Relationships between modern humans and Neanderthals

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 ancestryNature, 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?

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

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

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?

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

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

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

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?

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 COand 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

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 shelfNature, 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