Human impact on surface geological processes

I last wrote about sedimentation during the ‘Anthropocene’ a year ago (See: Sedimentary deposits of the ‘Anthropocene’, November 2019). Human impact in that context is staggeringly huge: annually we shift 57 billion tonnes of rock and soil, equivalent to six times the mass of the UKs largest mountain, Ben Nevis. All the world’s rivers combined move about 35 billion tonnes less. I don’t particularly care for erecting a new Epoch in the Stratigraphic Column, and even less about when the ‘Anthropocene’ is supposed to have started. The proposal continues to be debated 12 years after it was first suggested to the IUGS International Commission on Stratigraphy. I suppose I am a bit ‘old fashioned’, but the proposals is for a stratigraphic entity that is vastly shorter than the smallest globally significant subdivision of geological time (an Age) and the duration of most of the recorded mass extinctions, which are signified by horizontal lines in the Column. By way of illustration, the thick, extensive bed of Carboniferous sandstone on which I live is one of many deposited in the early part of the Namurian Age (between 328 and 318 Ma). Nonetheless, anthropogenic sediments of, say, the last 200 years are definitely substantial. A measure of just how substantial is provided by a paper published online this week (Kemp, S.B. et al. 2020. The human impact on North American erosion, sediment transfer, and storage in a geologic context. Nature Communications, v. 11, article 6012; DOI: 10.1038/s41467-020-19744-3).

‘Badlands’ formed by accelerated soil erosion.

Anthropogenic erosion, sediment transfer and deposition in North America kicked off with its colonisation by European immigrants since the early 16th century. First Americans were hunter-gatherers and subsistence farmers and left virtually no traces in the landscape, other than their artefacts and, in the case of farmers, their dwellings. Kemp and colleagues have focussed on late-Pleistocene alluvial sediment, accumulation of which seems to have been pretty stable for 40 ka. Since colonisation began the rate has increased to, at present, ten times that previously stable rate, mainly during the last 200 years of accelerated spread of farmland. This is dominated by outcomes of two agricultural practices – ploughing and deforestation. Breaking of the complex and ancient prairie soils, formerly held together by deep, dense mats of grass root systems, made even flat surfaces highly prone to soil erosion, demonstrated by the ‘dust bowl’ conditions of the Great Depression during the 1930s. In more rugged relief, deforestation made slopes more likely to fail through landslides and other mass movements. Damming of streams and rivers for irrigation or, its opposite, to drain wetlands resulted in alterations to the channels themselves and their flow regimes. Consequently, older alluvium succumbed to bank erosion. Increased deposition behind an explosion of mill dams and changed flow regimes in the reaches of streams below them had effects disproportionate to the size of the dams (see: Watermills and meanders, March 2008). Stream flow beforehand was slower and flooding more balanced than it has been over the last few hundred years. Increased flooding, the building of ever larger flood defences and an increase in flood magnitude, duration and extent when defences were breached form a vicious circle that quickly transformed the lower reaches of the largest American river basins.

North American rates of alluvium deposition since 40 Ka ago – the time axis is logarithmic. (Credit: Kemp et al., 2020; Fig. 2)

All this deserves documentation and quantification, which Kemp et al. have attempted at 400 alluvial study sites across the continent, measuring >4700 rates of sediment accumulation at various times during the past 40 thousand years. Such deposition serves roughly as a proxy for erosion rate, but that is a function of multiple factors, such as run-off of rain- and snow-melt water, anthropogenic changes to drainage courses and to slope stability. The scale of post-settlement sedimentation is not the same across the whole continent. In some areas, such as southern California, the rate over the last 200 years is lower than the estimated natural, pre-settlement rate: this example may be due to increased capture of surface water for irrigation of a semi-arid area so that erosion and transport were retarded. In others it seems to be unchanged, probably for a whole variety of reason. The highest rates are in the main areas of rain-fed agriculture of the mid-west of the US and western Canada.

In a nutshell, during the last century the North American capitalism shifted as much sediment as would be moved naturally in between 700 to 3000 years. No such investigation has been attempted in other parts of the world that have histories of intense agriculture going back several thousand years, such as the plains of China, northern India and Mesopotamia, the lower Nile valley, the great plateau of the Ethiopian Highlands, and Europe. This is a global problem and despite its continent-wide scope the study by Kemp et al. barely scratches the surface. Despite earnest endeavours to reduce soil erosion in the US and a few other areas, it does seem as if the damage has been done and is irreversible.

Earliest sign of a sense of aesthetics

Maybe because of the Covid-19 pandemic, there has been a dearth of interesting new developments in the geosciences over that last few months: the ‘bread and butter’ of Earth-logs. So instead of allowing a gap in articles to develop, and as a sign that I haven’t succumbed, this piece concerns one of the most intriguing discoveries in palaeoanthropology. In 1925 Wilfred Eitzman, a school teacher, investigated a cave in the Makapansgat Valley in Limpopo Province, South Africa that had been exposed by quarry workers.  His most striking discovery was a polished pebble made of very fine-grained, iron-rich silica, probably from a Precambrian banded iron formation. Being round and deeply pitted, it had clearly been subject to prolonged rolling and sand blasting in running water and wind. Eerily, whichever way it was viewed it bore a striking resemblance to a primate face: eyes, mouth, nose and, viewed from the rear, a disturbing, toothless grin. We have all picked up odd-looking pebbles on beaches or a river bank: I recently found a sandstone demon-cat (it even has pointy ears) when digging a new vegetable patch.

The Makapansgat Pebble. Inverted it still resembles a face and its obverse side does too.

What is different about the Makapansgat Pebble is that Eitzman found it in a cave-floor layer full of bones, including those of australopithecines. The cave is located in dolomitic limestone outcrops high in the local drainage system, so it’s unlikely that the pebble was washed into it. The nearest occurrence of banded iron formation is about 20 kilometres away, so something must have carried the pebble for a day or more to the cave. The local area has since yielded a superb palaeontological record of early hominin evolution, stimulated by  Eitzman’s finds. He gave the fossils and the pebble to Raymond Dart, the pioneer of South African palaeoanthropology. Dart named the hominin fossils Australopithecus prometheus because associated bones of other animals were covered in black stains that Dart eagerly regarded as signs of burning and thus cooking. When it became clear that the stains were of manganese oxide the name was changed to Au. africanus, the fossils eventually being dated to around 3 million years ago.

Dart was notorious for his showmanship, and the fossils and the Makapansgat Pebble ‘did the rounds’ and continue to do so. In 2016 the pebble was displayed with a golden rhino, a collection of apartheid-era badges and much more in the British Museum’s South Africa: the art of a nation exhibition. Well, is the pebble art? As it shows no evidence of deliberate working it can not be considered art, but could be termed an objet trouvé. That is, an ‘object found by chance and held to have aesthetic value to an artist’. The pebble’s original finder 3 million years ago must have found the 0.25 kg pebble sufficiently interesting to have carried it back to the cave, presumably because of its clear resemblance to a hominin head: in fact a multiple-faced head. Was it carried by a cave-dwelling australopithecine or an early member of genus Homo who left no other trace at Makapansgat? At an even earlier time a so-far undiscovered hominin did indeed make simple stone tools to dismember joints of meat on the shores of Lake Turkana in Kenya. It is impossible to know who for sure carried the pebble, nor to know why. Yet all living primates are curious creatures, so it is far from impossible that any member of the hominins in our line of descent would have collected portable curiosities.

Kerguelen Plateau: a long-lived large igneous province

It’s easy to think of the Earth’s largest outpourings of lava as being restricted to the continents; continental flood basalts with their spectacular stepped topography made up of hundreds of individual massive flows and intervening soil horizons. The Deccan Traps of western India are the epitome, having been so named by natural scientists of the late 18th century from the Swedish word for ‘stairs’ (trappa). Examples go back to the Proterozoic Era, younger ones still retaining much of their original form as huge plateaus. All began life within individual tectonic plates, although some presaged continental break-up and the formation of new oceanic spreading centres. They must have been spectacular events, up to millions of cubic kilometres of magma belched out in a few million years. They have been explained as manifestations of plumes of hot mantle rock rising from as deep as the core-mantle boundary. Unsurprisingly, the biggest continental flood-basalt outpourings coincided with mass extinction events. Otherwise known as large igneous provinces (LIPs), they are not the only signs of truly huge production of magma by partial melting in the mantle. The biggest LIP, with an estimated volume of 80 million km3, lies deep beneath the Western Pacific Ocean. To the northeast of New Guinea, the Ontong Java Plateau formed over a period of about 3 Ma in the mid-Cretaceous (~120 Ma) and blanketed one percent of the Earth’s solid surface with lavas erupted at a rate of 22 km3 per year. Possibly because this happened on the Pacific’s abyssal plains beneath around 4 km of sea water, there is little sign of any major perturbation of mid-Cretaceous life, but it is associated with evidence for global oceanic anoxia. Ontong Java isn’t the only oceanic LIP. Bearing in mind that oceanic lithosphere only goes back to the start of the Jurassic Period (200 Ma) – earlier material has largely been subducted – they are not as abundant as continental flood-basalt provinces. One of them is the Kerguelen Plateau 3000 km to the SE of Australia, which is about three times the area of Japan and the second largest LIP of the Phanerozoic Eon. The Plateau was split into two large fragments while sea-floor spreading progressed along the Southeast Indian Ridge.

Bathymetry of the Indian Ocean south-west of Australia, showing the Kerguelen Plateau and South-east Indian Ridge. The red arrows show the amount of sea-floor spreading on either side of the Ridge since it began to open. The pale blue area at the NE end of the arrow was formerly part of the Plateau (credit: Google Earth)

Long regarded as a microcontinental  fragment left when India parted company with Antarctica – based on isolated occurrences of gneisses – there is evidence that during the formation of the Kerguelen LIP the basalts rose above sea level. Because earlier radiometric dating of basalts from ocean-floor drill cores were of low quality, an Australian-Swedish group of geoscientists have re-evaluated those data and supplemented them with 25 new Ar-Ar dates from 12 sites (Jiang, Q. et al. 2020. Longest continuously erupting large igneous province driven by plume-ridge interaction. Geology, v. 48, online; DOI: 10.1130/G47850.1). Rather than a cluster of ages around a short time range as expected from the short life of most other LIPs, those from Kerguelen span 32 Ma during the Cretaceous (from 122 to 90 Ma). The magmatic pulse began at roughly the same time as that of Ontong Java, but continued for much longer. Smaller oceanic LIPs do seem to have lingered for unusually lengthy periods, but all seem to have constructed in several separate pulses. Large-volume eruption at Kerguelen was continuous for at least 32 Ma; the drilling did not penetrate the oldest of the plateau basalts. It seems that the Kerguelen LIP is unique in that respect and requires an explanation other than simply a mantle plume, however large.

Jiang et al. suggest a model of continuous interaction between a long-lived plume and the development of the Southeast Indian Ridge oceanic spreading centre. Their model involves the line of continental splitting between India and Antarctic taking place close to a major deep-mantle plume at around 128 Ma. There is nothing unique about that; incipient ocean rifting in the Horn of Africa and formation of the Red Sea and Gulf of Aden ridges is currently associated with the active Afar plume. This was followed by a kind of tectonic shuffling of the Ridge back and forth across the head of the Kerguelen plume: not far different from the Palaeogene North Atlantic LIP, where the mid-Atlantic Ridge and the still-active Iceland plume, except the ridge and plume seem more intimately involved there. However, there are probably many subtle relationships between plumes and various kind of oceanic plate margins that are still worth exploring. Since the first discovery of mantle plumes as an explanation for volcanic island chains (e.g. the Hawaiian chain) where volcanism becomes progressively older in the direction of plate movement, there is still much to discover.

See also: Magma .conveyor belt’ fuelled world’s longest erupting supervolcanoes (Science Daily, 4 November 2020)

More Denisovan connections

In 2006 mining operations in NE Mongolia uncovered a human skull cap with prominent brow ridges. After having been dubbed Mongolanthropus because of its primitive appearance and then suggested to be either a Neanderthal of Homo erectus. Radiocarbon dating in 2019 then showed the woman to be around 34,500 years old and the accompanying sequencing of its mtDNA assigned her to a widespread Eurasian haplotype of modern humans. Powdered bone samples ended up in Svante Paabo’s renowned ancient-DNA lab at the Max Planck Institute for Evolutionary Anthropology in Leipzig and yielded a full genome (Massilani, D. and 14 others 2020. Denisovan ancestry and population history of early East Asians. Science, v. 370, p. 579-583; DOI: 10.1126/science.abc1166). From this flowed some interesting genetic history.

Skull cap of a female modern human from Salkhit in Outer Mongolia, which superficially resembles those of Homo erectus from Java (Credit: Massilani et al. Fig 1a; © Institute of Archaeology, Mongolian Academy of Sciences)

First was a close overall resemblance to living East Asians and Native Americans, similar to that of an older individual from near Beijing, China. This confirmed the antiquity of the East Eurasian population’s split from that of the west, yet contained evidence of some interbreeding with West Eurasians to the extent of sharing 25% of DNA and with Neanderthals. The two specimens also contained evidence of Denisovan ancestry in their genomes, but fragments that are more akin to those in living people in East Asia than to those of Papuans and Aboriginal Australians: these were definitely cosmopolitan people! The simplest explanation is two distinct minglings with Denisovans: that involving ancestors of Papuans and Australian being the perhaps earlier, en route to their arrival at least 60 thousand years on what became an island continent in the run-up to the last glacial maximum. Be that as it may, two separate Denisovan populations interbred with modern human bands. Further genetic connections with ancient Northern Siberian humans suggests complex movement across the continent, probably inevitable because these hunter-gatherers would have followed prey animals on their seasonal migrations, which would have been longer than today because of climatic cooling. The same can be surmised for Denisovans which would have increased the chances of contact

See also: Denisovan DNA in the genome of early East Asians (Science Daily, 29 October 2020)

In May 2019 (Denisovan on top of the world) I wrote about a human lower jaw that a Buddhist monk had found in a cave at a height of 3.3 km on the Tibetan plateau. Analysis of protein traces in the teeth it retained suggested that it was Denisovan. Like the earlier small remnants from Siberia, dating this putative Denisovan precisely proved to be impossible. The jawbone was at least 160 ka old from the age of speleothem carbonate encrusting it. Excavation of the sediment layers from Baishiya Cave has enabled a large team of Chinese, Australian, US and Swedish scientists to try out the ‘environmental DNA’ approach pioneered by the Max Planck Institute for Evolutionary Anthropology (see: Detecting the presence of hominins in ancient soil samples, April 2017). The cave confirmed occupation by Denisovans from mtDNA found in layers dated using radiocarbon and optically stimulated luminescence methods. Denisovan mtDNA turned up in four layers dated at ~100, ~60 and possibly as young as 45 ka, as well at that from a variety of other mammals (Zhang, D. and 26 others. Denisovan DNA in Late Pleistocene sediments from Baishiya Karst Cave on the Tibetan Plateau. Science, v. 370, p. 584-587; DOI: 10.1126/science.abb6320).  Denisovans were clearly able to live at high elevations for at least 100 thousand years: long enough to evolve the metabolic processes essential to sustain living in low-oxygen conditions, which it has been suggested was passed on to ancestral modern Tibetans.

Up-to-date review of animals before the Cambrian ‘Explosion’

Artist’s impression of the Ediacaran Fauna (credit: Science)

Since I began this blog in 2000 one of my most regular topics concerns the animals of the latest Precambrian: the Ediacaran fauna. If you want to browse through the items use ‘Ediacaran’ in the Search Earth-logs box. New material and ideas about those precursors to modern life forms (and some that are still puzzling) appear on a regular basis. Science journalist Traci Watson has just summarised the latest developments in an essay for Nature. It is a nicely written and copiously illustrated piece with lots of links. Rather than precis her article, I suggest that you go straight to it, if the topic piques your interest.

(Watson, T. 2020. The bizarre species that are rewriting animal evolution. Nature, v. 586, p. 662-665; DOI: 10.1038/d41586-020-02985-z)

Environmental change and early-human innovation

Acheulean biface tools strewn on a bedding surface in the Olorgesailie Basin, Kenya (credit: mmercedes_78 Flickr)

The Olorgesailie Basin in Southern Kenya is possibly the world’s richest source for evidence of ancient stone-tool manufacture. For early humans, it certainly was rich in the necessary resources from which to craft tools. Lying in East Africa’s active rift system, its stratigraphy contains abundant beds of hydrothermal silica (chert), deposited by hot springs, and flows of fine grained lavas. Its sediments spanning the last 1.2 million years show that the Basin hosted lakes and extensive river systems for the earlier part of this period: it was rich in food resources too. The tools, together with bones from dismembered prey, bear witness to long-term human occupation, but hominin remains themselves have yet to be discovered. The time span suggests early occupation by Homo erectus, who probably manufactured Acheulean biface stone tools in large quantities that litter the surface at some archaeological sites.

There is a break in the stratigraphic sequence from about 500 to 320 thousand years ago caused by erosion during a period of tectonic uplift. Younger sediments reveal a striking change in archaeology. The earlier large cutting tools give way to a more diverse ‘toolkit’ of smaller tools produced by more sophisticated techniques than those used to make the Acheulean ‘hand axes’. In African archaeological parlance, the <320 ka-old tools mark the onset of the Middle Stone Age (NB not equivalent to the much younger Mesolithic of Europe). The sedimentary gap also marks what seems to have been very different human behaviour. The stone resources used in the 1.2 to 0.5 Ma sequence were local: no more than 5 km from the tool-yielding sites. After the gap a much more varied range of lithologies was used, from as far afield as 95 km. Not only that, but rock unsuitable for tools appears: soft pigments such as hematite.

The foregoing was known from three major papers that appeared in March 2018 (see: Human evolution and revolution in Africa, March 2018 – specifically the section Hominin cultural revolution 320,000 years ago). Now, many members of the teams who produced that published evidence report detailed analysis of samples from a deep drill core through the stratigraphy in a similar, nearby basin (Potts, R. and 21 others 2020. Increased ecological resource variability during a critical transition in hominin evolutionScience Advances, v. 6, article eabc8975; DOI:10.1126/sciadv.abc8975). As well as calibrating the timing of stratigraphic changes using 40Ar/39Ar dating from 22 volcanic layers, the team analysed sedimentary structures, body- and trace fossils, variations in sediment geochemistry, palaeobotany and carbon isotopes, to suggest variations in environmental conditions and ecology throughout the section in greater detail than previously achieved anywhere in Africa.

They conclude that as well as a change in topography resulting from the 500-320 ka period of tectonic uplift and erosion, the climate of this part of East Africa became more unstable. Combined, these two factors transformed the ecosystems of the Olorgesailie Basin. Between 1.2 to 0.5 Ma the Acheulean tool makers inhabited dominantly grassy plains with substantial, permanent lakes – a stable period of 700 thousand years, well suited to large herbivores and thus to these early humans. Tectonic and climatic change disrupted a ‘land of plenty’; the herbivores left to be replaced by smaller prey animals; vegetation shifted back and forth from grassland to woodland with the unstable climate; lakes became smaller and ephemeral. The problem in linking environmental change to changed human practices in this case, however, is the 180 thousand-year gap in the geological record. Lead author Richard Potts, director of the Human Origins Program at the Smithsonian’s National Museum of Natural History, and his team suggest that the change contributed to the ecological flexibility of the probable Homo sapiens who left the fancier, more diverse tools during the later phase. Yet 1.6 million years beforehand early H. erectus had sufficient flexibility to cross 30 to 40 degrees of latitude and end up on the shores of the Black Sea in Georgia! The likely late-stage H. erectus of Olorgesailie may have moved out around 500 ka ago and sometime later early H. sapiens moved in with new technology developed elsewhere. We know that the earliest known anatomically modern humans lived in Morocco at around 315 ka (see: Origin of anatomically modern humans, June 2017): but we don’t know what tools they had or where they went next. There are all sorts of possibilities that cannot be addressed by even the most intricate analysis of secondary evidence. The important issue seems, I think, to centre on the transition from erects to sapiens, in anatomical, cognitive and behavioural contexts, via some intermediary such as H. antecessor, to which this study can contribute very little. That needs complete stratigraphic records: ironically, the other basin from which the core was drilled is apparently more complete, especially for the 500 to 320 ka ‘gap’. That seems likely to offer more potential. Yet, such big questions also demand a much broader brush: perhaps on a continental scale. It’s to early to tell …

See also: Turbulent era sparked leap in human behavior, adaptability 320,000 years ago (Science Daily,21 October 2020)

How continental keels and cratons may have formed

There is Byzantine ring to the word craton: hardly surprising as it stems from the Greek kratos meaning ‘might’ or ‘strength’. Yes, the ancient cores of the continents were well named, for they are mighty. Some continents, such as Africa, have several of them: probably relics of very ancient supercontinents that have split and spread again and again. Cratons overlie what are almost literally the ‘keels’ of continents. Unlike other mantle lithosphere beneath continental crust (150 km on average) cratonic lithosphere extends down to 350 km and is rigid. Upper mantle rocks at that depth elsewhere are mechanically weaker and constitute the asthenosphere. Geologists only have evidence from the near-surface on which to base ideas of how cratons formed. Their exposed rocks are always Precambrian in age, from 1.5 to 3.5 billion years old, though in some cases they are covered by a thin veneer of later sedimentary rocks that show little sign of deformation. No cratons formed after the Palaeoproterozoic and they are the main repositories of Archaean rock. Their crust is thicker than elsewhere and dominated at the surface by crystalline rocks of roughly granitic composition. Cratons have the lowest amount of heat flowing out from the Earth’s interior; i.e. heat produced by the decay of long-lived radioactive isotopes of uranium, thorium and potassium. This relative coolness provides an explanation for the rigidity of cratons relative to younger continental lithosphere. Because granitic rocks are well-endowed with heat-producing isotopes, the implication of low heat flow is that the deeper parts of the crust are strongly depleted in them. As a result the deep mantle in cratonic keels is at higher pressure and lower temperature than elsewhere beneath the continental surface. Ideal conditions for the formation of diamonds in mantle rock, so that cratonic keels are their main source – they get to the surface in magma pipes when small amounts of partial melting take place in the lithospheric mantle.

The low heat flow through cratons beckons the idea that the heat-producing elements U, Th and K were at some stage driven from depth. An attractive hypothesis is that they were carried in low-density granitic magmas formed by partial melting of mantle lithosphere during the Precambrian that rose to form continental crust. Yet there is an abundance of younger granite plutons that are associated with thinner continental lithosphere. This seeming paradox suggests different kinds of magmagenesis and tectonics during the early Precambrian. Russian and Australian geoscientists have proposed an ingenious explanation (Perchuk, A.L. et al. 2020. Building cratonic keels in Precambrian plate tectonics. Nature, v. 586, p. 395-401; DOI: 10.1038/s41586-020-2806-7). The key to their hypothesis lies in the 2-layered nature of mantle keels beneath cratons, as revealed by seismic studies. Modelling of the data suggests that the layering resulted from different degrees of partial melting in the upper mantle during Precambrian subduction.

Development of a cratonic keel from melt-depleted lithospheric mantle during early Precambrian subduction. Mantle temperature is 250°C higher than it is today. The oceanic lithosphere being subducted in (a) has become a series of stagnant slabs in (b) (credit: Perchuk et al.; Fig. 2)

Perchuk et al. suggest that high degrees of partial melting of mantle associated with subduction zones produced the bulk of magma that formed the Archaean and Palaeoproterozoic crust. This helps explain large differences between the bulk compositions of ancient and more recent continental crust, which involves less melting. The residue left by high degrees of melting of mantle rock in the early Precambrian would have had a lower density than the rest of the mantle. While older oceanic crust at ancient subduction zones would be transformed to a state denser than the mantle as a whole and thus able to sink, this depleted lithospheric mantle would not. In its hot ductile state following partial melting, this mantle would be ‘peeled’ from the associated oceanic crust to be emplaced below. The figure shows one of several outcomes of a complex magmatic-thermomechanical model ‘driven’ by assumed Archaean conditions in the upper mantle and lithosphere An excellent summary of modern ideas on the start of plate tectonics and evolution of the continents is given by:Hawkesworth, C.J., Cawood, P.A. & Dhuime, B. 2020. The evolution of the continental crust and the onset of plate tectonics. In Topic: The early Earth crust and its formation, Frontiers in Earth Sciences; DOI: 10.3389/feart.2020.00326

Balanced boulders and seismic hazard

The seismometer invented by early Chinese engineer Zhang Heng

China has been plagued by natural disasters since the earliest historical writings. Devastating earthquakes have been a particular menace, the first recorded having occurred in 780 BC . During the Han dynasty in 132 CE, polymath Zhang Heng invented an ‘instrument for measuring the seasonal winds and the movements of the Earth’ (Houfeng Didong Yi, for short): the first seismometer. A pendulum mechanism in a large bronze jar activated one of eight dragons corresponding to the eight cardinal and intermediate compass directions (N, NE, E etc.) so that a bronze ball dropped from its mouth to be caught by a corresponding bronze toad. The device took advantage of unstable equilibrium in which a small disturbance will produce a large change: akin to a pencil balanced on its unsharpened end. Modern seismometers exploit the same basic principle of amplification of small motions. The natural world is also full of examples of unstable equilibrium, often the outcome of chemical and physical weathering. Examples are slope instability, materials that are on the brink of changing properties from those of a solid to a liquid state (thixotropic materials – see: Mud, mud, glorious mud August 2020) and rocks in which stress has built almost to the point of brittle failure: earthquakes themselves. But there are natural curiosities that not only express unstable equilibrium but have maintained it long enough to become … curious! Perched boulders, such as glacial erratics and the relics of slow erosion and weathering, are good examples. Seismicity could easily topple them, so that their continued presence signifies that large enough tremors haven’t yet happened.

A precarious boulder in coastal central California (credit: Anna Rood & Dylan Rood, Imperial College London)

Now it has become possible to judge how long their delicate existence has persisted, giving a clue to the long-term seismicity and thus the likely hazard in their vicinity (Rood, A.H. and 10 others 2020. Earthquake Hazard Uncertainties Improved Using Precariously Balanced Rocks. American Geological Union Advances, v. 1, ePDF e2020AV000182; DOI: 10.1029/2020AV000182). Anna Rood and her partner Dylan of Imperial College London, with colleagues from New Zealand, the US and Australia, found seven delicately balanced large boulders of silica-rich sedimentary rock in seismically active, coastal California. They had clearly withstood earthquake ground motions for some time. Using multiple photographs to produce accurate digital 3D renditions and modelling of resistance to shaking and rocking motions, the authors determined each precarious rock’s probable susceptibility to toppling as a result of earthquakes. How long each had withstood tectonic activity shows up from the mass-spectrometric determination of beryllium-10 isotopes produced by cosmic-ray bombardment of the outer layer. Comparing its surface abundance relative to that in the rock’s interior indicates the time since the boulders’ first exposure to cosmic rays. With allowance for former support from surrounding blocks, this gives a useful measure of the survival time of each boulder – its ‘fragility age’.

The boulder data provide a useful means of reducing the uncertainties inherent in conventional seismic hazard assessment, which are based on estimates of the frequency of seismic activity, the magnitude of historic ‘quakes, in most cases over the last few hundred years, and the underlying geology and tectonics. In the study area (near a coastal nuclear power station) the data have narrowed uncertainty down to almost a half that in existing risk models. Moreover, they establish that the highest-magnitude earthquakes to be expected every 10 thousand years (the ‘worst case scenario’) were 27% less than otherwise estimated. This is especially useful for coastal California, where the most threatening faults lie off shore and are less amenable to geological investigation.

See also:  Strange precariously balanced rocks provide earthquake forecasting clues. (SciTech Daily; 1 October 2020) 

Supernova at the start of the Pleistocene

This brief note takes up a thread begun in Can a supernova affect the Earth System? (August 2020). In February 2020 the brightness of Betelgeuse – the prominent red star at the top-left of the constellation Orion – dropped in a dramatic fashion. This led to media speculation that it was about to ‘go supernova’, but with the rise of COVID-19 beginning then, that seemed the least of our worries. In fact, astronomers already knew that the red star had dimmed many times before, on a roughly 6.4-year time scale. Betelgeuse is a variable star and by March 2020 it brightened once again: shock-horror over; back to the latter-day plague.

When stars more than ten-times the mass of the Sun run out of fuel for the nuclear fusion energy that keeps them ‘inflated’ they collapse. The vast amount of gravitational potential energy released by the collapse triggers a supernova and is sufficient to form all manner of exotic heavy isotopes by nucleosynthesis. Such an event radiates highly energetic and damaging gamma radiation, and flings off dust charged with a soup of exotic isotopes at very high speeds. The energy released could sum to the entire amount of light that our Sun has shone since it formed 4.6 billion years ago. If close enough, the dual ‘blast’ could have severe effects on Earth, and has been suggested to have caused the mass extinction at the end of the Ordovician Period.

Betelgeuse is about 700 light years away, massive enough to become a future supernova and its rapid consumption of nuclear fuel – it is only about 10 million years old – suggests it will do so within the next hundred thousand years. Nobody knows how close such an event needs to be to wreak havoc on the Earth system, so it is as well to check if there is evidence for such linked perturbations in the geological record. The isotope 60Fe occurs in manganese-rich crusts and nodules on the floor of the Pacific Ocean and also in some rocks from the Moon. It is radioactive with a half-life of about 2.6 million years, so it soon decays away and cannot have been a part of Earth’s original geochemistry or that of the Moon. Its presence may suggest accretion of debris from supernovas in the geologically recent past: possibly 20 in the last 10 Ma but with apparently no obvious extinctions. Yet that isotope of iron may also be produced by less-spectacular stellar processes, so may not be a useful guide.

There is, however, another short-lived radioactive isotope, of manganese (53Mn), which can only form under supernova conditions. It has been found in ocean-floor manganese-rich crusts by a German-Argentinian team of physicists  (Korschinek, G. et al. 2020. Supernova-produced 53Mn on Earth. Physical Review Letters, v. 125, article 031101; DOI: 10.1103/PhysRevLett.125.031101). They dated the crusts using another short-lived cosmogenic isotope produced when cosmic rays transform the atomic nuclei of oxygen and nitrogen to 10Be that ended up in the manganese-rich crusts along with any supernova-produced  53Mn and 60Fe. These were detected in parts of four crusts widely separated on the Pacific Ocean floor. The relative proportions of the two isotopes matched that predicted for nucleosynthesis in supernovas, so the team considers their joint presence to be a ‘smoking gun’ for such an event.

The 10Be in the supernova-affected parts of the crusts yielded an age of 2.58 ± 0.43 million years, which marks the start of the Pleistocene Epoch, the onset of glacial cycles in the Northern Hemisphere and the time of the earliest known members of the genus Homo. A remarkable coincidence? Possibly. Yet cosmic rays, many of which come from supernova relics, have been cited as a significant source of nucleation sites for cloud condensation. Clouds increase the planet’s reflectivity and thus act to to cool it. This has been a contentious issue in the debate about modern climate change, some refuting their significance on the basis of a lack of correlation between cloud-cover data and changes in the flux of cosmic rays over the last century. Yet, over the five millennia of recorded history there have been no records of supernovas with a magnitude that would suggest they were able to bathe the night sky in light akin to that of daytime. That may be the signature of one capable of affecting the Earth system. Thousands that warrant being dubbed a ‘very large new star’are recorded, but none that ‘turned night into day’. The hypothesis seems to have ‘legs’, but so too do others, such as the slow influence on oceanic circulation of the formation of the Isthmus of Panama and other parochial mechanisms of changing the transfer of energy around our planet

See also: Stellar explosion in Earth’s proximity, eons ago. (Science Daily; 30 September 2020.)

Severe COVID-19 associated with Neanderthal inheritance?

News broke in 2010 about evidence from modern and ancient DNA samples that showed some anatomically modern humans who left Africa before 40 thousand years ago to have interbred with the Neanderthal occupants of Eurasia (see: Yes, it seems that they did …; May 2010). In 2011 it turned out that the same had happened when AMH migrants in Asia met with Denisovans (see: Snippets on human evolution; November 2011). The resulting human hybrids went on to spread their new genes as they populated East Asia, Australasia and the Americas. Genomes of thousands of living people from these continents all show varying proportions – but generally less than about 5% – of genetic contributions from one or the other and in some cases both. Some of the modern humans who remained in Africa also had a similar opportunity. A few Neanderthals did set foot in Africa sharing their genes with its original inhabitants, but those venturing far from their normal range had already interbred with early ‘out-of-Africa’ AMH migrants about 150 to 100 thousand years ago, as had  AMH returning from Eurasia around 20 ka ago. Five widespread groups of modern Africans (but not all) carry up to 0.3% of the Neanderthal genome. Moreover, the ancestors of some living Africans had also exchanged genes with as-yet unknown archaic humans (see also: Everyone now has their Inner Neanderthal; February 2020).

Personally, I reacted to the news with a sense of pride. Neanderthals were tough, survivors of several hundred thousand years of climatic extremes hunting fearsome prey, and they probably had an intellect as advanced as that of the AMH with whom they mingled. My little bit of Neanderthal has conferred several advantages including resistance to Eurasian pathogens, but also has its downside, such as a tendency to depression and excessive blood clotting. But an unedited paper released in advance of publication by the journal Nature suggests that my pride turns out to include an unwelcome element of hubris.

Early in the COVID-19 pandemic, genetic research on over 3000 individuals, whose symptoms were severe enough for them to be hospitalised – a high proportion of whom sadly died – revealed that there was more to their being prone to severe symptoms than age, co-morbidities, gender and ethnicity. A short segment (around 50,000 base pairs long) on the DNA of their chromosome-3 is significantly associated with severe COVID-19 outcomes. Svante Pääbo, the leader of the team than reconstructed the Neanderthal and Denisovan genomes and discovered their presence in living people, and his co-author, Hugo Zeberg, have linked this segment to a stretch in the Neanderthal genome (Zeberg, H. & Pääbo, S. 2020. The major genetic risk factor for severe COVID-19 is inherited from Neanderthals. Nature, accelerated preview; DOI: 10.1038/s41586-020-2818-3). One gene included in the segment plays a role in the human immune response and another is linked to the way the coronavirus invades human cells, but how they influence health outcomes in COVID-19 victims is yet to be established. Sixteen percent of Europeans and 30% of South Asians carry the segment. If infected, they are at higher risk of severe outcomes than the rest of the population. That the relatively small segment still persists 40 ka after Neanderthals became extinct suggests that it has not always conferred high risk: probably because it once conferred significant advantages, perhaps by protecting against other, now extinct pathogens. A fitness benefit is passed on through natural selection

The pandemic has yet to run its course and genetic research, such as that by Zeberg and Pääbo, takes second place to that aiming at lessening the effects of the disease and developing vaccines that, hopefully, will wipe it out. The country-by-country statistics of COVID-19 morbidity and mortality are shaky, but an interesting feature may be emerging. Although there have been many cases in Africa, where health services are under developed, it seems that deaths as a proportion of infections are significantly lower there than in the more advanced countries. Hopefully that will continue, perhaps as a result of lower Neanderthal inheritance.

See also: Sample, I. 2020. Neanderthal genes increase risk of serious Covid-19, study claims. (The Guardian, 30 September 2020)