the impact’. But the timing does say something interesting about the return of life to the seas; in geological terms it was swift, if the forams are anything to go by. Since the tsunamis swept onto and drained the surrounding land masses a great deal of nutrient would have ended up in the sea awaiting organisms at the bottom of the food chain. Biomarker chemicals and trace fossils in the Ir-rich layer suggest  thriving bacterial communities, with forams, crustacea and larval fish.

The authors conclude ‘The clear association of the Ir anomaly within the Chicxulub impact structure and the recorded biotic response confirms the direct relationship between the impact event and the K-Pg mass extinction’. Whether that is accepted by those geoscientists with their eyes on the Deccan Trap hypothesis is not so certain …

Indian groundwater shortage threatens food production

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Farmers in India have been engaged in mass protests since September 2020. Their anger is directed at a series of laws introduced by the central government of Narendra Modi’s  Bharatiya Janata Party (BJP) that change farmers’ terms of trade. Agriculture in India also faces a future of reduced availability of groundwater on which farmers have become increasingly dependent, especially in the vast alluvial plains of the Ganges river system. The twin satellites of the Gravity Recovery  and Climate Experiment (GRACE), which chart changes in mass beneath the Earth’s surface, detected a major change in gravity over 3 million km2 of India’s largest area of agriculture in the northwestern Gangetic plains (Rodell, M. et al. 2009. Satellite-based estimates of groundwater depletion in India. Nature, v. 460, p.999-1002; DOI: 10.1038/nature08238). The data suggested a loss between 2002 and 2008 of around 109 cubic kilometres of water from the aquifers that support regional irrigation and the livelihoods of about 114 million people (see NASA summary). The loss of water and decline in well-water levels have continued since then.

Colour-coded GRACE data  from 2002 to 2008 showing the estimated drawdown in water levels in wells in NW India and NE Pakistan during this period. Green to dark-red colours indicate from 0 to 12 metres of decline (credit: Trent Schindler and Matt Rodell, NASA)

A recent comprehensive survey (Jain, M. and 8 others 2021. Groundwater depletion will reduce cropping intensity in India. Science Advances, v. 9, article eabd2849; DOI: 10.1126/sciadv.abd2849) uses satellite image and census data to document the actual changes in winter crops (those most dependent on irrigation) over the period 2001 to 2012. It roughly measures the realities of the unsustainable extraction of groundwater indicated by GRACE from 2002 to 2008. The study projects an average reduction of 20% in winter cropping across the whole of India, with some of the worst-hit areas being likely to experience a 68% loss. The dominant supplies of irrigation water are from countless tube wells and systems of canals supplied by dams or rivers. India has witnessed impressive gains in food production in the last half century, thanks to rapid and continuing growth in the number of tube wells driven by individual farmers. The livelihoods of about 600 million people depend on agriculture. There is no prospect of substituting either form of irrigation to maintain current levels of production. If increased canal supply was used to replace well water and reduce groundwater depletion, cropping intensity would still decline, albeit at about half the projected rate; however, that doesn’t take into account unpredictable droughts in surface water accumulation and movement.

Faced with this situation, it is hardly surprising that farmers fear for their families future and react massively to state intervention in their marketing and crop storage strategies.

For a wider context to the Indian agricultural crisis see also: The ecological roots of India’s farming crisis (Deutche Welle, 1 February, 2021)

Magnetic reversal and demise of the Neanderthals?

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A rumour emerged last week that the Neanderthals met their end as one consequence of an extraterrestrial, possibly even extragalactic influence. Curiously, it stems from a recent discovery in New Zealand, where of course Neanderthals never set foot and nor did anatomically modern humans, the ancestors of Maori people, until a mere 800 years ago. It started with an ancient log from a kauri tree (Agathis australis), a species that Maoris revere. Found in excavations of boggy ground, the log weighed about 60 tons, so it was a valuable commodity, especially as it is illegal to fell living kauri trees. The wood is unaffected by burial and insect attack, has a regular grain and colour throughout, so is ideal for monumental Maori sculpture. Such swamp kauri also preserves their own life history in annual growth rings, and the log in question has 1700 of them. Using growth rings to chart climate variation gives the most detailed records of the recent past, provided the wood can be dated. Matching growth ring records from several trees of different ages is key to charting local climate with annual precision over several millennia.

An ancient kauri tree log recovered by swampland excavations in New Zealand. (Credit: Jonathan Palmer, in Voosen 2021)

Radiocarbon dating indicates that this particular kauri tree was growing around 42 thousand years ago. That is close to the upper limit for using 14C concentration in organic matter to determine age because the isotope has a short half-life (5730 years). In this case samples of the log would contain only about 0.7 % of its original complement of radioactive carbon. Cosmic rays generate 14C when they hit nitrogen atoms in the atmosphere and it enters CO2 and thus the carbon cycle. Carbon dioxide taken up by photosynthesis to contribute carbon to plants contains only about one part per trillion of 14C. Consequently wood as ancient as that in the kauri log contains almost vanishingly small amounts, yet it can still be measured using mass spectrometry to yield an accurate radiometric age.

The particularly interesting thing about the 42 ka date is that it coincides with the timing of the last reversal of the Earth’s magnetic field, known as the Laschamps event. The kauri tree bears detailed witness through its growth rings to the environmental effects of a decrease in that field to almost zero as the poles flipped. The bulk of cosmic rays are normally deflected away from the Earth by the geomagnetic field, but during a reversal a great many more pass through the atmosphere, the most energetic reaching the surface and the biosphere. The kauri growth rings record fluctuations in the generation of 14C by their passage and thereby the geomagnetic field strength, which was only 6% of normal levels from 42.3 to 41.6 ka (Cooper, A. and 32 others  2021. A global environmental crisis 42,000 years ago. Science, v. 371, p. 811-818; DOI: 10.1126/science.abb8677). This coincided with an unrelated succession of periods of low solar activity and a reduced solar ‘wind’, which also provides some cosmic-rayprotection when activity is at normal levels; a ‘double whammy’. One consequence would have been destruction of stratospheric ozone by cosmic rays and thus increased ultraviolet exposure at ground level.

Combined with the highly precise growth-ring dating, the climatic changes over the 1700 year lifetime of the kauri tree can be linked to other records of environmental change. These include glacial ice- and lake-bed cores together with stalactite layers. Apparently, the Laschamps geomagnetic reversal coincided with abrupt shifts in wind belts and precipitation, perhaps triggering major droughts in the southern continents. Highly plausible, but some of the other speculations are less certain. For instance, some time around 42 ka, but far from well-established, Australia’s marsupial megafauna experienced major extinctions, the Neanderthals disappear from the fossil record and modern humans started decorating caves in Europe (20 ka after they did in Indonesia). In fact, speculation becomes somewhat silly, with suggestions that early Europeans went to live in caves because of increased exposure to UV (they knew, did they, while Neanderthals didn’t?), their painting and, by implication, their entire culture shifting through the shock and awe of mighty displays of the aurora borealis. Just because the number 42 is (or was), according to the late Douglas Adams’s Hitchhiker’s Guide to the Galaxy, ‘the answer to life, the universe and everything’, the authors tag the episode as the ‘Adams Event’. In their summary for The Conversation they include an animation with a quintessential Stephen Fry narrative, which Earth-logs readers can judge for themselves. Perhaps ‘Lockdown Trauma’ has a lot more to answer for, other than upsurges in Zoom conferences, knitting and gourmet experimentation …

See also: Voosen, P. 2021. Kauri trees mark magnetic flip 42,000 years ago. Science, v. 371, p. 766; DOI: 10.1126/science.371.6531.766

When the Arctic Ocean was filled with fresh water

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The salinity of surface water at high latitudes in the North Atlantic is a critical factor in its sinking to draw warm, low-latitude water northwards in the Gulf Stream while contributing to the southwards flow of North Atlantic Deep Water along the ocean floor. One widely supported hypothesis for rapid cooling events, such as the Younger Dryas, is the shutdown of this thermohaline circulation (Review of thermohaline circulation, February 2002). That may happen when surface seawater at high latitudes is freshened and made less dense by rapid melting or break-up of continental ice sheets, or through the release of vast amounts of fresh water from glacially dammed lakes. The climatic decline leading to the last glacial maximum at around 20 ka was punctuated by irregular episodes known as Dansgaard-Oeschger and Heinrich Events that have been attributed to such hiccups in thermohaline processes. In this context, a whole new barrel of fish has been opened up by a geochemical study of the top few metres of sediments on the Arctic Ocean floor (Geibert, W. et al. 2021. Glacial episodes of a freshwater Arctic Ocean covered by a thick ice shelf. Nature, v. 590, p. 97–102; DOI: 10.1038/s41586-021-03186-y), particularly their content of an isotope of thorium (230Th).

Being radioactive (half-life ~75 ka), 230Th is useful in working out sediment deposition rates, especially as it is insoluble and adheres to dust grains. The isotope is a decay product of uranium, yet it not only forms on land from uranium in hard rocks, eventually to be transported into marine sediments, but from uranium dissolved in seawater too. Interestingly, the amount of uranium that can enter seawater in solution depends on water salinity. Fresh water, especially that locked up in glacial ice, has very low concentrations of uranium. Consequently, ordinary seawater adds additional 230Th to sediments whereas fresh water does not. An excess of the isotope in marine sediments signifies their deposition from salty water, but those deposited in fresh water carry no excess. In the course of analysing deep-sea cores from the floors of the Arctic Ocean and the northernmost part of the North Atlantic, Walter Geibert and colleagues at the Alfred Wegener Institute in Bremerhaven, and the University of Bremen, Germany revealed a series of sediment layers that were devoid of excess 230Th. This suggests that twice, probably in periods between 150 to 131 and 70 to 62 ka, water in the Arctic Ocean and the connected Nordic Sea was entirely fresh. In two cores the evidence suggests a third, restricted occurrence of fresh water fill at about 15 ka.

The most likely explanation is that the fresh-water episodes marked the development of major ice shelves, similar to those still present around Antarctic; i.e. floating or grounded ice of glacial origin (not sea ice). That had been anticipated, but not previously proved for the northern polar region. The outlets from the Arctic Ocean basin to the Pacific and North Atlantic Oceans are marked by barriers of shallow seabed. One is the Bering Straits, which became the Beringia land bridge that facilitated animal and human migrations from Siberia to North America when sea level fell as continental ice sheets grew. The other is the Greenland-Scotland Ridge formed by volcanism connected to the Icelandic hot spot as the North Atlantic opened. It is possible that the suggested ice shelves grounded on these ridges, to effectively dam and isolate the Arctic Ocean. Fresh water from melting land ice would ‘pond’ beneath the ice shelves, floating on denser salt water and eventually expelling it from much of the polar marine basin. A side effect of this would have been partially to accumulate and isolate the oxygen-isotope proportions that characterise snow and glacial ice. Remember that the light 16O isotope is preferentially extracted from sea water during evaporation, to become stored in glacial ice sheets so that the proportion of the heavier 18O increases in ocean water; δ18O is therefore an important proxy for glacial waxing and waning and thus the fluctuations of global sea level. Trapping a proportion of water of glacial origin in isolated Arctic Ocean water and ice shelves would explain discrepancies in the oxygen-isotope records of successive ice ages. Also, if the ice shelves periodically broke up, fresh water derived from them and ponded in the deepest Arctic Ocean basin could change the salinity of surface ocean water elsewhere – being lower density that fresh water would ‘float’.

The work of Geibert and colleagues may well result in a great deal of head scratching among palaeoclimatologists and perhaps new ideas on the dynamics of ice age climates.

See also: Hoffmann, S. 2021. The Arctic Ocean might have been filled with freshwater during ice ages. Nature, v. 590, p. 37-38; DOI: 10.1038/d41586-021-00208-7

And here’s another snippet: Neanderthal link to our brain

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Elizabeth Pennisi reports on a ‘Petri-dish’ experiment that substitutes a Neanderthal gene for a modern human one in a culture of human brain tissue. It gives some idea of how our very close relative may have thought differently from us. Pennisi, E. 2021. Neanderthal-inspired ‘minibrains’ hint at what makes modern humans special. Science, online news item; DOI:10.1126/science.abh0331

Worth a read: Genes that prepared fish to invade the land

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Elizabeth Pennisi comments on three comparative studies of the genetics of modern fish and terrestrial tetrapods in the latest online issue of Science News. Apparently some fish genes were, perhaps fortuitously, ‘multipurpose’. They may have been exploited during the Devonian colonisation of land to help evolution of limbs, lungs and aspects of the nervous system to adapt shallow-water fishes to climb out onto dry land. (Pennisi, E. 2021. Fish had the genes to adapt to life on land—while they were still swimming the seas. Science, News 10 February 2021; DOI: 10.1126/science.abg9265).

The ancestry of our opposable thumbs

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Since the appearance of smart phones and the explosion of social media our thumbs have found a new niche; typing while holding a mobile. At a desktop keyboard, most of us don’t use thumbs very much, unless we have mastered fast touch typing, but for a huge variety of manual tasks thumbs are essential. The first makers of sophisticated stone tools must have been able to grip between fingers and thumb to manipulate the materials from which they were made and to perform the various stages in creating a razor sharp edge. To do that, as most of us are aware, the tip of the thumb must be capable of touching the tips of all four fingers; an opposable thumb is essential for the ‘precision grip’. Being able to tell when opposable thumbs evolve depends, of course, on finding hand-bone fossils. Being made of many bones disarticulated hands are a lot more fragile than long bones or those of the skull. Complete fossil hands are rare, as are feet, but a number have been found more or less complete. Whichever hominin had evolved opposable thumbs, their potential would have given them a considerable advantage over those that hadn’t.

The main muscles that control the movements of modern human fingers and thumb (Credit: Wikipedia)

Simply comparing the shapes of fossilised bones of fingers and thumbs with those of modern humans and other living primates has, so far, not proved capable of resolving with certainty which hominin groups either did or did not have opposable thumbs. The key lies in the muscles that operate them. It has become commonplace to reconstruct faces and even whole bodies from fairly complete skeletal remains by modelling musculature from the positioning and shape of the points of attachment of muscles to bone. But that become increasingly difficult for the small-scale and intricate attachments in hands. The critical muscle for opposable thumbs is known as the Opponens pollicis (the Latin for thumb is Digitus pollex); a small triangular muscle that operates in conjunction with three others (with pollicis in their Latin names).

Fotios Karakostis and six colleagues from German, Swiss and Greek universities have devised software that can model muscles in 3-D (F.A. Karakostis et al. 2021. Biomechanics of the human thumb and the evolution of dexterity. Current Biology, v.31,  online; DOI: 10.1016/j.cub.2020.12.041). Based on the anatomy of human and chimpanzee hand muscles and the positions of their attachment to individual bones, they have been able to establish a series of parameters that clearly distinguish the morphological and probably functional characteristics of the thumbs of these living primates. Complete sets of thumb bones from four Neanderthal skeletons show that they were significantly, but only slightly, different from anatomically modern humans. Those from three species of Australopithecus (africanus, sediba and afarensis) lie between ours and chimps’, with significantly closer affinity to chimpanzees. It seems that australopithecines of whatever age were not equipped with opposable thumbs and were possible tool producers and users with the very limited capabilities of modern chimps; holding, pounding and poking. A single set of hominin thumb bones from about two million years ago that were found in the famous Swartkrans Cave in South Africa show just as close affinity in thumb opposability to humans as do Neanderthals. So at 2 Ma there was a hominin species sufficiently dextrous to make and use sophisticated tools. The problem is, the bones are not directly associated with others and have been ascribed by different authors either to H. habilis or Paranthropus robustus. Interestingly, this paranthropoid has also been suggested (controversially) to have been the first known hominin to use fire, and it also used digging sticks. No one has ever suggested that the genus Homo descended from a paranthropoid ancestor or vice versa; these massively jawed beings did coexist with early humans in East Africa for over a million years. The other hominin who left hands in the geological record was Homo naledi; a controversial species because it was found in a barely accessible cave chamber, and took a while to date. This context gave rise to the notions that it was the direct ancestor of humans and that it buried its dead in a special place. However, it turned out to be relative recent, at about 280 ka (see: Homo naledi: an anti-climax; May 2017). Homo naledi does seem to have had opposable thumbs, but there is no associated evidence to suggest either tool making or use.

Fascinating as the methodology outlined by Karakostis et al. is, their findings do not take early human capabilities very much further than what is already known. Tools were made and used as far back as 3.3 Ma ago, and we know that H. habilis was doing this by about 2.6 Ma; i.e. long before the first evidence for opposable thumbs, and who had them first is uncertain. What is clear is that sophisticated tools, such as the bifacial Acheulian artifacts whose manufacture demands great dexterity, only appeared after the potential for nimble dexterity (about 1.8 Ma). The same goes for the first migration out of Africa, at about the same time, which demanded resourcefulness that may have sprung from the ability to manipulate natural materials effectively and carefully

See also: Handwerk B. 2012. How dexterous thumbs may have helped shape evolution two million years ago. (Smithsonian Magazine, 28 January 2021); Bower, B. 2021. Humanlike thumb dexterity may date back as far as 2 million years ago. (Science News, 28 January 2021)

Global warming: an important revision

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Part of the turmoil surrounding the issue of anthropogenic global warming hinges on whether or not observed changes in annual mean global temperature since the Industrial Revolution may be due to natural climatic cycles similar to those that operated previously during the Holocene Epoch. Actual measurements of temperatures of the air, sea surface and so on date only as far back at the early 18th century when thermometers were invented. Getting an idea of natural climate change through the 11.65 thousand years since the end of the last period of extensive glaciation depends on a variety of indirect measurements or proxies for temperature. For sea-surface temperature (SST) the proxy of choice is based on the way that surface-dwelling organisms, specifically planktic foraminifera, extract magnesium and calcium from sea water to construct their tests (shells). The warmer the sea surface the more magnesium is incorporated as a trace element into the calcium carbonate that forms their tests. The Mg/Ca ratio in planktic foram tests recovered from sea-floor sediment layers changes in a reliably precise fashion with warming and cooling. Following the Younger Dryas frigid millennium this proxy suggests that the average sea-surface temperature at mid-latitudes in the North Atlantic rose to a maximum of 0.5°C above the present value between 10 to 6 thousand years ago. After this Holocene Climate Optimum the sea surface seems to have cooled until very recently. Much the same pattern has been recorded in sediment cores from many parts of the world. Another approach is based on the varying amount of solar heating modelled by the Milankovich theory of astronomical climatic forcing and a variety of other forcing factors, such as albedo changes and the greenhouse effect. The two sets of data, one measured the other based on well-accepted simulations, do not agree; the modelling suggests a steady rise in SST throughout the Holocene and no climatic optimum. This conundrum either casts doubt on computer modelling of climate forcing, otherwise reliable on the broader time scale, or on some unsuspected aspect of the Mg/Ca palaeothermometer. The second could involve some kind of bias.

Plots of global mean sea-surface temperature estimates during the Holocene: blue – based on the Mg/Ca ratios in the tests of planktic foraminifera; red – the Mg/Ca data corrected for seasonal bias (the pale blue and pink areas encompass the full range of mid-latitude marine records); grey – modelling based on all potential forcing factors, including anthropogenic greenhouse emissions. (credit: Jennifer Hertzberg, 2021; Fig 1)

Samatha Bova of Rutgers University, USA, and colleagues from the US and China have examined the possibility of seasonal bias in estimates of SSTs from West Pacific ocean floor sediment cores off New Guinea  (Bova, S. et al. 2021. Seasonal origin of the thermal maxima at the Holocene and the last interglacial. Nature, v. 589, p. 548–553; DOI: 10.1038/s41586-020-03155-x). First they examined the Mg/Ca proxy record from the last, Eemian interglacial episode (128-115 ka), on the grounds that astronomical modelling indicated much stronger seasonal contrasts in solar warming during that period, whereas other forcing factors were comparatively weak. By calculating the varying sensitivity of the older Mg/Ca record to seasonal factors they were able to devise a method of correcting such records for seasonal bias and apply it to the Holocene data from northeast New Guinea. The corrected Holocene SST record lacks the previously suspected climate optimum and its peak at ~8000 years ago. Instead, it reveals a continuous warming trend throughout the Holocene. The early part is far cooler than previously indicated by uncorrected SST thermometry. That may have resulted from the increased reflection of solar radiation – albedo forcing – from a larger area of remnant ice sheets on high-latitude parts of continents than was present during the warmer early-Eemian interglacial. Final melting of the great ice sheets of the Northern Hemisphere took until about 6500 years ago, when albedo effects would be roughly the same as at present. Thereafter, rising levels of atmospheric greenhouse gases warmed the planet towards modern levels.

Bova et al’s findings fundamentally change the context for modelling future climate change, and also for the interpretation of all previous interglacials, palaeotemperature records from which remain uncorrected. It seems likely that none of them had an early warm episode. As regards the future; climate modelling will have to change its parameters. For climate-change sceptics; two of their favourite arguments have been questioned. There are no longer signs of major, natural  ups and downs in the early Holocene that might suggest that current warming is simply repeating such fluctuations. The other aspect of the Holocene climate conundrum, that greenhouse gases increased naturally since 6000 years ago while global mean SSTs declined, has been removed from the sceptics’ arguments

See also: Hertzberg, J. 2021. Palaeoclimate puzzle explained by seasonal variation. Nature, v. 589, p. 521-522; DOI: 10.1038/d41586-021-00115-x. Kiefer, P. 2021. Earth used to be cooler than we thought, which changes our math on global warming, Popular Science, 28 January 2021

How flowering plants may have regulated atmospheric oxygen

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Ultimately, the source of free oxygen in the Earth System is photosynthesis, but that is the result of a chemical balance in the biosphere and hydrosphere that operates at the surface and just beneath it in sediments. Burial of dead organic carbon in sedimentary rocks allows free oxygen to accumulate whereas weathering and oxidation of that carbon, largely to CO2, tends to counteract oxygen build-up. The balance is reflected in the current proportion of 21% oxygen in the atmosphere. Yet in the past oxygen levels have been much higher. During the Carboniferous and Permian periods it rose dramatically to an all-time high of 35% in the late Permian (about 250 Ma ago). This is famously reflected in fossils of giant dragonflies and other insects from the later part of the Palaeozoic Era.  Insects breathe passively by tiny tubes (trachea) through whose walls oxygen diffuses, unlike active-breathing quadrupeds that drive air into lung alveoli to dissolve O2 directly in blood. Insect size is thus limited by the oxygen content of air; to grow wing spans of up to 2 metres a modern dragon fly’s body would consist only of trachea with no room for gut; it would starve.

Woman holding a reconstructed Late Carboniferous dragonfly (Namurotypus sippeli)

During the early Mesozoic oxygen fell rapidly to around 15% during the Triassic then rose through the Jurassic and Cretaceous Periods to about 30%, only to fall again to present levels during the Cenozoic Era. Incidentally, the mass extinction at the end of the Cretaceous (the K-Pg boundary event) was marked in the marine sedimentary record by unusually high amounts of charcoal. That is evidence for the Chixculub impact being accompanied by global wild fires that a high-oxygen atmosphere would have encouraged. The high oxygen levels of the Cretaceous marked the emergence of modern flowering plants – the angiosperms. Six British geoscientists have analysed the possible influence on the Earth System of this new and eventually dominant component of the terrestrial biosphere. (Belcher, C.M. et al. The rise of angiosperms strengthened fire feedbacks and improved the regulation of atmospheric oxygen. Nature Communications, v. 12, article 503; DOI 10.1038/s41467-020-20772-2)

The episodic occurrence of charcoal in sedimentary rocks bears witness to wildfires having affected terrestrial ecosystems since the decisive colonisation of the land by plants at the start of the Devonian 420 Ma ago. Fire and vegetation have since gone hand in hand, and the evolution of land plants has partly been through adaptations to burning. For instance the cones of some conifer species open only during wildfires to shed seeds following burning. Some angiosperm seeds, such as those of eucalyptus, germinate only after being subject to fire . The nature of wildfires varies according to particular ecosystems: needle-like foliage burns differently from angiosperm leaves; grassland fires differ from those in forests and so on. Massive fires on the Earth’s surface are not inevitable, however. Evidence for wildfires is absent during those times when the atmosphere’s oxygen content has dipped below an estimated 16%. The current oxygen level encourages fires in dry forest during drought, as those of Victoria in Australia and California in the US during 2020 amply demonstrated. It is possible that with oxygen above 25% dry forest would not regenerate without burning in the next dry season. Wet forest, as in Brazil and Indonesia, can burn under present conditions but only if set alight deliberately. Evidence of a global firestorm after the K-Pg extinction implies that tropical rain forest burns easily when oxygen is above 30%. So, how come the dominant flora of Earth’s huge tropical forests – the flowering angiosperms – evolved and hung on when conditions were ripe for them to burn on a massive scale?

Early angiosperms had small leaves suggesting small stature and growth in stands of open woodland [perhaps shrubberies] that favoured the fire protection of wetlands. ‘Weedy’ plants regenerate and reach maturity more quickly than do those species that are destined to produce tall trees. With endemic wildfires, tree-sized plants – e.g. the gymnosperms of the Mesozoic – cannot attain maturity by growing above the height of flames. Diminutive early angiosperms in a forest understory would probably outcompete their more ancient companions.  Yet to become the mighty trees of later rain forests angiosperms must somehow have regulated atmospheric oxygen so that it declined well below the level where wet forest is ravaged by natural wild fires. The oldest evidence for angiosperm rain forest dates to 59 Ma, when perhaps more primitive tropical trees had been almost wiped-out by wildfires. Did angiosperms also encourage wildfires, that consumed oxygen on a massive scale, as well as evolving to resist their affects on plant growth? Claire Belcher et al. suggest that they did, through series of evolutionary steps. Key to their stabilising oxygen levels at around 21%, the authors allege, was angiosperms’ suppression of weathering of phosphorus from rocks and/or transfer of that major nutrient from the land to the oceans. On land nitrogen is the most important nutrient for biomass, whereas phosphorus is the limiting factor in the ocean. Its reduction by angiosperm dominance on land thereby reduces carbon burial in ocean sediments. In a very roundabout way, therefore, angiosperms control the key factor in allowing atmospheric build-up of oxygen; by encouraging mass burning and suppressing carbon burial.  Today, about 84 percent of wildfires are started by anthropogenic activities. As yet we have little, if any, idea of how such disruption of the natural flora-fire system is going to affect future ecosystems. The ‘Pyrocene’ may be an outcome of the ‘Anthropocene’ …

The oldest known impact structure (?)

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That large, rocky bodies in the Solar System were heavily bombarded by asteroidal debris at the end of the Hadean Eon (between 4.1 to 3.8 billion years ago) is apparent from the ancient cratering records that they still preserve and their matching with dating of impact-melt rocks on the Moon. Being a geologically dynamic planet, the Earth preserves no tangible, indisputable evidence for this Late Heavy Bombardment (LHB), and until quite recently could only be inferred to have been battered in this way. That it actually did happen emerged from a study of tungsten isotopes in early Archaean gneisses from Labrador, Canada (see: Tungsten and Archaean heavy bombardment, August 2002; and Did mantle chemistry change after the late heavy bombardment? September 2009). Because large impacts deliver such vast amounts of energy in little more than a second (see: Graveyard for asteroids and comets, Chapter 10 in Stepping Stones) they have powerful consequences for the Earth System, as witness the Chicxulub impact off the Yucatán Peninsula of Mexico that resulted in a mass extinction at the end of the Cretaceous Period. That seemingly unique coincidence of a large impact with devastation of Earth’s ecosystems seems likely to have resulted from the geology beneath the impact; dominated by thick evaporite beds of calcium sulfate whose extreme heating would have released vast amounts of SO2 to the atmosphere. Its fall-out as acid rain would have dramatically affected marine organisms with carbonate shells. Impacts on land would tend to expend most of their energy throughout the lithosphere, resulting in partial melting of the crust or the upper mantle in the case of the largest such events.

The further back in time, the greater the difficulty in recognising visible signs of impacts because of erosion or later deformation of the lithosphere. With a single, possible exception, every known terrestrial crater or structure that may plausibly be explained by impact is younger than 2.5 billion years; i.e. they are post-Archaean. Yet rocky bodies in the Solar System reveal that after the LHB the frequency and magnitude of impacts steadily decreased from high levels during the Archaean; there must have been impacts on Earth during that Eon and some may have been extremely large. In the least deformed Archaean sedimentary sequences there is indirect evidence that they did occur, in the form of spherules that represent droplets of silicate melts (see: Evidence builds for major impacts in Early Archaean; August 2002, and Impacts in the early Archaean; April 2014), some of which contain unearthly proportions of different chromium isotopes (see: Chromium isotopes and Archaean impacts; March 2003). As regards the search for very ancient impacts, rocks of Archaean age form a very small proportion of the Earth’s continental surface, the bulk having been buried by younger rocks. Of those that we can examine most have been subject to immense deformation, often repeatedly during later times.

The Archaean geology of part of the Akia Terrane (Manitsoq area) in West Greenland. The suggested impact structure is centred on the Finnefjeld Gneiss (V symbols) surrounded by highly deformed ultramafic to mafic igneous rocks. (Credit: Jochen Kolb, Karlsruhe Institute of Technology, Germany)

There is, however, one possibly surviving impact structure from Archaean times, and oddly it became suspected in one of the most structurally complex areas on Earth; the Akia Terrane of West Greenland. Aeromagnetic surveys hint at two concentric, circular anomalies centred on a 3.0 billion years-old zone of grey gneisses (see figure) defining a cryptic structure. It is is surrounded by hugely deformed bodies of ultramafic and mafic rocks (black) and nickel mineralisation (red). In 2012 the whole complex was suggested to be a relic of a major impact of that age, the ultramafic-mafic bodied being ascribed to high degrees of impact-induced melting of the underlying mantle. The original proposers backed up their suggestion with several associated geological observations, the most crucial being supposed evidence for shock-deformation of mineral grains and anomalous concentrations of platinum-group metals (PGM).

A multinational team of geoscientists have subjected the area to detailed field surveys, radiometric dating, oxygen-isotope analysis and electron microscopy of mineral grains to test this hypothesis (Yakymchuck, C. and 8 others 2020. Stirred not shaken; critical evaluation of a proposed Archean meteorite impact in West Greenland. Earth and Planetary Science Letters, v. 557, article 116730 (advance online publication); DOI: 10.1016/j.epsl.2020.116730). Tectonic fabrics in the mafic and ultramafic rocks are clearly older than the 3.0 Ga gneisses at the centre of the structure. Electron microscopy of ~5500 zircon grains show not a single example of parallel twinning associated with intense shock. Oxygen isotopes in 30 zircon grains fail to confirm the original proposers’ claims that the whole area has undergone hydrothermal metamorphism as a result of an impact. All that remains of the original suggestion are the nickel deposits that do contain high PGM concentrations; not an uncommon feature of Ni mineralisation associated with mafic-ultramafic intrusions, indeed much of the world’s supply of platinoid metals is mined from such bodies. Even if there had been an impact in the area, three phases of later ductile deformation that account for the bizarre shapes of these igneous bodies would render it impossible to detect convincingly.

The new study convincingly refutes the original impact proposal. The title of Yakymchuck et al.’s paper aptly uses Ian Fleming’s recipe for James Bond’s tipple of choice; multiple deformation of the deep crust does indeed stir it by ductile processes, while an impact is definitely just a big shake. For the southern part of the complex (Toqqusap Nunaa), tectonic stirring was amply demonstrated in 1957 by Asger Berthelsen of the Greenland Geological Survey (Berthelsen, A. 1957. The structural evolution of an ultra- and polymetamorphic gneiss-complex, West Greenland. Geologische Rundschau, v. 46, p. 173-185; DOI: 10.1007/BF01802892). Coming across his paper in the early 60s I was astonished by the complexity that Berthelsen had discovered, which convinced me to emulate his work on the Lewisian Gneiss Complex of the Inner Hebrides, Scotland. I was unable to match his efforts. The Akia Terrane has probably the most complicated geology anywhere on our planet; the original proposers of an impact there should have known better …

Tsunami risk in East Africa

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The 26 December 2004 Indian Ocean tsunami was one of the deadliest natural disasters since the start of the 20th century, with an estimated death toll of around 230 thousand. Millions more were deeply traumatised, bereft of homes and possessions, rendered short of food and clean water, and threatened by disease. Together with that launched onto the seaboard of eastern Japan by the Sendai earthquake of 11 March 2011, it has spurred research into detecting the signs of older tsunamis left in coastal sedimentary deposits (see for instance: Doggerland and the Storegga tsunami, December 2020). In normally quiet coastal areas these tsunamites commonly take the form of sand sheets interbedded with terrestrial sediments, such as peaty soils. On shores fully exposed to the ocean the evidence may take the form of jumbles of large boulders that could not have been moved by even the worst storm waves.

Sand sheets attributed to a succession of tsunamis, interbedded with peaty soils deposited in a swamp on Phra Thong Island, Thailand. Note that a sand sheet deposited by the 2004 Indian Ocean tsunami is directly beneath the current swamp surface (Credit: US Geological Survey)

Most of the deaths and damage wrought by the 2004 tsunami were along coasts bordering the Bay of Bengal in Indonesia, Thailand, Myanmar, India and Sri Lanka, and the Nicobar Islands. Tsunami waves were recorded on the coastlines of Somalia, Kenya and Tanzania, but had far lower amplitudes and energy so that fatalities – several hundred – were restricted to coastal Somalia. East Africa was protected to a large extent by the Indian subcontinent taking much of the wave energy released by the magnitude 9.1 to 9.3 earthquake (the third largest recorded) beneath Aceh at the northernmost tip of the Indonesian island of Sumatra. Yet the subduction zone that failed there extends far to the southeast along the Sunda Arc. Earthquakes further along that active island arc might potentially expose parts of East Africa to far higher wave energy, because of less protection by intervening land masses.

This possibility, together with the lack of any estimate of tsunami risk for East Africa, drew a multinational team of geoscientists to the estuary of the Pangani River  in Tanzania (Maselli, V. and 12 others 2020. A 1000-yr-old tsunami in the Indian Ocean points to greater risk for East Africa. Geology, v. 48, p. 808-813; DOI: 10.1130/G47257.1). Archaeologists had previously examined excavations for fish farming ponds and discovered the relics of an ancient coastal village. Digging further pits revealed a tell-tale sheet of sand in a sequence of alluvial sediments and peaty silts and fine sands derived from mangrove swamps. The peats contained archaeological remains – sherds of pottery and even beads. The tsunamite sand sheet occurs within the mangrove facies. It contains pebbles of bedrock that also litter the open shoreline of this part of Tanzania. There are also fossils; mainly a mix of marine molluscs and foraminifera with terrestrial rodents fish, birds and amphibians. But throughout the sheet, scattered at random, are human skeletons and disarticulated bones of male and female adults, and children. Many have broken limb bones, but show no signs of blunt-force trauma or disease pathology. Moreover, there is no sign of ritual burial or weaponry; the corpses had not resulted from massacre or epidemic. The most likely conclusion is that they are victims of an earlier Indian Ocean tsunami. Radiocarbon dating shows that it occurred at some time between the 11th and 13th centuries CE. This tallies with evidence from Thailand, Sumatra, the Andaman and Maldive Islands, India and Sri Lanka for a major tsunami in 950 CE.

Computer modelling of tsunami propagation reveals that the Pangani River lies on a stretch of the Tanzanian coast that is likely to have been sheltered from most Indian Ocean tsunamis by Madagascar and the shallows around the Seychelles Archipelago. Seismic events on the Sunda Arc or the lesser, Makran subduction zone of eastern Iran may not have been capable of generating sufficient energy to raise tsunami waves at the latitudes of the Tanzanian coast much higher than those witnessed there in 2004, unless their arrival coincided with high tide – damage was prevented in 2004 because of low tide levels. However, the topography of the Pangani estuary may well amplify water level by constricting a surge. Such a mechanism can account for variations of destruction during the 2011 Tohoku-Sendai tsunami in NE Japan.

If coastal Tanzania is at high risk of tsunamis, that can only be confirmed by deeper excavation into coastal sediments to check for multiple sand sheets that characterise areas closer to the Sunda Arc. So far, that in the Pangani estuary is the only one recorded in East Africa

Weak lithosphere delayed the formation of continents

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There are very few tangible signs that the Earth had continents at the surface before about 4 billion years (Ga) ago. The most cited evidence that they may have existed in the Hadean Eon are zircon grains with radiometric ages up to 4.4 Ga that were recovered from much younger sedimentary rocks in Western Australia. These tiny grains also show isotopic anomalies that support the existence of continental material, i.e. rocks of broadly granitic composition, only 100 Ma after the Earth formed (see: Zircons and early continents no longer to be sneezed at; February 2006). So, how come relics of such early continents have yet to be discovered in the geological record? After all granitic rocks – in the broad sense – which form continents are so less dense than the mantle that modern subduction is incapable of recycling them en masse. Indeed, mantle convection of any type in the hotter Earth of the Hadean seems unlikely to have swallowed continents once they had formed. Perhaps they are hiding in another guise among younger rocks of the continental crust. But, believe me; geologists have been hunting for them, to no avail, in every scrap of existing continental crust since 1971 when gneisses found in West Greenland by New Zealander Vic McGregor turned out to be almost 3.8 Ga old. This set off a grail-quest, which still continues, to negate James Hutton’s ‘No vestige of a beginning …’ concept of geological time.

There is another view. Early continental lithosphere may have returned to the mantle piece by piece by other means. One that has been happening since the Archaean is as debris from surface erosion and its transportation to the ocean floor, thence to be subducted along with denser material of the oceanic lithosphere. Another possibility is that before 4 Ga continental lithosphere had far less strength than characterised it in later times; it may have been continually torn into fragments small enough for viscous drag to defy buoyancy and consign them into the mantle by convective processes. Two things seem to confer strength on continental lithosphere younger than 4 billion years: its depleted surface heat flow and heat-production that stem from low concentrations of radioactive isotopes of uranium, thorium and potassium in the lower crust and sub-continental mantle; bolstering by cratons that form the cores of all major continents. Three geoscientists at Monash University in Victoria, Australia have examined how parts of early convecting mantle may have undergone chemical and thermal differentiation (Capitanio, F.A. et al. 2020. Thermochemical lithosphere differentiation and the origin of cratonic mantle.  Nature, v. 588, p. 89-94; DOI: 10.1038/s41586-020-2976-3). These processes are an inevitable outcome of the tendency for mantle melting to begin as it becomes decompressed when pressure decreases when it rises during convection. Continual removal of the magmas produced in this way would remove not only much of the residue’s heat-producing capacity – U, Th and K preferentially enter silicate melts – but also its content of volatiles, especially water. Even if granitic magmas were completely recycled back to the mantle by the greater vigour of the hot, early Earth, at least some of the residue of partial melting would remain. Its dehydration would increase its viscosity (strength). Over time this would build what eventually became the highly viscous thick mantle roots (tectosphere) on which increasing amounts of the granitic magmas could stabilise to establish the oldest cratons. Over time more and more such cratonised crust would accumulate, becoming increasingly unlikely to be resorbed into the mantle. Although cratons are not zoned in terms of the age of their constituent rocks, they do jumble together several billion years’ worth of continental crust in what used to be called ‘the Basement Complex’.

Development of depleted and viscous sub-continental mantle on the early Earth – a precedes b – TTG signifies tonalite-trondhjemite-granodiorite rocks typical of Archaean cratons (Credit, Capitanio et al.; Fig 5)

Early in this process, heat would have made much of the lithosphere too weak to form rigid plates and the tectonics with which geologists are so familiar from the later parts of Earth’s history. The evolution that Capitanio et al. propose suggests that the earliest rigid plates were capped by Archaean continental crust. That implies subduction of oceanic lithosphere starting at their margins, with intra-oceanic destructive plate margins and island arcs being a later feature of tectonics. It is in the later, Proterozoic Eon that evidence for accretion of arc terranes becomes obvious, plastering their magmatic products onto cratons, further enlarging the continents.

Thawing permafrost, release of carbon and the role of iron

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Projected shrinkage of permanently frozen ground i around the Arctic Ocean over the next 60 years

Global warming is clearly happening. The crucial question is ‘How bad can it get?’ Most pundits focus on the capacity of the globalised economy to cut carbon emissions – mainly CO2 from fossil fuel burning and methane emissions by commercial livestock herds. Can they be reduced in time to reverse the increase in global mean surface temperature that has already taken place and those that lie ahead? Every now and then there is mention of the importance of natural means of drawing down greenhouse gases: plant more trees; preserve and encourage wetlands and their accumulation of peat and so on. For several months of the Northern Hemisphere summer the planet’s largest bogs actively sequester carbon in the form of dead vegetation. For the rest of the year they are frozen stiff. Muskeg and tundra form a band across the alluvial plains of great rivers that drain North America and Eurasia towards the Arctic Ocean. The seasonal bogs lie above sediments deposited in earlier river basins and swamps that have remained permanently frozen since the last glacial period. Such permafrost begins at just a few metres below the surface at high latitudes down to as much as a kilometre, becoming deeper, thinner and more patchy until it disappears south of about 60°N except in mountainous areas. Permafrost is melting relentlessly, sometimes with spectacular results broadly known as thermokarst that involves surface collapse, mudslides and erosion by summer meltwater.

Thawing permafrost in Siberia and associated collapse structures

Permafrost is a good preserver of organic material, as shown by the almost perfect remains of mammoths and other animals that have been found where rivers have eroded their frozen banks. The latest spectacular find is a mummified wolf pup unearthed by a gold prospector from 57 ka-old permafrost in the Yukon, Canada. She was probably buried when a wolf den collapsed. Thawing exposes buried carbonaceous material to processes that release CO2­, as does the drying-out of peat in more temperate climes. It has long been known that the vast reserves of carbon preserved in frozen ground and in gas hydrate in sea-floor sediments present an immense danger of accelerated greenhouse conditions should permafrost thaw quickly and deep seawater heats up; the first is certainly starting to happen in boreal North America and Eurasia. Research into Arctic soils had suggested that there is a potential mitigating factor. Iron-3 oxides and hydroxides, the colorants of soils that overlie permafrost, have chemical properties that allow them to trap carbon, in much the same way that they trap arsenic by adsorption on the surface of their molecular structure (see: Screening for arsenic contamination, September 2008).

But, as in the case of arsenic, mineralogical trapping of carbon and its protection from oxidation to CO2 can be thwarted by bacterial action (Patzner, M.S. and 10 others 2020. Iron mineral dissolution releases iron and associated organic carbon during permafrost thaw. Nature Communications, v. 11, article 6329; DOI: 10.1038/s41467-020-20102-6). Monique Patzner of the University of Tuebingen, Germany, and her colleagues from Germany, Denmark, the UK and the US have studied peaty soils overlying permafrost in Sweden that occurs north of the Arctic Circle. Their mineralogical and biological findings came from cores driven through the different layers above deep permafrost. In the layer immediately above permanently frozen ground the binding of carbon to iron-3 minerals certainly does occur. However, at higher levels that show evidence of longer periods of thawing there is an increase of reduced iron-2 dissolved in the soil water along with more dissolved organic carbon – i.e. carbon prone to oxidation to carbon dioxide. Also, biogenic methane – a more powerful greenhouse gas – increases in the more waterlogged upper sediments. Among the active bacteria are varieties whose metabolism involves the reduction of insoluble iron in ferric oxyhdroxide minerals to the soluble ferrous form (iron-2). As in the case of arsenic contamination of groundwater, the adsorbed contents of iron oxyhydroxides are being released as a result of powerful reducing conditions.

Applying their results to the entire permafrost inventory at high northern latitudes, the team predicts a worrying scenario. Initial thawing can indeed lock-in up to tens of billion tonnes of carbon once preserved in permafrost, yet this amounts to only a fifth of the carbon present in the surface-to-permafrost layer of thawing, at best. In itself, the trapped carbon is equivalent to between 2 to 5 times the annual anthropogenic release of carbon by burning fossil fuels. Nevertheless, it is destined by reductive dissolution of its host minerals to be emitted eventually, if thawing continues. This adds to the even vaster potential releases of greenhouse gases in the form of biogenic methane from waterlogged ground. However, there is some evidence to the contrary. During the deglaciation between 15 to 8 thousand years ago – except for the thousand years of the Younger Dryas cold episode – land-surface temperatures rose far more rapidly than happening at present. A study of carbon isotopes in air trapped as bubbles in Antarctic ice suggests that methane emissions from organic carbon exposed to bacterial action by thawing permafrost were much lower than claimed by Patzner et al. for present-day, slower thawing (see: Old carbon reservoirs unlikely to cause massive greenhouse gas release, study finds. Science Daily, 20 February 2020) – as were those released by breakdown of submarine gas hydrates.

Origin of life: some news

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For self-replicating cells to form there are two essential precursors: water and simple compounds based on the elements carbon, hydrogen, oxygen and nitrogen (CHON). Hydrogen is not a problem, being by far the most abundant element in the universe. Carbon, oxygen and nitrogen form in the cores of stars through nuclear fusion of hydrogen and helium. These elemental building blocks need to be delivered through supernova explosions, ultimately to where water can exist in liquid form to undergo reactions that culminate in living cells. That is only possible on solid bodies that lie at just the right distance from a star to support average surface temperatures that are between the freezing and boiling points of water. Most important is that such a planet in the ‘Goldilocks Zone’ has sufficient mass for its gravity to retain water. Surface water evaporates to some extent to contribute vapour to the atmosphere. Exposed to ultraviolet radiation H2O vapour dissociates into molecular hydrogen and water, which can be lost to space if a planet’s escape velocity is less than the thermal vibration of such gas molecules. Such photo-dissociation and diffusion into outer space may have caused Mars to lose more hydrogen in this way than oxygen, to leave its surface dry but rich in reddish iron oxides.

Despite liquid water being essential for the origin of planetary life it is a mixed blessing for key molecules that support biology. This ‘water paradox’ stems from water molecules attacking and breaking the chemical connections that string together the complex chains of proteins and nucleic acids (RNA and DNA). Living cells resolve the paradox by limiting the circulation of liquid water within them by being largely filled with a gel that holds the key molecules together, rather than being bags of water as has been commonly imagined. That notion stemmed from the idea of a ‘primordial soup’, popularised by Darwin and his early followers, which is now preserved in cells’ cytoplasm. That is now known to be wrong and, in any case, the chemistry simply would not work, either in a ‘warm, little pond’ or close to a deep sea hydrothermal vent, because the molecular chains would be broken as soon as they formed. Modern evolutionary biochemists suggest that much of the chemistry leading to living cells must have taken place in environments that were sometimes dry and sometimes wet; ephemeral puddles on land. Science journalist Michael Marshall has just published an easily read, open-source essay on this vexing yet vital issue in Nature (Marshall, M. 2020. The Water Paradox and the Origins of Life. Nature, v. 588, p. 210-213; DOI: 10.1038/d41586-020-03461-4). If you are interested, click on the link to read Marshall’s account of current origins-of-life research into the role of endlessly repeated wet-dry cycles on the early Earth’s surface. Fascinating reading as the experiments take the matter far beyond the spontaneous formation of the amino acid glycine found by Stanley Miller when he passed sparks through methane, ammonia and hydrogen in his famous 1953 experiment at the University of Chicago. Marshall was spurred to write in advance of NASA’s Perseverance Mission landing on Mars in February 2021. The Perseverance rover aims to test the new hypotheses in a series of lake sediments that appear to have been deposited by wet-dry cycles  in a small Martian impact crater (Jezero Crater) early in the planet’s history when surface water was present.

Crystals of hexamethylenetetramine (Credit: r/chemistry, Reddit)

That CHON and simple compounds made from them are aplenty in interstellar gas and dust clouds has been known since the development of means of analysing the light spectra from them. The organic chemistry of carbonaceous meteorites is also well known; they even smell of hydrocarbons. Accretion of these primitive materials during planet formation is fine as far as providing feedstock for life-forming processes on physically suitable planets. But how did CHON get from giant molecular clouds into such planetesimals. An odd-sounding organic compound – hexamethylenetetramine ((CH2)6N4), or HMT – formed industrially by combining formaldehyde (CH2O) and ammonia (NH3) – was initially synthesised in the late 19th century as an antiseptic to tackle UTIs and is now used as a solid fuel for lightweight camping stoves, as well as much else besides. HMT has a potentially interesting role to play in the origin of life.  Experiments aimed at investigating what happens when starlight and thermal radiation pervade interstellar gas clouds to interact with simple CHON molecules, such as ammonia, formaldehyde, methanol and water, yielded up to 60% by mass of HMT.

The structure of HMT is a sort of cage, so that crystals form large fluffy aggregates, instead of the gases from which it can be formed in deep space. Together with interstellar silicate dusts, such sail-like structures could accrete into planetesimals in nebular star nurseries under the influence of  gravity and light pressure. Geochemists from several Japanese institutions and NASA have, for the first time, found HMT in three carbonaceous chondrites, albeit at very low concentrations – parts per billion (Y. Oba et al. 2020. Extraterrestrial hexamethylenetetramine in meteorites — a precursor of prebiotic chemistry in the inner Solar System. Nature Communications, v. 11, article 6243; DOI: 10.1038/s41467-020-20038-x). Once concentrated in planetesimals – the parents of meteorites when they are smashed by collisions – HMT can perform the useful chemical ‘trick’ of breaking down once again to very simple CHON compounds when warmed. At close quarters such organic precursors can engage in polymerising reactions whose end products could be the far more complex sugars and amino acid chains that are the characteristic CHON compounds of carbonaceous chondrites. Yasuhiro Oba and colleagues may have found the missing link between interstellar space, planet formation and the synthesis of life through the mechanisms that resolve the ‘water paradox’ outlined by Michael Marshall.

See also: Scientists Find Precursor of Prebiotic Chemistry in Three Meteorites (Sci-news, 8 December 2020.)

 

How like the Neanderthals are we?

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An actor made-up to resemble a Neanderthal man in a business suit traveling on the London Underground. (Source: screen-grab from BBC2 Neanderthals – Meet Your Ancestors)

In the most basic, genetic sense, we were sufficiently alike for us to have interbred with them regularly and possibly wherever the two human groups met. As a result the genomes of all modern humans contain snips derived from Neanderthals (see: Everyone now has their Inner Neanderthal; February 2020). East Asian people also carry some Denisovan genes as do the original people of Australasia and the first Americans. Those very facts suggest that members of each group did not find individuals from others especially repellent as potential sexual partners! But that covers only a tiny part of what constitutes culture. There is archaeological evidence that Neanderthals and modern humans made similar tools. Both had the skills to make bi-faced ‘hand axes’ before they even met around 45 to 40 ka ago.  A cave (La Grotte des Fées) near Châtelperron to the west of the French Alps that was occupied by Neanderthals until about 40 ka yielded a selection of stone tools, including blades, known as the Châtelperronian culture, which indicates a major breakthrough in technology by their makers. It is sufficiently similar to the stone industry of anatomically modern humans (AMH) who, around that time, first migrated into Europe from the east (Aurignacian) to pose a conundrum: Did the Neanderthals copy Aurignacian techniques when they met AMH, or vice versa? Making blades by splitting large flint cores is achieved by striking the cores with just a couple of blows with a softer tool. At the very least Neanderthals had the intellectual capacity to learn this very difficult skill, but they may have invented it (see: Disputes in the cavern; June 2012). Then there is growing evidence for artistic abilities among Neanderthals, and even Homo erectus gets a look-in (see: Sophisticated Neanderthal art now established; February 2018).

Reconstructed burial of a Neanderthal individual at La Chappelle-aux-Saints (Credit: Musée de La Chapelle-aux-Saints, Corrèze, France)

For a long time, a pervasive aspect of AMH culture has been ritual. Indeed much early art may be have been bound up with ritualistic social practices, as it has been in historic times. A persuasive hint at Neanderthal ritual lies in the peculiar structures – dated at 177 ka – found far from the light of day in the Bruniquel Cave in south-western France (see: Breaking news: Cave structures made by Neanderthals; May 2016). They comprise circles fashioned from broken-off stalactites, and fires seem to have been lit in them. The most enduring rituals among anatomically modern humans have been those surrounding death: we bury our dead, thereby preserving them, in a variety of ways and ‘send them off’ with grave goods or even by burning them and putting the ashes in a pot. A Neanderthal skeleton (dated at 50 ka) found in a cave at La Chappelle-aux-Saints appears to have been buried and made safe from scavengers and erosion. There are even older Neanderthal graves (90 to 100 ka) at Quafzeh in Palestine and Shanidar in Iraq, where numerous individuals, including a mother and child, had been interred. Some are associated with possible grave goods, such as pieces of red ochre (hematite) pigment, animal body parts and even pollen that suggests flowers had been scattered on the remains. The possibility of deliberate offerings or tributes and even the notion of burial have met with scepticism among some palaeoanthropologists. One reason for the scientific caution is that many of the finds were excavated long before the rigour of modern archaeological protocols

Recently a multidisciplinary team involving scientists from France, Belgium, Italy, Germany, Spain and Denmark exhaustively analysed the context and remains of a Neanderthal child found in the La Ferrassie cave (Dordogne region of France) in the early 1970s  (Balzeau, A. and 13 others 2020. Pluridisciplinary evidence for burial for the La Ferrassie 8 Neandertal child. Scientific Reports, v. 10, article 21230; DOI: 10.1038/s41598-020-77611-z). Estimated to have been about 2 years old, the child is anatomically complete. Bones of other animals found in the same deposit were less-well preserved than those of the child, adding weight to the hypothesis that a body, rather than bones, had been buried soon after death. Luminescence dating of the sediments enveloping the skeleton is considerably older than the radiocarbon age of one of the child’s bones. That is difficult to explain other than by deliberate burial. It is almost certain that a pit had been dug and the child placed in it, to be covered in sediment. The skeleton was oriented E-W, with the head towards the east. Remarkably, other Neanderthal remains at the La Ferrassie site also have heads to the east of the rest of their bones, suggesting perhaps a common practice of orientation relative to sunrise and sunset.

It is slowly dawning on palaeoanthropologists that Neanderthal culture and cognitive capacity were not greatly different from those of anatomically modern humans. That similar beings to ourselves disappeared from the archaeological record within a few thousand years of the first appearance of AMH in Europe has long been attributed to what can be summarised as the Neanderthals being ‘second best’ in many ways. That may not have been the case. Since the last glaciation something similar has happened twice in Europe, which analysis of ancient DNA has documented in far more detail than the disappearance of the Neanderthals. Mesolithic hunter-gatherers were followed by early Neolithic farmers with genetic affinities to living people in Northern Anatolia in Turkey – the region where growing crops began. The DNA record from human remains with Neolithic ages shows no sign of genomes with a clear Mesolithic signature, yet some of the genetic features of these hunter-gatherers still remain in the genomes of modern Europeans. Similarly, ancient DNA recovered from Bronze Age human bones suggests almost complete replacement of the Neolithic inhabitants by people who introduced metallurgy, a horse-centred culture and a new kind of ceramic – the Bell Beaker. This genetic group is known as the Yamnaya, whose origins lie in the steppe of modern Ukraine and European Russia. In this Neolithic-Bronze Age population transition the earlier genomes disappear from the ancient DNA record. Yet Europeans still carry traces of that earlier genetic heritage. The explanation now accepted by both geneticists and archaeologists is that both events involved assimilation and merging through interbreeding. That seems just as applicable to the ‘disappearance’ of the Neanderthals

See also: Neanderthals buried their dead: New evidence (Science Daily, 9 December 2020)

Doggerland and the Storegga tsunami

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Britain is only an island when sea level stands high; i.e. during interglacial conditions. Since the last ice age global sea level have risen by about 130 m as the great northern ice sheets slowly melted. That Britain could oscillate between being part of Europe and a large archipelago as a result of major climatic cycles dates back only to between 450 and 240 ka ago. Previously it was a permanent part of what is now Europe, as befits its geological identity, joined to it by a low ridge buttressed by Chalk across the Dover Strait/Pas de Calais. All that remains of that are the white cliffs on either side. The drainage of what became the Thames, Seine and Rhine passed to the Atlantic in a much larger rive system that flowed down the axis of the Channel. Each time an ice age ended the ridge acted as a dam for glacial meltwater to form a large lake in what is now the southern North Sea. While continuous glaciers across the northern North Sea persisted the lake remained, but erosion during interglacials steadily wore down the ridge. About 450 ka ago it was low enough for this pro-glacial lake to spill across it in a catastrophic flood that began the separation. Several repeats occurred until the ridge was finally breached (See: When Britain first left Europe; September 2007). Yet sufficient remained that the link reappeared when sea level fell. What remains at present is a system of shallows and sandbanks, the largest of which is the Dogger Bank roughly halfway between Newcastle and Denmark. Consequently the swamps and river systems that immediately followed the last ice age have become known collectively as Doggerland.

The shrinkage of Doggerland since 16,000 BCE (Credit: Europe’s Lost Frontiers Project, University of Bradford)

Dredging of the southern North Sea for sand and gravel frequently brings both the bones of land mammals and the tools of Stone Age hunters to light – one fossil was a skull fragment of a Neanderthal. At the end of the Younger Dryas (~11.7 ka) Doggerland was populated and became a route for Mesolithic hunter-gatherers to cross from Europe to Britain and become transient and then permanent inhabitants. Melting of the northern ice sheets was slow and so was the pace of sea-level rise. A continuous passage across Dogger Land  remained even as it shrank. Only when the sea surface reached about 20 m below its current level was the land corridor breached bay what is now the Dover Strait, although low islands, including the Dogger Bank, littered the growing seaway. A new study examines the fate of Doggerland and its people during its final stage (Walker, J. et al. 2020. A great wave: the Storegga tsunami and the end of Doggerland? Antiquity, v. 94, p. 1409-1425; DOI: 10.15184/aqy.2020.49).

James Walker and colleagues at the University of Bradford, UK, and co-workers from the universities of Tartu, Estonia, Wales Trinity Saint David and St Andrews, UK, focus on one devastating event during Doggerland’s slow shrinkage and inundation. This took place around 8.2 ka ago, during the collapse of a section of the Norwegian continental edge. Known as the Storegga Slides (storegga means great edge in Norse), three submarine debris flows shifted 3500 km3 of sediment to blanket 80 thousand km2 of the Norwegian Sea floor, reaching more than half way to Iceland.  Tsunami deposits related to these events occur along the coast western Norway, on the Shetlands and the shoreline of eastern Scotland. They lie between 3 and 20 m above modern sea level, but allowing for the lower sea level at the time the ‘run-up’ probably reached as high as 35 m: more than the maximum of both the 26 December 2004 Indian Ocean tsunami and that in NW Japan on 11 March 2011. Two Mesolithic archaeological sites definitely lie beneath the tsunami deposit, one close to the source of the slid, another near Inverness, Scotland. At the time part of the Dogger Bank still lay above the sea, as did a wide coastal plain and offshore islands along England’s east coast. This catastrophic event was a little later than a sudden cooling event in the Northern Hemisphere. Any Mesolithic people living on what was left of Doggerland would not have survived. But quite possibly they may already have left as the climate cooled substantially

A seabed drilling programme financed by the EU targeted what lies beneath more recent sediments on the Dogger Bank and off the embayment known as The Wash of Eastern England. Some of the cores contain tsunamis deposits, one having been analysed in detail in a separate paper (Gaffney, V. and 24 others 2020. Multi-Proxy Characterisation of the Storegga Tsunami and Its Impact on the Early Holocene Landscapes of the Southern North Sea. Geosciences, v. 10, online; DOI: 10.3390/geosciences10070270). The tsunami washed across an estuarine mudflat into an area of meadowland with oak and hazel woodland, which may have absorbed much of its energy. Environmental DNA analysis suggests that this relic of Doggerland was roamed by bear, wild boar and ruminants. The authors also found evidence that the tsunamis had been guided by pre-existing topography, such as the river channel of what is now the River Great Ouse. Yet they found no evidence of human occupation. Together with other researchers, the University of Bradford’s Lost Frontiers Project have produced sufficient detail about Doggerland to contemplate looking for Mesolithic sites in the excavations for offshore wind farms.

See also: Addley, E. 2020.  Study finds indications of life on Doggerland after devastating tsunamis. (The Guardian, 1 December 2020); Europe’s Lost Frontiers website