Using lasers to map landslide risk

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As radar stands for radio detection and ranging, so lidar signifies light detection and ranging. In one respect the two are related: they are both active means of remote sensing and illuminate the surface, rather than passively monitoring solar radiation reflected from the surface or thermal radiation emitted by it. The theory and practice of imaging radar that beams microwaves at a surface and analyses the returning radiation are fiendishly complex. For a start microwave beams are directed at an angle towards the surface. Lidar is far simpler being based on an aircraft -mounted laser that sends pulses vertically downwards and records the time taken for them to be reflected from the surface back to the aircraft. The method measures the distance from aircraft to the ground surface and thus its topographic elevation. Lidar transmits about 100 thousand pulses per, so the resulting digital elevation model has remarkably good spatial resolution (down to 25 cm) and can measure surface elevation to the nearest centimetre. The technique is becoming popular: the whole of England and much of the nations of Scotland, Wales and Northern Ireland now have lidar coverage with 1 metre resolution.

The first thing the laser pulses encounter is the vegetation canopy, from which some are reflected back to the instrument. Others penetrate gaps in the canopy, to be reflected by the ground surface, so they take slightly longer to return. If the penetrating pulses are digitally separated from those reflected by vegetation, they directly map the elevation of the solid ground surface or the terrain. These data produce a  digital terrain model (DTM) whereas the more quickly returning pulses map the height and structure of the ground cover, if there is any. Both products are useful, the first to map topographic and geological features, the details of which are hidden to conventional remote sensing, the second to assess vegetation. The great advantage of a DTM is that image processing software can simulate illumination and shading of the terrain from different directions and angles to improve interpretation. Aerial photography has but a single direction and angle of solar illumination, depending on the time of day, the season and the area’s latitude. Stereoscopic viewing of overlapping photographic images does yield topographic elevation, and photogrammetric analysis produces a digital elevation model, but its usefulness is often compromised  by ground cover in vegetated terrain and by shadows. Also its vertical resolution is rarely better than 1 m. Another factor that limits terrain analysis using aerial photographs and digital images from satellites is the ‘patchwork-quilt’ appearance of farmed land that results from sharp boundaries between fields that contain different crops, bare ploughed soil and grassland. Together with spatial variation of natural vegetation, both ‘camouflage’ physical features of the landscape.

A cliff collapse in July 2023 at Seatown, Dorset England

In the field, areas of what is known as ‘mass wasting’, such as landslides, landslips, rockfalls, debris flows and solifluction, show topographic features that are characteristic of the processes involved.  They can be mapped by careful geological surveys. But are overlooked, being masked by vegetation cover such as woodland or because slower downslope movement of soil has smoothed out their original landforms. Potentially devastating mass wasting is encouraged by increased moisture content of soils and rocks that lie beneath steep slopes. Moisture provides lubrication that gravitational forces can exploit to result in sudden disruption of slopes and the movement of huge masses of Earth materials. Large areas of upland Britain show evidence of having experienced such mass wasting in the past. Some continue to move, such as that in the Derbyshire Peak District on the slopes of Mam Tor, as do cases on rugged parts of Britain’s coast where underlying rocks are weak and coastal erosion is intense (see image above).

It is thought that many of the mass-wasting features in Britain were initiated at the start of the Holocene. Prior to that, during the Younger Dryas cooling event, near-surface Earth materials were gripped solid by permafrost. Sudden warming at about 11.7 ka ago melted deeply frozen ground to create ideal conditions for mass wasting. In the last eleven thousand years the surface has come to a more or less stable gravitational balance. Yet heavy, sustained rainfall may reactivate some of the structures or trigger new ones. The likelihood of increased annual rainfall as the climate warms will undoubtedly increase the risk of more and larger instances of mass wasting. Indeed such an acceleration is happening now.

The most risky places are those with a history of landslides etc. So detailed mapping of such risk-prone ground is clearly needed. The UK has a large number of sites where mass wasting has been recorded, and below are lidar images of three of the most spectacular. Undoubtedly, there are other areas where no recent movements have been recorded, but which may ‘go off’ under changed climatic conditions. One of the best documented risky areas is in the English West Midlands within the new city of Telford. It follows the flanks of the River Severn as it passes through the Ironbridge Gorge that was cut by subglacial meltwater after the last glacial maximum. This area is also recognised as having been the birthplace of the Industrial Revolution. In 1714 Abraham Darby pioneered the use of coke in iron smelting and mass production of cast iron at Coalbrookdale a few kilometres to the east. The Severn also powered numerous forges and other heavy industries in the 18th and 19th centuries.  Industrial activity and townships in the Gorge have been plagued by large-scale mass wasting throughout subsequent history and no doubt long before. An excellent illustrated guide to the area has been produced by the Shropshire Geological Society (Rayner, C. et al. 2007. A Geological Trail through the landslides of Ironbridge Gorge, Proceedings of the Shropshire Geological Society, v. 12, p. 39-52; ISSN 1750-8568)

Lidar DTM illuminated from the west for the Severn Gorge near Ironbridge, Telford, Shropshire, UK. Lips of four major landslides shown by ‘No Entry’ signs. Initiated at the beginning of the Holocene, they continue to be active to this day, the southernmost slide having obliterated a tile factory and workers’ dwellings at Jackfield in 1952
Lidar DTM illuminated from the NW for the Alport Valley in the Peak District of North Derbyshire, UK. This includes the largest landslide complex in England, known as Alport Castles from the huge displaced sandstone blocks in the area of mass wasting.
An active landslide near Castleton, Derbyshire, UK. Note the defences of an Iron Age hillfort on Mam Tor that have been cut as the head of the landslide retreated westwards, as have medieval field walls. The relics of a major road that has been repeatedly disrupted and then destroyed following decades of maintenance can also be seen in the debris flow: it was abandoned in the 1970s.

Water in unexpected places. 1: Atmosphere

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As a liquid, solid or in gaseous form water is everywhere in the human environment: even in the driest deserts it rains at some time and they may become tangibly humid. Water vapour moves most quickly in the atmosphere because of continual circulation. But 99% of all the Earth’s gaseous water resides in the lowest part, the troposphere. In that layer temperature decreases upwards to around -70°C, reflected by the lapse rate, so that water vapour condenses out as liquid or ice at low altitudes in the tangible form of clouds. So as altitude increases the air becomes increasingly cold and dry until it reaches what is termed the tropopause, the boundary between the troposphere and the stratosphere. This lies at altitudes between 6 km at the poles and 18 km in the tropics. Higher still, counter intuitively, the stratospheric air temperature rises. This is due to the production of ozone (O3) as oxygen (O2) interacts with UV radiation. Ozone absorbs UV thereby heating the thin stratospheric air. The tropopause is therefore an efficient ‘cold trap’ for water vapour, thereby preventing Earth from losing its surface water. Any that does pass through rises to the outer stratosphere where solar radiation dissociates it into oxygen and hydrogen, the latter escaping to space. So for most of the time the stratosphere is effectively free of water.

57 km high eruption plume and surrounding shock wave of Hunga Tonga-Hunga Ha’apai volcano one hour after explosion began on 15 January 2022: from the Himawari-8 satellite. The image is about 350 km across. Islands in red, the main island of Tonga being slightly to the south of the centre.

On 14 to 15 January 2022 the formerly shallow submarine Hunga Tonga-Hunga Ha’apai volcano in the Tonga archipelago of the South Pacific underwent an enormous explosive eruption (see an animation of the event captured by the Japanese weather satellite Himawari-8). The explosion was the largest in the atmosphere ever recorded by modern instruments, dwarfing even nuclear bomb tests, and the most powerful witnessed since that of Krakatoa in 1883. But, as regards global media coverage, it was a one-trick pony, trending for only a few days. It did launch tsunami waves that spanned the whole of the Pacific Ocean, but resulted in only 6 fatalities and 19 people injured. However, Hunga Tonga-Hunga Ha’apai managed to punch through the tropopause and in doing so, it changed the chemistry and dynamics of the stratosphere during the following year. A group of researchers from Harvard University and the University of Maryland used data from NASA’s Aura satellite to investigate changes in stratigraphic chemistry after the eruption (Wilmouth, D.M. et al. 2023. Impact of the Hunga Tonga volcanic eruption on stratospheric composition. Proceedings of the National Academy of Sciences, v. 120, article e23019941; DOI: 10.1073/pnas.2301994120). The Microwave Limb Sounder (MLS) carried by Aura measures thermal radiation emitted in the microwave region from the edge of the atmosphere, as revealed by Earth’s limb – seen at the horizon from a satellite. Microwave spectra from 0.12 to 2.5 mm in wavelength enable the concentrations of a variety of gases present in the atmosphere to be estimated along with temperature and pressure over a range of altitudes.

The team used MLS data for the months of February, April, September and December following the eruption to investigate its effects on the stratosphere n from 30°N to the South Pole. These data were compared with the averages over the previous 17 years. What emerged was a highly anomalous increase in the amount of water vapour between 0 and 30°S (the latitude band that includes the volcano) beginning in February 2022 and persisting until December 2023, the last dates of measurements. By April the peak showed up and persisted north of the Equator and at mid latitudes of the Southern Hemisphere and by December over Antarctica. It may well be present still. The estimated mass of water vapour that the eruption jetted into the stratosphere was of the order of 145 million tons along with about 0.4 million tons of SO2, the excess water helping accelerate the formation of highly reflective sulfate aerosols. Associated chemical changes were decreases in ozone (~ -14%) and HCl (~ -22%) and increases in ClO (>100%) and HNO3 (43%). Hunga Tonga-Hunga Ha’apai therefore changed the stratosphere’s chemistry and a variety of chemical reactions. As regards the resulting physical changes, extra water vapour together with additional sulfate aerosols should have had a cooling effect, leading to changes in its circulation with associated decrease in ozone in the Southern Hemisphere and increased ozone in the tropics. Up to now, the research has not attempted to match the chemical changes with climatic variations. The smaller 15 June 1991 eruption of Mount Pinatubo on the Philippine island of Luzon predated the possibility of detailed analysis of its chemical effects on the stratosphere. Nevertheless the material that is injected above the tropopause resulted in a global ‘volcanic winter’, and a ‘summer that wasn’t’ in the following year. The amount of sunlight reaching the surface fell by up to 10%, giving a 0.4 decrease in global mean temperature. Yet there seem to have been no media stories about such climate disruption in the aftermath of Hunga Tonga-Hunga Ha’apai. That is possibly because the most likely effect is a pulse of global warming in the midst of general alarm about greenhouse emissions, the climatically disruptive effect of the 2023 El Niño and record Northern Hemisphere temperature highs in the summer of 2023. Volcanic effects may be hidden in the welter of worrying data about anthropogenic global climate change.   David Wilmouth and colleagues hope to follow through with data from 2023 and beyond to track the movement of the anomalies, which are expected to persist for several more years. Their research is the first of its kind, so quite what its significance will be is hard to judge.

Aftershocks of ancient earthquakes

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Any major earthquake is likely to be followed by aftershocks. Survivors of seismic devastation live in dread of them for weeks, even months. In reality the fault responsible for the initial event continues to move for longer than that. Commonly, aftershock activity dies down in magnitude and frequency over time, sometimes after a few weeks and in other cases much later to reach ‘normal background seismicity’ for the associated tectonic setting. Near a major plate boundary, such as the San Andreas Fault system in coastal California or the mid-Atlantic Ridge in Iceland, there is a continual risk of damaging seismic events, but the area around each major event becomes less risky a few tens of years afterwards. For instance, the Loma Prieta area on the San Andreas became quiescent sixteen years after the October 1989 Magnitude 6.9 earthquake that wrought havoc in San Francisco – and interrupted a Major League baseball match in the city. The December 1954, Magnitude 7.3 Dixie Valley earthquake in the active extensional zone of Nevada had a longer period of instability: 48 years. There is no fixed period for the aftermath, seismicity ‘stops when it stops’.

Earthquakes of greater than Magnitude 2.5 in eastern North America (see key to magnitudes at lower right). Those shown in blue date from 1568 to 1979, those in red between 1980 and 2016. (Credit: Chen & Liu, Fig 1)

Sometimes devastating earthquakes take place in what seem to be the least likely places: in tectonically ‘stable’ continental plate interiors. A Magnitude 7.9 earthquake in Sichuan Province, central China on 12 May 2008 left 86 thousand dead or missing, 374 thousand injured and 4.2 million homeless. It occurred in a region whose ancient fault systems had had little if any historic activity. One of the best studied records of seismic events in the middle of a continent is in the Mississippi River valley at the Missouri-Kentucky border, USA, near the town of New Madrid. This experienced three major earthquakes in 1811 and 1812 at Magnitudes estimated from 7.0 to 7.4. Seismicity there has continued ever since. Others that occurred long ago in the ‘stable’  North American continental crust were in South Carolina (1886) and southern Quebec, Canada (1663). They and the subsequent, lesser earthquakes that define clusters up to 250 km around them have been studied using spatial statistics (Chen, Y. & Liu, M. 2023. Long-Lived Aftershocks in the New Madrid seismic Zone and the Rest of Stable North America. Journal of Geophysics Research: Solid Earth, v. 128; DOI: 10.1029/2023JB026482). Yuxuan Chen and Mian Lui of Wuhan University, China and the University of Missouri, USA considered the dates of historic events, their estimated magnitudes and their proximity to other events in each cluster. The closer two events are the greater the chance that the later one is an aftershock of the first, although the relationship may also indicate a long-lived deformation process responsible for both. The authors suggest that this ‘nearest-neighbour’ approach may reveal that up to 65% of earthquakes in the New Madrid zone between 1980 and 2016 are aftershocks of the 1811-1812 major earthquake cluster, and a significant number of modern events in South Carolina could similarly relate to the 1886 Charleston earthquake. On the other hand, small modern earthquakes in Quebec are more likely to be part of the regional seismic background than to have any relationship to the large 17th century event.

Earthquakes are manifestations of deep-seated processes, most usually the build-up and release of strain in the lithosphere. If such processes persist they can result in long-lived earthquake swarms. So both delayed aftershocks and a high background of seismicity can contribute to the mapped clusters of historic events: a blend of relics of the past and modern deformation. They are yet to be detected in earthquake records associated with tectonic plate boundaries. A long history of movements within continents suggests that it is possible that long-delayed aftershocks may masquerade as foreshocks that presage greater events that are pending. Chen and Liu’s nearest-neighbour approach may therefore distinguish false alarms from real risk of major seismic motions.

See also: Some of today’s earthquakes may be aftershocks from quakes in the 1800s. Eureka|Alert, 13 November 2023

Relics of the Moon-forming impact?

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Close to the core-mantle boundary (CMB) there are two extensive zones up to 10 km thick in the lower mantle. They have seismic-wave speeds that are much lower than expected at such depths: hence their being termed large low-velocity provinces (LLVPs). Seismic velocities being inversely proportional to the density of the material through which such waves travel, these zones have anomalously high density. The LLVPs have remained enigmatic since they were first discovered. Some have suggested that they are relics of dense subducted banded iron formations (see also: Curiously low-velocity material at the core-mantle boundary; March 2005) or simply piles of subducted slabs with an eclogite component that have gradually accumulated through Earth’s long history of  plate tectonics. An alternative is that LLVPs may be connected to geochemical evidence for a heterogeneous lower mantle and perhaps are relics of Earth’s earliest history.

An artist’s impression of the collision between Theia and the proto-Earth. (Credit: Hernán Cañellas, Nature)

The Moon-forming event about 4,500 Ma ago (for more information search the Planetary Science annual logs index) that probably involved a collision between the proto-Earth and another, Mars-sized planet – dubbed ‘Theia’ – is an alternative explanation for LLVPs. Maybe they are chunks of that planet that became embedded in the early Earth’s mantle. Many geochemical approaches to such an obvious origin are inconclusive, however. The latest attempt to model the processes involved in such a planetary truck crash using computer simulation does suggest that LLVPs may indeed be relics of Theia material that sank through the molten mass that became Earth’s mantle after the collision (Yuan, Q. et al. 2023. Moon-forming impactor as a source of Earth’s basal mantle anomalies. Nature v. 623, p. 95–99; DOI: 10.1038/s41586-023-06589-1).

Qian Yuan of the California Institute of Technology, and colleagues from China, USA and the UK based their approach on geochemical anomalies in plume related ocean-island basalts. These included distinctly non-terrestrial isotopic proportions of the noble gases neon and xenon, similar to those in lunar basalts., which in turn are more iron-rich than most basalts and thus 2-3% denser. The initial assumption in their modelling was that during the collision fragments of Theia peppered the magma ocean that became Earth upper mantle. These were thoroughly mixed in this molten zone as it convected before solidifying. But melts derived from some of the fragments could have penetrated the solid mantle below 1400 km depth as blobs, to retain their chemically anomalous integrity. Being dense, the blobs could slowly sink to accumulate at the CMB to form the two LLVPs. An animation of the processes revealed by Yuan et al.’s modelling can be viewed here.

See also: Oza, A. 2023. Strange blobs in Earth’s mantle are relics of a massive collision. Nature v. 623; DOI: 10.1038/s41586-023-06589-1

Extreme scientific showing-off: Hominin fossils in space

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Good illustrations of self publicity and soaring ambition are the private space programmes of oligarchs Elon Musk (SpaceX), Jeff Bezos (Blue Origin) and Richard Branson (Virgin Galactic). For a cool US$65 million a ‘civilian’ can get a trip to the International Space Station on SpaceX; a one-hour suborbital flight on Blue Origin will cost US$300,000, with luck having Bezos as a companion; a reservation on Virgin Galactic for a 1 hour trip to the ‘edge of space’ (~100 km up) now costs US$624,000. It’s a tourist trip for the very, very rich only … but even the long-dead can go … or bits of them. On 8 September 2023 aboard Virgin Galactic flight Tim Nash, a South African billionaire had in his pocket a sturdy tube containing a thumb bone of Homo naledi and the collarbone of Australopithecus sediba. Nash reportedly said afterwards, “I am humbled and honoured to represent South Africa and all of humankind as I carry these precious representations of our collective ancestors”.

Reconstructed head of a somewhat annoyed Homo naledi. Credit: John Gurche, Mark Thiessen, National Geographic.

Nash was entrusted with these unique fossils by Lee Berger, Professor in Palaeoanthropology at Witwatersrand University, South Africa and a National Geographic Explorer-in-Residence. Berger recovered fossils of both species from limestone caves in the UNESCO World Heritage Site grandly named the Cradle of Humankind near Johannesburg. He is no stranger to controversy, and this venture cooked up with Nash seems to aim at promotion of South African achievements rather than having any scientific purpose. It has backfired spectacularly (see: McKie, R. 2023. ‘Callous, reckless, unethical’: scientists in row over rare fossils flown into space. The Observer, 22 October 2023). Comments from the anthropological world, six national and international bodies and perhaps the leading hominin specialist Professor Chris Stringer of the Natural History Museum in London include the words and phrases “callous”, “unethical”, “extraordinarily poorly thought-out”, “a publicity stunt”, “reckless” and “utterly irresponsible”. The caper breaks the South African, indeed international, scientific rule that fossils can only be allowed to travel for scientific purposes, applied consistently by similarly hominin-rich African countries such as Ethiopia, Kenya and Tanzania.

But, Hey, that’s how you get on in the world … isn’t it?

Plate tectonics loses another of its pioneers: W. Jason Morgan

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The theory of plate tectonics had a long gestation. Continental drift, one of its central tenets, was first proposed by the meteorologist Alfred Wegener in 1912. Apart from a few enthusiasts of such a dynamic aspect of geology, such as Alex du Toit and Arthur Holmes, the majority of geoscientists remained with the non-revolutionary fixist ideology of their Victorian predecessors. Wegener’s stumbling block was his proposed driving mechanism – polflucht (flight from the poles) – which assumed that supercontinents had formed in polar regions to be subject to centrifugal force resulting from Earth’s rotation. This broke them apart to be driven towards the Equator. Such a mechanism being easily invalidated, most contemporary geologists preferred to ‘throw Wegener’s  baby out with the bathwater’. Yet every piece of his evidence that continents had moved around and most of his ideas about the nature of their movements were steadily verified and amplified over the next six decades, which attracted more curious and flexible scientists. What is now the central paradigm of the Earth Sciences had to wait for a set of major discoveries in the 1950s and ‘60s enabled by emerging technologies, such as the magnetometers used by Fred Vine and Drummond Matthews to discover sea-floor magnetic striping and thus sea-floor spreading. Their breakthrough presented a plausible mechanism for continental drift and launched a near frenzy of collaborative research among a global milieu of young geoscientists, one of whom being W. Jason Morgan.

W. Jason Morgan outside the Department of Earth Sciences, Princeton University. (Credit: Denise Applewhite, Princeton University)

His initial interest was in the great fracture zones on the floors of the Atlantic and Pacific Oceans. He grasped that each of them was very nearly a great circle. This was a central key to unifying seafloor spreading and continental drift – to move across a spherical surface every point on the seafloor had to follow such a path. Morgan recognised that the fracture zones could only result from rigid plates having to fracture to accommodate that motion. Using spherical geometry he was able to link together ridges, trenches and these huge transform faults with poles of rotation and triple junctions to predict plate motions in a quantitative manner. That insight provided a key to active earthquakes, mountain belts and volcanoes. His scientific unification was a result of genius: in just a few weeks Morgan established the fundamentals of what became known as plate tectonics.

W. Jason Morgan was one of the revolutionaries who made geology dynamic and launched its resurrection from the boring province of damp field workers in anoraks tramping across tracts of extremely puzzling rocks and structures, noses to the ground. He died at the age of 87 on 31 July 2023.

You can read an obituary by his former research student Richard Hey and his son Jason Phipps Morgan together with a fuller account of his career on Wikipedia.

North America occupied by modern humans during the Last Glacial Maximum

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White Sands National Park in New Mexico, USA is notorious for being adjacent to the site at which the first nuclear weapon was tested (code name Trinity) on 16 July 1945. Four weeks later two such bombs killed between 129,000 and 226,000 people at Hiroshima (6 August 1945) and Nagasaki (9 August 1945). The area is one of spectacular geology, the white sand being made of gypsum (CaSO4) grains precipitated from lake water supplied by rivers that had dissolved the mineral from Permian evaporites in the surrounding mountains. Subsequent wind erosion created a large, white dune field: the main attraction. Though a national park that has been proposed for UNESCO World Heritage Centre, the park itself is surrounded by military installations including the nuclear test site.

Gypsum sand dunes in White Sands National Park USA. (Credit: Wikipedia)

As in most evaporite basins, the White Sands’ gypsum sediments built up layer-by-layer through deposition of clays during successive inundations followed by evaporation of CaSO4 rich water. Animals crossing the basin were likely to leave trackways, which subsequent sedimentary cycles could preserve in stratigraphic order. Examples had been found in the early 20th century, revealing the former presence of the late-Pleistocene megafauna: Columbian mammoths, ground sloths, ancient camels, dire wolves, lions, and sabre-toothed cats. One set of dire wolf prints found in the 2010s contained seeds that yielded a radiocarbon age of 18 ka. More recently, 61 human footprint tracks turned up in layers that also displayed signs of megafauna crossing the lake flats, in one case showing convincing signs of hunters having followed a giant ground sloth (Bennett, M.R. 2021 and 13 others 2021. Evidence of humans in North America during the Last Glacial Maximum. Science, v. 373, p. 1528-1531; doi: 10.1126/science.abg7586). Interestingly, many of the human tracks seem to have been made by teenagers and children with only a few made by adults. Dating of seeds in the sediment layers – and in some footprints – yielded 23 to 21 ka radiocarbon ages. This evidence suggested human occupation of New Mexico long before those who left Clovis-style artifacts around 13 ka and others who preceded them. However, the seeds that were dated are those of an aquatic grass (Ruppia cirrhosa), which may have absorbed older carbon from groundwater permeating the evaporite sediments. Being robust, the seeds could also have been transported by wind back and forth from plants that lived before the animals and humans left their marks in the saline flats. Such is the importance of the White Sands fossil trackways that a team of US and British geologists, some of whom authored Bennett et al. 2021, have sought to refute doubts of their antiquity (Pigati, J.S. and 10 others 2023. Independent age estimates resolve the controversy of ancient human footprints at White Sands. Science, v. 382, p. 73-75; DOI: 10.1126/science.adh5007).

Human footprints (arrowed) preserved on three sediment surfaces of the White Sands clay-gypsum sequences; i.e. at three times in their depositional sequence. (Credit: from Pigati et al.; Fig 1)

The researchers cut trenches into the layered clay-gypsum to reveal human footprints on three successive surfaces at the site where Ruppia seeds had provided very old, but disputed ages. They supplemented the earlier evidence by 14C dating of pollen grains blown into the prints from terrestrial plants and optically stimulated luminescence ages (time of last exposure to sunlight) of detrital quartz grains in the evaporites. The pollen dating gave ages from 23.4 to 22.6 ka, the minimum quartz OSL age being 21.5 ka. Similar ages from three different methods are pretty convincing evidence that humans were active in New Mexico during the Last Glacial Maximum (LGM), and that absorption of older carbon from groundwater had not affected the Ruppia seeds.

The Asia to America migration, which led these hunters to what the abundant megafauna trackways suggest were rich pickings around the White Sands palaeo-lake, must have been earlier still. High-latitude North America was almost certainly a vast, frigid desert for thousands of years leading up to the LGM. Another implication of the remarkable finds in the gypsum beds is that migration most probably involved a coastal or even a maritime route along the Eastern Pacific shore to reach more habitable lower latitudes.

See also: Earliest Americans, and plenty of them. Earth-logs, 27July 2020; Prillaman, M. 2023. Human footprints in New Mexico really may be surprisingly ancient, new dating shows. Science News, 5 October 2023.

Repeated climate and ecological stress during the run-up to the K-Pg extinction

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The Cretaceous-Palaeogene mass extinction is no longer an event that polarises geologists’ views between a slow volcanic driver (The Deccan large igneous province) and a near instantaneous asteroid impact (Chicxulub). There is now a broad consensus that both processes were involved in weakening the Late Cretaceous biosphere and snuffing out much of it around 66 Ma ago. Yet is still no closure as regards the details. From a palaeontologist’s standpoint the die-off varied dramatically between major groups of animals. For instance, the non-avian dinosaurs disappeared completely while those that evolved to modern birds did not. Crocodiles came through it largely unscathed unlike aquatic dinosaurs. In the seas those animals that lived in the water column, such as ammonites, were far more affected than were denizens of the seafloor. But much the same final devastation was visited on every continent and ocean. However, lesser and more restricted extinctions occurred before the Chicxulub impact.

Scientists from Norway, Canada, the US, Italy, the UK and Sweden have now thrown light on the possibility that climate change during the last half-million years of the Cretaceous may have been eroding biodiversity and disrupting ecosystems (Callegaro, S. et al. 2023. Recurring volcanic winters during the latest Cretaceous: Sulfur and fluorine budgets of Deccan Traps lavas. Science Advances, v. 9, article eadg8284; DOI: 10.1126/sciadv.adg8284). Almost inevitably, they turned to the record of Deccan volcanism that overlapped the K-Pg event, specifically the likely composition of the gases that the magmas may have belched into the atmosphere. Instead of choosing the usual suspect carbon dioxide and its greenhouse effect, their focus was on sulfur and fluorine dissolved in pyroxene grains from 15 basalts erupted in the 10 Formations of the Deccan flood-basalt sequence. From these analyses they were able to estimate the amounts of the two elements in the magma erupted in each of these 10 phases.

Exposed section through a small part of the Deccan Traps in the Western Ghats of Maharashtra, India. (Credit: Gerta Keller, Princeton University)

The accompanying image of a famous section through the Deccan Traps SE of Mumbai clearly shows that 15 sampled flows could reveal only a fraction of the magmas’ variability: there are 12 flows in the foreground alone. The mountain beyond shows that the pale-coloured sequence is underlain by many more flows, and the full Deccan sequence is about 3.5 km thick. Clearly, flood-basalt volcanism is in no way continuous, but builds up from repeated lava flows that can be as much as 50 m thick. Each of them is capped by a red, clay-rich soil or bole – from the Greek word bolos (βόλος) meaning ‘clod of earth’. Weathering of basalt would have taken a few centuries to form each bole. Individual Deccan flows extend over enormous areas: one can be traced for 1500 km. At the end of volcanism the pile extended over roughly 1.5 million km2 to reach a volume of half a million km3.

Fluorine is a particularly toxic gas with horrific effects on organisms that ingest it. In the form of hydrofluoric acid (HF) – routinely used to dissolve rock – it penetrates tissue very rapidly to react with calcium in the blood to form calcium fluoride. This causes very severe pain, bone damage and other symptoms of skeletal fluorosis. The 1783-4 eruption of the Laki volcanic fissure in Iceland emitted an estimated 8,000 t of HF gas that wiped out more than half the domestic animals as a result of their eating contaminated grass. The famine that followed the eruption killed 20 to 25% of Iceland’s people: exhumed human skeletons buried in the aftermath show the distinctive signs of endemic skeletal fluorosis. This small flood-basalt event had global repercussions, as the Wikipedia entry for Laki documents. Volcanic sulfur emissions in the form of SO2 gas react with water vapour to form sulphuric acid aerosols in a reflective haze. If this takes place in the stratosphere as a result of powerful eruptions, as was the case with the 1991 Pinatubo eruption in the Philippines, the high-altitude haze lingers and spreads. This results in reduced solar warming: a so-called ‘volcanic winter’. In the Pinatubo aftermath global temperatures fell by about 0.5°C during 1991-3. Unsurprisingly, volcanic sulfur emissions also result in acid rainfall. Moreover, inhaling the sulphur-rich haze at low altitudes causes victims to choke as their respiratory tissues swell: an estimated 23,000 people in Britain died in this way when the 1783-4 Laki eruption haze spread southwards Sara Calegaro and colleagues found that the fluorine and sulfur contents of Deccan magmas fluctuated significantly during the eruptive phases. They suggest that fluorine emissions were far above those from Laki, perhaps leading to regional fluorine toxicity around the site of the Deccan flood volcanism but not extinctions. Global cooling due to sulphuric acid aerosols in the stratosphere is suggested to have happened repeatedly, albeit briefly, as eruption waxed and waned during each phase. Magmas rich in volatiles would have been more likely to erupt explosively to inject SO2 to stratospheric altitudes (above 10 to 20 km). The authors do not attempt to model when such cooling episodes may have occurred: data from only 15 levels in the Deccan Traps do not have the time-resolution to achieve that. They do, however, show that this large igneous province definitely had the potential to generate ‘volcanic winters’ and toxic episodes. Time and time again ecosystems globally and regionally would have experienced severe stress, the most important perhaps being disruption of the terrestrial and marine food chains.

A way for early humans to leave Africa for Eurasia via the Middle East

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Without seafaring skills and sturdy boats, ancient humans had only two options to leave Africa for Eurasia: by crossing the Straits of Bab el Mandab at the southern end of the Red Sea and from the Nile delta to the Levant at its northern end. Both would have been difficult. The first route demanded extremely low sea level drawn down by continental ice accumulation to narrow the sea crossing, the earliest in the last glacial cycle being around 70 ka ago. The northern route, with no sea crossing, was potentially achievable throughout the history of the genus Homo. But that way is beset to the north and east by deserts with large tracts that today lack natural water sources. To leave Africa by that route seems the most obvious, being reached along the well-watered Nile valley or the Red Sea coast with its abundant marine resources. Yet moving eastwards to Arabia and further would have required climatic windows of opportunity to ensure well-watered corridors: it would be impossible today without an infrastructure of wells; and edible resources are extremely sparse. Remains of anatomically modern humans (AMH) as old as 200 ka and others in the period between 130 to 85 ka have been found around the eastern shores of the Mediterranean. Either of the two routes could have led them there during periods of increased humidity, perhaps in a series of migratory pulses. In the case of an exodus across the Straits of Bab el Mandab, people could have moved northwards along the Red Sea coast of modern Yemen and Arabia to the Levant. However, the record is patchy, and there is no direct fossil evidence to suggest they went further, into southern Asia or Europe in these earlier times. Each early venture may also have ended in extinction.  The first presence of AMH in Asia and Europe, seems to have been tens of thousand years later: about 75 ka and 45 ka, respectively, so far as we know.

Left: Satellite image of the Arabia and the Levant, showing the possible northern (red) and southern migration routes (blue) and sites that yielded various palaeoclimatic signs of formerly wet areas, Homo sapiens fossils and stone tools (see key). Right colour-coded map of topographic elevation for the study area in the Levant with sites that reveal palaeoclimatic and anthropological information. (Credit: Abbas et al., Fig 1)

Research in the Arabian Peninsula has early recorded human presence from discarded stone artefacts at widely scattered sites, as far east as the UAE and Oman, but whether these were carried by AMH or other human groups is uncertain. Yet geological research suggests that even in the presently forbidding Empty Quarter of Saudi Arabia there were from time to time abundant springs, river networks and even lakes: occasionally climate changes made much of Arabia habitable. Researchers from the University of Southampton (UK) and Shantou University (China), together with colleagues in Jordan, Australia and the Czech Republic have documented further evidence for ‘green’ episodes on the Jordan Plateau – part of the currently hyperarid  Arabian interior (Abbas, M. and 10 others 2023. Human dispersals out of Africa via the Levant. Science Advances, v.9, article eadi6838; DOI: 10.1126/sciadv.adi6838).

Three sites in Jordan reveal wetland sediments incised by now dry channels or wadis, one of which yielded stone tools Luminescence dating of wetland sediment grains shows the times when they were last exposed to sunlight: some between 86 to 65 ka, others between 57 to 43 ka. Together with data from the rest of Arabia the sites help roughly to define routes that would have permitted human migration, though not the actual directions that early AMH might have travelled or their destinations – if any. They may just have wandered around surviving on the resources that they found during short periods of amenable local climate, and vegetation much as do desert dwellers today. Actually to exit Arabia to southern Asia would require migration around what is now the Persian Gulf, where relevant data are lacking and likely to remain so while poor security for research prevails. To get to Europe would require a much more intricate journey through large mountainous tracts to reach the shores of the Black Sea.

See also: Early human migrants followed lush corridor-route out of Africa. Science Daily. 4 October 2023

Sudden climate change: a warning from 8 millennia ago

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Mesolithic hunter-gatherers in Britain must have had a very hard time around 8.2 thousand years age. The whole area around the North Atlantic experienced sudden climatic cooling of around 3.3°C together with drought that lasted about 70 years. To make things worse shortly afterwards, coasts around the North were devastated by a tsunami generated by a submarine landslide off western Norway. That event exceeded the maximum coast ‘run up’ of both the 26 December 2004 Indian Ocean tsunami and that in NW Japan on 11 March 2011. Doggerland, then in the central North Sea was devastated by a catastrophic event of a few days duration. It littered the seabed with the bones of its megafauna and even Mesolithic tools recovered by trawlers from its surviving relic the shallow Dogger Bank. It seems the tsunami arrived just as climate was warming back to ‘normal’ Holocene conditions: for many foragers, surely, a last straw.

The cooling episode has been attributed to perturbation of the Atlantic Meridional Overturning Circulation (AMOC) as a result of meltwater discharge during the deglaciation of the Laurentide Ice Sheet (see: Just when you think it’s going to turn out alright… November 2009).The event may have unfolded in a similar fashion to the trigger for the Younger Dryas and the succession of warming-cooling episodes known as Dansgaard-Oeschger events that interrupted the otherwise relentless global cooling towards the last glacial maximum (see: Review of thermohaline circulation; February 2002). The physics that set off such climatic ‘hiccups’ is that freshening of surface seawater reduces its density, so that it cannot sink to be replaced by denser saline water ‘dragged’ northwards from warmer latitudes. That currently takes the form of the Gulf Stream with its warming influence, particularly in the eastern North Atlantic and even beyond Norway’s North Cape, responsible for much warmer winters than at similar latitudes on the western side. The culprit  had long been suggested to be the drainage of a huge lake dammed by the ice sheet that covered most of eastern Canada during late stages of deglaciation. Seemingly the best candidate was Lake Agassiz trapped by the early Holocene ice front in Manitoba – the largest proglacial lake known anywhere.

Colour coded topographic elevation of North America showing the maximum extent of Lake Agassiz and four possible routes for its drainage: north-west to the Arctic Ocean via the Mackenzie River; south to the Gulf of Mexico via the Mississippi valley; east to the North Atlantic via the Great Lakes and St Laurence River; north to the North Atlantic via Hudson Bay. (Credit: ©Sheffield University)

The present landforms of central Canada show evidence for several outflow directions at different times, Including to the northwest to reach the Arctic Ocean at the onset of the Younger Dryas. Until recently there was little detailed evidence for the flow volume and timing of its drainage around 8 to 9 ka. Providing the details in the context of the short-lived event around 8.2 ka requires accurate data over a mere 200 years able to reveal a change in sea level to a precision of better than a few tens of centimetre. Any site on the shores of the North Atlantic would do, provided it satisfies these criteria. Geographers from universities in York, Leeds, Sheffield and Oxford, UK selected the small estuary of the River Ythan in NE Scotland. There, a continuous sand unit just above fine-grained intertidal tidal muds marks the knife-sharp time datum of the Storegga tsunami (Rush, G. et al. 2023. The magnitude and source of meltwater forcing of the 8.2 ka climate event constrained by relative sea-level data from eastern Scotland. Quaternary Science Advances, v. 12, article 100119; DOI: 10.1016/j.qsa.2023.100119).

Cores of the intertidal sediments from beneath the present Ythan salt marsh contain plant remains that yielded precise radiocarbon dates at several stratigraphic levels from which to derive an age-depth model for the age range of interest. The buried sediments are also rich in marine microfossils (foraminifera and diatoms) that thrive in estuaries at a variety of depths.  These enabled fluctuations in relative sea level during the build-up of the intertidal sediments to be constrained at unprecedented resolution and precision for a three thousand year period from 9.5 to 6.5 ka. The authors show that there were two episodes of rapid sea-level rise over that time: between 8.53 and 8.37 ka (~2.4 m at 13 mm yr-1) and 8.37 to 8.24 ka (~ 0.6 m at 4 mm yr-1) – these would have been global increases in sea level.

Despite its vast size, it turns out that Lake Agassiz would have been unable to result in sea-level rises of that magnitude so quickly merely through outflow. Rush et al. suggest that the huge  and rapid addition of fresh water to the North Atlantic involved flow of lake water towards Hudson Bay, beneath the ice sheet, causing it to collapse and melt, followed by completion of Lake Agassiz’s emptying in the second stage. It took a long drawn-out ‘freshening’ of the North Atlantic surface water ultimately to shut down the Atlantic Meridional Overturning Circulation, thereby depriving high latitudes of its east-side warming effect by the Gulf Stream.

Sea level has been rising since the early 20th century mainly through the melting of Greenland’s ice cap together with a substantial amount of thermal expansion while global climate has been warming. Between 1901 and 2018 the rise has amounted to 15 to 25 cm at a rate of 1 to 2 mm yr-1. The AMOC is possibly weaker now than at any time during the last millennium (Zhu, C. et al. 2023. Likely accelerated weakening of Atlantic overturning circulation emerges in optimal salinity fingerprint. Nature Communications, v. 14, article 1245; DOI: 10.1038/s41467-023-36288-4). Yet increases in freshening of the northernmost parts of the North Atlantic are now being added to by annual increases in the melting of polar sea ice, which is salt-free. The AMOC may be approaching a tipping point, because warming is accelerating over Greenland at around 1.5°C each year: faster than most of the rest of the world. In 2021 it rained for the first time ever recorded at the ice cap’s summit (3.2 km above sea level). A ‘perturbation’ of the AMOC would add chaos to the dominantly linear view of global warming taken by climatologists. That could launch frigidity and drought at mid northern latitudes as it did eight millennia ago: the opposite of what is currently feared.

See also: Unlocking Ancient Climate Secrets – Melting Ice Likely Triggered Climate Change Over 8,000 Years Ago. Scitechdaily 16 September 2023.

Direct signs of what caused the Palaeocene-Eocene thermal maximum

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Until about 56 Ma ago North America and Europe were connected: one of the last relics of the Pangaea supercontinent. Oxygen isotopes and magnesium/calcium ratios in the tests of both surface- and bottom-dwelling foraminifera suggest that around that time global mean surface temperature increased by about 5 to 6°C within 10 to 20 thousand years. The rate of global warming was comparable to that currently being induced by human activities. The Palaeocene-Eocene thermal maximum (PETM) is seen by climatologists as a dreadful warning of times to come in the not so distant future. The PETM event marks the most dramatic biological changes since the mass extinction at the Cretaceous-Palaeogene boundary 10 million years earlier. They included the rapid expansions of mammals and land plants and major extinction of deep-water foraminifera. The PETM also coincided with an equally profound excursion in the δ13C of carbon-rich strata of that age, whose extreme negative value marks the release of a huge mass of previously buried organic carbon into the atmosphere. It was probably methane, much more potent at delaying heat loss to space than carbon dioxide – methane has more than 80 times the warming effect of carbon dioxide. Since CH4 is soon oxidised to CO2 and H2O estimates of atmospheric greenhouse gas levels are generally expressed in terms of CO2. The PETM release was equivalent to about 4.4 x 1013metrictons over 50 ka; on average 0.24 gigatons per year compared with 0.51 Gt from energy-related sources in 2022.

During the Palaeocene, areas around the present North Atlantic were subject to basaltic continental volcanism before the rifting that opened the North Atlantic from 62 to 58 Ma. Magmatism, dominated by intrusions, began again at the Palaeocene-Eocene boundary from 56 to 54 Ma, linked to the start of continental rifting. Both episodes suggest a rising mantle plume. Once the rift had truly opened volcanism became restricted to the mid Atlantic ridge and a mantle plume remains active beneath Iceland. After geoscientists became aware of the PETM and its coincidence with North Atlantic igneous activity many palaeoclimatologists suggested methane release from organic-rich sediments heated by intrusion of basaltic sills below the opening seaway (but see 2022 post on alternative hypotheses). As with so many extreme geological events, choosing a most-likely scenario depends ultimately on tangible evidence. A convincing sign has been demonstrated dramatically in a recent study by a multinational team of geophysicists, oceanographers, geochemists, palaeontologists and sedimentologists (Berndt, C. and 35 others 2023. Shallow-water hydrothermal venting linked to the Palaeocene–Eocene Thermal Maximum. Nature Geoscience, v. 16, p. 803–809; DOI: 10.1038/s41561-023-01246-8).

Three-dimensional view of seismic reflection data off western Norway. The greytone lower part is a vertical ‘slice’. The coloured part shows the depth variation of sediments that fill hydrothermal vent systems beneath a horizontal unconformity. (Credit: Berndt et al, Fig 1b)

The breakthrough by Berndt et al. stemmed from a detailed 3-D seismic survey off the coast of Norway. It revealed an unconformity at the P-E boundary beneath which were clear signs of hundreds of large pockmarks, up to 80 m deep. Seismic reflection from older sediments beneath the unconformity showed the distinctive presence of intrusive sills of igneous rocks. The consortium drilled 20 boreholes into the seabed beneath the survey area. Five of them penetrated crater-like features to yield cores through the sediments that had filled them. The fills were muds, which were interleaved beds of volcanic ash in the sequences marking the P-E boundary suggesting an igneous influence. Organic remains in the muds established the depositional timing of several distinct layers and also gave clues to their depositional conditions. Those spanning the 50 ka of the PETM were dominated by plant debris, pollen and spores, together with abundant marine diatoms that live in very shallow water. Laminations in the muds dip radially inwards towards the deeper parts of some craters to define funnel-like structures. In others the sediments have been domed upwards. The sediments and their structures closely resemble those in blow-out craters formed during petroleum drilling accidents and in onshore maar volcanoes produced by sudden explosive eruptions on land. The pockmarks formed suddenly, to be filled by mobilised mud and volcanic ash.

The evidence points to explosive vents formed by massive degassing of deeper sediments induced by igneous intrusions. Such systems are common around active ocean-floor rifts: ‘black-‘ and ‘white smokers’, but those off Norway formed in shallow water. That has an important bearing on their potency during the PETM. Deep hydrothermal systems may emit methane, but it is oxidised to CO2 in seawater. Those very close to the surface vent their gas almost directly into the atmosphere before such oxidation can consume methane. Intrusive sills also underlie the eastern continental margin of Greenland, so such explosive hydrothermal vents may have been widespread during the initial rifting of the North Atlantic’.

An evolutionary bottleneck and the emergence of Neanderthals, Denisovans and modern humans

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The genetic diversity of living humans, particularly among short, repetitive segments of DNA, is surprisingly low. As they are passed from generation to generation they have a high chance of mutation, which would be expected to create substantial differences between geographically separated populations. In the late 1990s and early 2000s some researchers attributed the absence of such gross differences to the human gene pool having been reduced to a small size in the past, thereby reducing earlier genetic variation as a result of increased interbreeding among survivors. They were able to assess roughly when such a population ‘bottleneck’ took place and the level to which the global population fell. Genetic analysis of living human populations seemed to suggest that around 74 ka ago the global human population fell to as little as 10 thousand individuals. A potential culprit was the catastrophic eruption of the Toba supervolcano in Sumatra around that time, which belched out 800 km3 of ash now found as far afield as the Greenland and Antarctic ice caps. Global surface temperature may have fallen by 10°C for several years to decades. Subsequent research has cast doubt on such a severe decline in numbers of living hummans; for instance archaeologists working in SE India found much the same numbers of stone tools above the Toba ash deposit as below it (see: Toba ash and calibrating the Pleistocene record: December 2012). Other, less catastrophic explanations for the low genetic diversity of modern humans have also been proposed. Nevertheless, environmental changes that placed huge stresses on our ancestors may repeatedly have led to such population bottlenecks, and indeed throughout the entire history of biological evolution.

An improved method of ‘back-tracking’ genetic relatedness among living populations, known as fast infinitesimal time coalescence or ‘FitCoal’, tracks genomes of individuals back to a last common ancestor. In simple language, it expresses relatedness along lineages to find branching points and, using an assumed mutation rate, estimates how long ago such coalescences probably occurred. The more lineages the further back in time FitCoal can reach and the greater the precision of the analysis. Moreover it can suggest the likely numbers of individuals, whose history is preserved in the genetics of modern people, who contributed to the gene pool at different branching points. Our genetics today are not restricted to our species for it is certain that traces of Neanderthal and Denisovan ancestry are present in populations outside of Africa. African genetics also host ‘ghosts’ of so-far unknown distant ancestors. So, the FitCoal approach may well be capable of teasing out events in human evolution beyond a million years ago, if sufficient data are fed into the algorithms. A team of geneticists based in China, Italy and the US has recently applied FitCoal to genomic sequences of 3154 individual alive today (Hu, W.and 8 others 2023. Genomic inference of a severe human bottleneck during the Early to Middle Pleistocene transition. Science, v. 381, p. 979-984; DOI I: 10.1126/science.abq7487). Their findings are startling and likely to launch controversy among their peers.

Their analyses suggest that between 930 and 813 ka ago human ancestors passed through a population bottleneck that involved only about 1300 breeding individuals. Moreover they remained at the very brink of extinction for a little under 120 thousand years. Interestingly, the genetic data are from people living on all continents, with no major differences between the analyses for geographically broad groups of people in Africa and Eurasia. Archaeological evidence, albeit sparse, suggests that ancient humans were widely spread across those two continental masses before the bottleneck event. The date range coincides with late stages of the Mid-Pleistocene climatic transition (1250 to 750 ka) during which glacial-interglacial cycles changed from 41 thousand-year periods to those that have an average duration of around 100 ka. The transition also brought with it roughly a doubling in the mean annual temperature range from the warmest parts of interglacials to the frigid glacial maxima: the world became a colder and drier place during the glacial parts of the cycles.

Genomes for Neanderthals and Denisovans suggest that they emerged as separate species between 500 and 700 ka ago. Their common ancestor, possibly Homo heidelbergensis, H. antecessor or other candidates (palaeoanthropologists habitually differ) may well have constituted the widespread population whose numbers shrank dramatically during the bottleneck. Perhaps several variants emerged because of it to become Denisovans, Neanderthals and, several hundred thousand years later, of anatomically modern humans. Yet it would require actual DNA from one or other candidate for the issue of last common ancestor for the three genetically known ‘late’ hominins to be resolved. But Hu et al. have shown a possible means of accelerated hominin evolution from which they may have emerged, at the very brink of extinction.

Oxygen-isotope record and global temperature changes over the last 5 million years, green lines showing the times dominated by 41 and 100 ka climatic cycles. The mid-Pleistocene climatic transition is shown in pink (Credit: Robert A Rohde)

There is a need for caution, however. H. erectus first appeared in the African fossil record about 1.8 Ma ago and subsequently spread across Eurasia to become the most ‘durable’ of all hominin species. Physiologically they seem not to have evolved much over at least a million years, nor even culturally – their biface Acheulean tools lasted as long as they did. They were present in Asia for even longer, and apparently did not dwindle during the mid-Pleistocene transition to the near catastrophic levels as did the ancestral species for living humans. The tiny global population suggested by Hu et al. for the latter also hints that their geographic distribution had to be very limited; otherwise widely separated small bands would surely have perished over the 120 ka of the bottleneck event. Yet, during the critical period from 930 to 813 ka even Britain was visited by a small band of archaic humans who left footprints in river sediments now exposed at Happisburgh in Norfolk. Hu et al. cite the scarcity of archaeological evidence from that period – perhaps unwisely – in support of their bottleneck hypothesis. There are plenty of other gaps in the comparatively tenuous fossil and archaeological records of hominins as a whole.

The discovery of genetic evidence for this population bottleneck is clearly exciting, as is the implication that it may have been the trigger for evolution of later human species and the stem event for modern humans. Hopefully Hu et al’s work will spur yet more genetic research along similar lines, but there is an even more pressing need for field research aimed at new human fossils from new archaeological sites.

See also: Ashton, N. & Stringer, C. 2023. Did our ancestors nearly die out? Science (Perspectives), v. 381, p. 947-948; DOI: 10.1126.science.adj9484.

Ikarashi, A. 2023. Human ancestors nearly went extinct 900,000 years ago. Nature, v. 621; DOI: 10.1038/d41586-023-02712-4

Di Vicenzo, F & Manzi, G. 2023. An evolutionary bottleneck and the emergence of Neanderthals, Denisovans and modern humans. Homo heidelbergensis as the Middle Pleistocene common ancestor of Denisovans, Neanderthals and modern humans. Journal of Mediterranean Earth Sciences, v, 15, p. 161-173; DOI: 10.13133/2280-6148/18074

When and why did the North American Pleistocene megafauna collapse?

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The US city of Los Angeles, originally known as El Pueblo de Nuestra Señora la Reina de los Ángeles (The Town of Our Lady the Queen of the Angels), was founded in 1781 by 44 Spanish settlers. It remained a small cattle-centred town after the annexation of California from Mexico by the USA in 1847. Once it was reached by the transcontinental Southern Pacific railroad in 1876 it had the potential for growth. But it took the discovery of oil within its limits in 1892 for its population to increase rapidly. The Los Angeles City Oil Field became the top producer in California with 200 separate oil companies crammed cheek by jowl by 1901. Now only one remains, producing just 3.5 barrels per day. That crude oil was there for the taking was pretty obvious as bitumen seeps had long been exploited by native people and the original Spanish colonists. The oilfield was developed near one such seep: the Rancho La Brea tar pits.

Rancho La Brea tar pit and derricks of the Los Angeles City Oil Field in 1901

By 1901 perfectly preserved bones of a huge variety of animals – 231 vertebrate species – as well as plants and invertebrates began to be collected from the continually roiling pond of bitumen. Thousands of specimens have been collected since then, both predators and prey of all sizes. Famous for mastodons and sabre-toothed cats, La Brea is a repository of almost the entire western Californian fauna through much of the Late Pleistocene: before about 100 ka the area lay beneath the Pacific Ocean. Tar pits are traps for unwary animals of any kind, especially as shallow water often hides the danger. Carnivores seeking easy, abundant food end up trapped too.

Because of the anaerobic nature of bitumen, bacterial decay is suppressed. Many of the bones still contain undegraded collagen: the most abundant protein in mammals, which can be dated using the radiocarbon method. So, despite the lack of stratigraphy in the tar pits, it is possible to track the history of the ecosystem by painstaking dating of individual fossils (OKeefe, F.R and 18 others 2023. Pre–Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift. Science, v. 381, article eabo3594; DOI: 10.1126/science.abo3594). Robin OKeefe and colleagues dated 169 specimens of eight large mammal species most commonly found in the bitumen: sabre-toothed cat (Smilodon fatalis); dire wolf (Aenocyon dirus); coyote (Canis latrans); American lion (Panthera atrox); ancient bison (Bison antiquus); western horse (Equus occidentalis); Harlan’s ground sloth (Paramylodon harlani); and yesterday’s camel (Camelops hesternus).

The authors focussed on precisely dated specimens spanning the 15.6 to 10.0 ka time range. This would allow the disappearance times of individual species to be compared with stages in the rapid change in the Californian climate during post glacial maximum warming, those during the Younger Dryas abrupt cooling (12.9 to 11.7 ka) and the earliest Holocene warming that succeeded it. The first to go extinct were the camels and giant sloths about 13.6 ka ago. At 13.2 ka the other mammals declined very rapidly, the two remaining herbivores vanishing more quickly than the four predators. By 12.9 ka the only surviving species of the chosen eight was the coyote. So seven members of the Pleistocene mammalian megafauna became extinct before the onset of the Younger Dryas cold millennium.

Part of the team examined pollen from a core through sediments deposited in a lake 100 km south of La Brea. They found that flora, and probably climate, had not changed at the time of camel and sloth extinctions around 13.6 ka. However a 300 year period between 13.2 and 12.9 ka witnessed a collapse in deciduous tree species while conifers, grasses and drought-tolerant shrubs increased. A woodland ecosystem had been replaced by semi-arid chaparral. Another feature of the lake-bed sediments was that charcoal fragments increased explosively during that 300-year episode that ended both the woodland ecosystem and the megafauna that exploited it: undoubtedly three centuries of regular wildfires. What remained was the chaparral ecosystem based on drought-tolerant, fire-adapted plants.

Were the megafauna collapse and a change in ecology results of a climatic harbinger for the Younger Dryas cool millennium, or some other cause? Interestingly, tangible evidence for the Clovis hunting culture of North America, which has long been implicated in the faunal ‘extirpation’, does not appear until 12.9 ka, and in California neither does any implicating other human groups. Yet evidence is accumulating for much earlier entry of humans into North America. Occupation sites are very rare on land, but human presence here and there implies such earlier migration, probably along the west coast that avoided the frigid interior further north than California. The question posed by OKeefe ­et al. is, ‘Were the fires ignited by humans over a 300 year period just before the Younger Dryas’? It remains to be confirmed … First human arrivals coinciding with evidence for wildfires in Australia, New Zealand and a few other areas do suggest that it is a possibility. There needs to be a motive, such as producing lush clearings in forest to attract game, or removing cover to make hunting easier. In this case, the fires immediately preceded a global climatic downturn with terrestrial drying, so they may have had natural causes: the potentially incendiary chaparral flora had been increasing steadily beforehand and decreased rapidly after the evidence for wildfires

See also: Price, M. 2023.  Death by fire. Science, v. 381, p. 724-727; DOI: 10.1126/science.adk3291

A book on archaeology, radiocarbon dating, ancient DNA, and how modern humans evolved

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Since 2001 Tom Higham, now Professor of Scientific Archaeology at the University of Vienna, helped develop new ways of refining radiocarbon dating at Oxford University’s Research Lab for Archaeology and the History of Art. Specifically his lab learned how to remove contamination of ancient samples by recent carbon and to reduce the detection limit of their accelerator mass spectrometer for the 14C atoms that remained from when they were in living organisms. The Oxford Radiocarbon Accelerator Unit pushed sample dates to the absolute limit of the method: around 50 thousand years. Being among the very best, the ORAU had a path beaten to its doors by archaeologists from across the world keen to get the most believable dates for their samples. Equally, Higham engaged in the field work itself and in the interpretation of other data from sites, such as ancient DNA. An outcome of Higham’s energetic efforts over two decades is his book The World Before Us: How Science is Revealing a New Story of Our Human Origins (paperback edition 2022, Penguin Books,ISBN-10: ‎0241989051). One reviewer commented ‘The who, what, where, when and how of human evolution’.

The World Before Us is not only comprehensive and eminently clear for the lay-reader, but it is more exciting than any science book that I have read. For the moment, it is the latest ‘word’ on early, anatomically modern humans and on the closely related Neanderthals and Denisovans. Its core is about how these three key groups ‘rubbed along’ once they met  in the Late Pleistocene. As an amateur interested in palaeoanthropology, I have tried to keep pace with all the developments in the field since 2001 through Earth-logs, but Higham shows just how much I have missed that is important to the human story. If you have followed my many posts on human evolution and migrations with interest, read his book for a great deal more and a coherent story of how things stand.

News about when subduction began

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Tangible signs of past subduction take the form of rocks whose mineralogy shows that they have been metamorphosed under conditions of high pressure and low temperature, and then returned to the surface somehow. Ocean-crust basaltic rocks become blueschist and eclogite. The latter is denser than mantle peridotite so that oceanic lithosphere can sink and be recycled. That provides the slab-pull force, which is the major driver of plate tectonics. Unfortunately, neither blueschists nor eclogites are found in metamorphic complexes older than about 800 Ma. This absence of direct proof of subduction and thus modern style plate tectonics has resulted in lively discussion and research seeking indirect evidence for when it did begin, the progress of which since 2000 you can follow through the index for annual logs about tectonics. An interesting new approach emerged in 2017 that sought a general theory for the evolution of silicate planets, which involves the concept of ‘lid tectonics’. A planet in a stagnant-lid phase has a lithosphere that is weak as a result of high temperatures: indeed so weak and warm that subduction was impossible. Stagnant-lid tectonics does not recycle crustal material back to its source in the mantle and it simply builds up the lithosphere. Once planetary heat production wanes below a threshold level that permits a rigid lithosphere, parts of the lid can be driven into the mantle. The beginnings of this mobile-lid phase and thus plate tectonics of some kind involves surface materials in mantle convection: the may be recycled.

Cartoon of possible Hadean stagnant lid tectonics, dominated by mantle plumes. (Credit: Bédard, J.H. 2018, Fig 3B, DOI: 10.1016/j.gsf.2017.01.005)

A group of geochemists from China, Canada and Australia have sought evidence for recycled crustal rocks from silicon and oxygen isotopes in the oldest large Archaean terrane, the  4.0 Ga old Acasta Gneiss Complex in northern Canada (Zhang, Q. and 10 others 2023. No evidence of supracrustal recycling in Si-O isotopes of Earth’s oldest rocks 4 Ga ago. Science Advances, v.9, article eadf0693; DOI: 10.1126/sciadv.adf0693). Silicon has three stable isotopes 28Si, 29Si, and 30Si. As happens with a number of elements, various geochemical processes are able to selectively change the relative proportions of such isotopes: a process known as isotope fractionation. As regards silicon isotopes used to chart lithosphere recycling, the basic steps are as follows: Organisms that now remove silicon from solution in seawater to form their hard parts and accumulate in death as fine sediments like flint had not evolved in the Archaean. Because of that reasonable supposition it has been suggested that seawater during the Archaean contained far more dissolved silicon than it does now. Such a rich source of Si would have entered Archaean oceanic crust and ocean-floor sediments to precipitate silica ‘cement’. The heaviest isotope 30Si would have left solution more easily than the lighter two. Should such silicified lithosphere have descended to depths in the mantle where it could partially melt the anomalously high 30Si would be transferred to the resulting magmas.

Proportions of 30Si in zircons, quartz and whole rock for Acasta gneisses (coloured), other Archaean areas (grey) and Jack Hills zircons (open circles. Vertical lines are error bars. (Credit: simplified from Zhang et al. Fig 1)

Stable-isotope analyses by Zhang et al. revealed that zircon and quartz grains and bulk rock samples from the Acasta gneisses, with undisturbed U-Pb ages, contain 30Si in about the same proportions relative to silicon’s other stable isotopes as do samples of the mantle. So it seems that the dominant trondhjemite-tonalite-granodiorite (TTG) rocks that make up the oldest Acasta gneisses were formed by partial melting of a source that did not contain rocks from the ocean crust. Yet the Acasta Gneiss Complex also contains younger granitic rocks (3.75 to 3.50 Ga) and they are significantly more enriched in 30Si, as expected from a deep source that contained formerly oceanic rocks. A similar ‘heavy’ silicon-isotope signature is also found in samples from other Archaean terranes that are less than 3.8 Ga old. Thus a major shift from stagnant-lid tectonics to the mobile-lid form may have occurred at the end of the Hadean. But apart from the Acasta Gneiss Complex only one other, much smaller Hadean terrane has been discovered, the 4.2 Ga Nuvvuagittuq Greenstone Belt. It occupies a mere 20 km2 on the eastern shore of Hudson Bay in Canada, and appears to be a sample of Hadean oceanic crust. It does include TTG gneisses, but they are about 3.8 Ga old and contain isotopically heavy silicon. So it seems unlikely that testing this hypothesis with silicon-isotope data from other Hadean gneissic terranes will be possible for quite a while, if at all.

Geochemical evidence for the origin of eukaryotes

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Along with algae, jellyfish, oak trees, sharks and nearly every organism that can be seen with the naked eye, we are eukaryotes. The cells of every member of the Eukarya, one of the three great domains of life, all contain a nucleus – the main location of genetic material – and a variety of other small bodies known as organelles, such as the mitochondria of animals and the chloroplasts of plant cells. The vast bulk of organisms that we can’t see unaided are prokaryotes, divided into the domains of Bacteria and Archaea. Their genetic material floats around in their cells’ fluid. The DNA of eukaryotes shares some stretches with prokaryotes, but no prokaryotes contain any eukaryote genetic material. This suggests that the Eukarya arose after the Bacteria and Archaea, and also that they are a product of evolution from prokaryotes, probably by several combining in symbiotic relationships inside a shared cell membrane. Earth-logs has followed developments surrounding this major issue since 2002, as reflected in some of the posts linked to what follows. 

While prokaryotes can live in every conceivable environment at the Earth’s surface and even in a few kilometres of crust beneath, the vast majority of eukaryotes depend on free oxygen for their metabolism. Logically, the earliest of the Eukarya could only have emerged when oxygen began to appear in the oceans following the Great Oxidation Event around 2.4 billion years ago. That is more than a billion years after the first prokaryotes had left their geological signature in the form of curiously bulbous, layered carbonate structures (stromatolites), probably formed by bacterial mats. The oldest occur in the Archaean rocks of Western Australia as far back as 3.5 Ga, and disputed examples have been found in the 3.7 Ga Isua sediments of West Greenland. The oldest of them are thought to have been produced through the anoxygenic photosynthesis of purple bacteria (See: Molecular ‘fossils’ and the emergence of photosynthesis; September 2000), suggested by organic molecules found in kerogen from early Archaean sediments. Later stromatolites (<3.0 Ga) have provided similar evidence for oxygen-producing cyanobacteria.

Acritarchs are microfossils of single-celled organisms made of kerogen that have been found in sediments up to 1.8 billion years old. Features protruding from their cell walls distinguish them from prokaryote cells, which are more or less ‘smooth’: acritarchs have been considered as possible early eukaryotes. Yet the oldest undisputed eukaryote microfossils – red and green algae – are much younger (about 1.0 Ga). A means of estimating an age for the crown group from which every later eukaryote organism evolved – last eukaryotic common ancestor (LECA) – is to use an assumed rate of mutation in DNA to deduce the time when differences in genetics between living eukaryotes began to diverge: i.e. a ‘molecular clock’. This gives a time around 2 Ga ago, but the method is fraught with uncertainties, not the least being the high possibility of mutation rates changing through time. So, when the Eukarya arose is blurred within the so-called ‘boring billion’ of the early Proterozoic Eon. A way of resolving this uncertainty to some extent is to look for ‘biomarker’ chemicals in the geological record that provide a ‘signature’ for eukaryotes.

A new study has been undertaken by a group of Australian, German and French scientists to analyse sediments ranging in age from 635 to 1640 Ma from Australia, China, Asia, Africa, North and South America (Brocks, J.J and 9 others 2023. Lost world of complex life and the late rise of the eukaryotic crown. Nature, v. 618, p. 767–773; DOI: 10.1038/s41586-023-06170-w; contact for PDF). Their chosen biomarkers are sterols (steroids) that regulate eukaryote cell membranes. Some prokaryotes also synthesise steroids but all of them produce hopanepolyols (hopanoids), which eukaryotes do not. The key measures for the presence/absence of eukaryote remains in ancient sea-floor sediments is thus the relative proportions of preserved steroids and hopanoids, together with those for the breakdown products of both – steranes and hopanesthat are, crudely speaking, carbon ‘skeletons’ of the original chemicals.

Proportions of biomarkers in sediments from present to 1.64 Ga. Cholesteroids – reds; ergosteroids – blues; stigmasteroids – greens; protosteroids magentas, hopanoids – yellows; unsampled – grey. Snowball glaciations are shown in pale blue. (Credit: Simplified from Figure 3 in Brocks et al.)

Interpretation of the results by Jochen Brocks and colleagues is complicated, and what follows is a summary based partly on an accompanying Nature News & Views article(Kenig, F. 2023. The long infancy of sterol biosynthesis. Nature, v. 618, p. 678-680; DOI: 10.1038/d41586-023-01816-1). The conclusions of Brocks et al. are surprising. First, the break-down products of steroids (saturated steranes) that can be attributed to crown eukaryotes (left on the figure above) are only present in sediments going back to about 200 Ma before the first Snowball Earth event (~900 Ma). Before that only hopanes formed by hopanoid degradation are present: a suggestion that LECA only appeared around that time – the authors suggest sometime between 1 and 1.2 Ga. That is far later than the time when eukaryotes could have emerged: i.e. once there was available oxygen after the Great Oxidation Event (~2.4 to 2.2 Ga). So what was going on before this? The authors broke new ground in analysis of biomarkers by being able to detect signs of the presence of actual hopanoids and steroids of several different kinds. Steroids were present as far back as 1.6 Ga in the oldest sediments that were analysed.

Steroids of crown eukaryotes are represented by cholesteroids, ergosteroids and stigmasteroids. All three are present throughout the Phanerozoic Eon and into the time of the Ediacaran Fauna that began 630 Ma ago. In that time span they generally outweigh hopanoids, thus reflecting the dominance of eukaryotes over prokaryotes. Back to about 900 Ma, only cholesteroids are present, together with archaic forms that are not found in living Eukarya, termed protosteroids.  Before that, only protosteroids are found. Moreover, these archaic steroids are not present in sediments that follow the Snowball Earth episodes (the Cryogenian Period).

Thus, it is possible that crown group eukaryotes – and their descendants, including us – evolved from and completely replaced an earlier primitive form (acritarchs?) at around the time of the greatest climatic changes that the Earth had experienced in the previous billion years or more. Moreover, the Cryogenian and Ediacaran Periods seem to show a rapid emergence of stigmasteroid- and ergosteroid production relative to cholesteroid: perhaps a result of explosive evolution of the Eukarya at that time. The organisms that produced protosteroids were present in variable amounts throughout the Mesoproteroic. Clearly there need to be similar analyses of sediments going back to the Great Oxygenation Event and the preceding Archaean to see if the protosteroid producers arose along with increasing levels of molecular oxygen. The ‘boring billion’ (2.0 to 1.0 Ga) may well be more interesting than previously thought.

Early modern human fossils from a Laotian cave and the eastward ‘out-of-Africa’ migration

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Finding human fossils in SE Asia is rare because its tropical climate generally results in decomposition of bones. Up to now the oldest known anatomically modern human (AMH) found beyond the Middle East is from Australia and has been dated to 65 ka. Other, less convincing candidates for the earliest appearance of AMH in Asia are scattered teeth found in Chinese caves that yielded dates of up to 139 ka: their assignment to AMH and the reliability of their dating are disputed. Now a large team of scientists from the USA, Germany, Australia, South Africa, France, Denmark and Laos have unearthed convincing but fragmented AMH bones among a jumble of diverse animal fossils in sediment flooring Tam Pà Ling cave  in northern Laos (Friedline, S.E. and 30 others 2023. Early presence of Homo sapiens in Southeast Asia by 86–68 kyr at Tam Pà Ling, Northern Laos. Nature Communications, v. 14, article 3193; DOI: 10.1038/s41467-023-38715-y). Several dating techniques reveal ages of the AMH samples that range from 46 to 77 ka, and potentially as far back as 86 ka. It is conceivable that the oldest are from the population that subsequently reached Australia. Far to the west of Laos in Greece, Israel and Arabia an earlier AMH presence goes back as far as 90 to 210 ka. Moreover, palaeoclimatic studies suggest many opportunities for eastward migration since 290 ka ago that AMH emigrants may have exploited. Once beyond regions around Arabia and the Gulf, which were periodically hyperarid, the journey to the rest of Asia was probably continuously habitable throughout the last two glacial-interglacial cycles.

Entrance to Tam Pà Ling cave in northern Laos (credit: Demeter et al.; Fig S1)

Another aspect of the AMH record in southern and SE Asia is that the individuals represented seem to have been anatomically very varied (Demeter, F. et al. 2023. Early Modern Humans and Morphological Variation in Southeast Asia: Fossil Evidence from Tam Pa Ling, Laos. PLOS ONE, v. 10, article e0121193. DOI:10.1371/journal.pone.0121193). This may suggest that migration was by significantly different groups at different times. Oddly, the earliest known examples have more ‘modern’ characteristics than younger ones that appear somewhat ‘archaic’. The age of the fossils conflicts with the 60 ka age reconstructed from genetic evidence for the main diffusion across Eurasia and Australasia. One possibility is that there were several pre-60 ka migrations, descendents of these early populations having been replaced or assimilated by a later, larger numbers of AMH migrants. At 74 ka the Sumatran Toba supervolcano erupted about 2,800 km3 of ash to blanket a vast area and cause global cooling that could have more than decimated migrating AMH groups. Alternatively the 60 ka ‘genetics’ date is not correct, as suggested by the minimum date of 65 ka for the earliest Australians. Such a conflict of evidence will surely spur further excavation: as one researcher observed about Laos, ‘There are thousands of caves to explore’.

See also: Coleman, J. 2023. Laos cave fossils prompt rethink of human migration map. Nature, v.618; DOI: 10.1038/d41586-023-01903-3; Ashworth, J. 2023. Fossils reveal early modern humans in southeast Asia 77,000 years ago. Natural History Museum’s Discover, 15 June 2023.

Did Precambrian BIFs ‘fall’ into the mantle to trigger mantle plumes?

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How the Earth has been shaped has depended to a large extent on a very simple variable among rocks: their density. Contrasts in density between vast rock masses are expressed when gravity attempts to maintain a balance of forces. The abrupt difference in elevation of the solid surface at the boundaries of oceans and continents – the Earth’s hypsometry – stems from the contrasted densities of continental and oceanic crust: the one dominated by granitic rocks (~2.8 t m-3) the other by those of basaltic composition (~ 3.0 t m-3). Astronomers have estimated that Earth’s overall density is about 5.5 t m-3 – it is the densest planet in the Solar System. The underlying mantle makes up 68% of Earth’s mass, with a density that increases with depth from 3.3 to 5.4 t m-3 in a stepwise fashion, at a number of discontinuities, because mantle minerals undergo changes induced by pressure. The remaining one third of Earth’s mass resides in the iron-nickel core at densities between 9.5 to 14.5 t m-3. Such density layering is by no means completely stable. Locally increased temperatures in mantle rocks reduce their density sufficiently for masses to rise convectively to be replaced by cooler ones, albeit slowly. By far the most important form of convection affecting the lithosphere involves the resorption of oceanic lithosphere plates at destructive margins, which results in subduction. This is thought to be due to old, cold oceanic basalts undergoing metamorphism as pressure increases during subduction. They are transformed at depth to a mineral assemblage (eclogite) that is denser (3.4 to 3.5 t m-3) than the enveloping upper mantle. That density contrast is sufficient for gravity to pull slabs of oceanic lithosphere downwards. This slab-pull force is transmitted through oceanic lithosphere that remains at the surface to become the dominant driver of modern plate tectonics. As a result, extension of the surface oceanic lithosphere at constructive margins draws mantle upwards to partially melt at reduced pressure, thus adding new basaltic crust at mid-ocean rift systems to maintain a form of mantle convection. Seismic tomography shows that active subducted slabs become ductile about 660 km beneath the surface and below that no earthquakes are detected. Quite possibly, the density of the reconstituted lithospheric slab becomes less than that of the mantle below the 660 km discontinuity. So the subducted slab continues by moving sideways and buckling in response to the ‘push’ from its rigid upper parts above. But it has been suggested that some subducted slabs do finally sink to the core-mantle boundary, but that is somewhat conjectural.

Typical banded iron formation

There are sedimentary rocks whose density at the surface exceeds that of the upper mantle: banded iron formations (BIFs) that contain up to 60% iron oxides (mainly Fe2O3) and have an average density at the surface of around 3.5 t m-3. BIFs formed mainly in the late Archaean and early Proterozoic Eons  (3.2 to 1.0 Ga) and none are known from the last 400 Ma. They formed when soluble iron-2 (Fe2+) – being added to ocean water by submarine hydrothermal activity –was precipitated as Fe3+ in the form of iron oxide (Fe2O3) where oxygen was present in ocean water. With little doubt this happened only in shallow marine basins where cyanobacteria that appeared about 3.5 Ga ago had sufficient sunlight to photosynthesise. Until about 2.4 Ga the atmosphere and thus the bulk of ocean water contained very little oxygen so the oceans were pervaded by soluble iron so that BIFs were able to form wherever such biological activity was going on. Conceivably (but not proven), that BIF-forming biochemical reaction may even have operated far from land in ocean surface water, slowly to deposit Fe2O3 on the deep ocean floor. After 2.4 Ga oxygen began to build in the atmosphere after the Great Oxidation Event had begon. That time was also when the greatest production of BIFs took place. Strangely, the amount of BIF in the geological record fell during the next 600 Ma to rise again to a very high peak at 1.8 Ga. Since there must have been sufficient soluble iron and an increasing amount of available oxygen for BIFs to form throughout that ‘lean’ period the drop in BIF formation is paradoxical. After 1.0 Ga BIFs more or less disappear. By then so much oxygen was present in the atmosphere and from top to bottom in ocean water that soluble iron was mostly precipitated at its hydrothermal source on the ocean floor. Incidentally, modern ocean surface water far from land contains so little dissolved iron that little microbiological activity goes on there: iron is an essential nutrient so the surface waters of remote oceans are effectively ‘wet deserts’.

Plots of probability of LIPs and BIFs forming at the Earth’s surface during Precambrian times, based on actual occurrences (Credit: Keller, et al., modified Fig 1A)

Spurred by the fact that if a sea-floor slab dominated by BIFs was subducted it wouldn’t need eclogite formation to sink into the mantle, Duncan Keller of Rice University in Texas and other US and Canadian colleagues have published a ‘thought experiment’ using time-series data on LIPs and BIFs compiled by other geoscientists (Keller, D.S. et al. 2023. Links between large igneous province volcanism and subducted iron formations. Nature Geoscience, v. 16, article; DOI: 10.1038/s41561-023-01188-1.). Their approach involves comparing the occurrences of 54 BIFs through time with signs of activity in the mantle during the Palaeo- and Mesoproterozoic Eras, as marked by large igneous provinces (LIPs) during that time span. To do this they calculated the degree of correlation in time between BIFs and LIPs. The authors chose a minimum area for LIPs of 400 thousand km2 – giving a total of 66 well-dated examples. Because the bulk of Precambrian flood-basalt provinces, such as occurred during the Phanerozoic, have been eroded away, most of their examples are huge, well-dated dyke swarms that almost certainly fed such plateau basalts. Rather than a direct time-correlation, what emerged was a match-up that covered 74% of the LIPs with BIFs that had formed about 241 Ma earlier. They also found a less precise correlation between LIPs associated with 241 Ma older BIFs and protracted periods of stable geomagnetic field, known as ‘superchrons’. These are thought by geophysicists to be influenced by heat flow through the core-mantle boundary (CMB).

The high bulk density of BIFs at the surface would be likely to remain about 15 % greater than that of peridotite as pressure increased with depth in the mantle. Such slabs could therefore penetrate the 660 mantle discontinuity. Their subduction would probably result in their eventually ‘piling up’ in the vicinity of the CMB. The high iron content of BIFs may also have changed the way that the core loses heat, thereby triggering mantle plumes. Certainly, there is a complex zone of ultra-low seismic velocities (ULVZ) that signifies hot, ductile material extending above the CMB. Because BIFs’ high iron-content makes them thermally highly conductive compared with basalts and other sediments, they may be responsible. Clearly, Keller et al’s hypothesis is likely to be controversial and they hope that other geoscientists will test it with new or re-analysed geophysical data. But the possibility of BIFs falling to the base of the mantle spectacularly extends the influence of surface biological processes to the entire planet. And, indeed, it may have shaped the later part of its tectonic history having changed the composition of the deep mantle. The interconnectedness of the Earth system also demands that the consequences – plumes and large igneous provinces – would have fed back to the Precambrian biosphere. See also: Iron-rich rocks unlock new insights into Earth’s planetary history, Science Daily, 2 June 2023

New drill core penetrates the Mohorovičić Discontinuity (the ‘Moho’)

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In 1909 Croatian geophysicist Andrija Mohorovičić examined seismograms of a shallow earthquake that shook the area around Zagreb. To his surprise the by-then familiar time sequence of P-waves followed by the slower S-waves appeared twice on seismic records up to 800 km away. The only explanation that he could come up with was that the first arrivals had travelled directly through the crust to the detector whereas the second set must have followed a longer path: it had travelled downwards to be refracted to reach the surface when it met rocks denser than those at the surface. His analysis revealed a sharp boundary between the Earth’s crust and its mantle at a depth of about 54 km below what was then Yugoslavia. Later workers confirmed this discovery and honoured its discoverer by naming it the Mohorovičić Discontinuity. Difficulty with pronouncing his name resulted in a geological nickname: ‘the Moho’. It can be detected everywhere: at 20 to 90 km beneath the continental surface and 5 to 10 km beneath the ocean floor, thus distinguishing between continental and oceanic crust.

In the late 1950s accelerating geological and oceanographic research that would culminate in the theory of plate tectonics turned its focus on drilling down to the Moho in much the same way as a lust for space travel spawned getting to the Moon. The difference was that the proposers of what became known as the Mohole Project were members of what amounted to a geoscientific glee club (The American Miscellaneous Society), which included a member of the well-financed US National Science Foundation’s Earth Science Panel. The idea emerged shortly after the Soviet Union had launched the Sputnik satellite and rumours emerged that it was proposing deep drilling into the continental crust beneath the Kola Peninsula.  The Mohole’s initial target was the 3.9 km deep floor of the Caribbean off Guadalupe in Mexico and required advanced methods of stabilisation for a new oceanographic ship that was to host the drilling rig.

Huge (tens of metres high) pillars or ‘chimneys’ of carbonates formed by the Lost City hydrothermal vent near the mid-Atlantic ridge (Credit: ETH Zurich)

The Mohole was spudded in 1961, but the deepest of five holes reached only 200 m beneath the sea floor. It recovered Miocene sediments and a few metres of basalt. Deep water drilling was somewhat more complicated than expected and about US$ 57 million was spent fruitlessly. The project was disbanded in 1966 with considerable acrimony and schadenfreude. Nonetheless, the Mohole fiasco made technical advances and did demonstrate the feasibility of offshore drilling. The petroleum industry benefitted and so did oceanography with the globe-spanning deep-sea drilling of ocean floor sediments. The sediment cores produced the 200 million-year exquisitely detailed record of climate change and vast amounts of geochemical data from the basaltic oceanic crust. In 2005 JOIDES (the Joint Oceanographic Institutions for Deep Earth Sampling) had another crack at the Moho. That venture centred on the intersection of the Mid-Atlantic Ridge and the Atlantis Fracture Zone close to the ‘Lost City’ hydrothermal vent. The area around the vent is the site of a huge low-angled extensional fault that has partly dragged the basaltic ocean crust off the mantle beneath causing it to bulge. This provided an excellent opportunity to drill through the Moho. All went well, but 54 days of drilling yielded 1.4 km of basalt but nothing resembling mantle rock. So, again, the Moho had thwarted Science (and research economics). But finally it is beginning to reveal it secrets (see: Voosen, P. 2023. Ocean drillers exhume a bounty of mantle rocks. Science, v. 380 (News) p. 876-877; DOI: 10.1126/science.adi9899

The area around the ‘Lost City’ vent was originally chosen for drilling to examine the chemical processes going on there. Hydrogen emitted by serpentinisation of mantle rocks can combine with carbon monoxide in hydrothermal fluids to create a wide variety of organic compounds, which could be the initial building blocks for the origin of life. As part of the International Ocean Drilling Programme JOIDES decided to launch IODP Expedition 399 to re-examine the area around ‘Lost City’ in more detail. The expedition first tried to continue drilling the 2005 hole, but failed yet again. Finally a new drill site aimed at penetrating the extensional detachment. Within a few days the drill bit punched into mantle rocks and over a 6-week period the expedition had recovered a kilometre of core. The technical accounts for each week of drilling give a flavour of what it must be like to be a part of such a ship-borne expedition as well as describing what emerged in the drill core. It seems like a bit of a jumble, dominated by the mineral olivine– the principal characteristic of the ultramafic mantle – almost pure in the rock dunite and mixed with pyroxenes in various kinds of peridotite. There are also coarse-grained rocks that contain plagioclase feldspar, which cut through the ultramafic materials – gabbros, troctolites and norites.  They are relics of intrusive basaltic magmas that did not make it to the seabed. The samples are variably altered by interaction with watery hydrothermal fluids, with lots of serpentine, talc and even asbestos: the drilling presented a health hazard for a few days. The rocks have been metamorphosed under pressure-temperature conditions of greenschist to amphibolite facies and subject to ductile deformation, probably because of the effect of extensional deformation. Whatever, there is plenty of material to be analysed, including for signs of microbial activity. So, the dreams of a 1950s academic drinking fraternity (they were all men!) have finally been realised. But since those pre-plate-tectonic times many geologists have seen and collected much the same, even putting their index fingers on the Moho itself in the time-honoured fashion. Intricate 3-D geology in ophiolite complexes such as that in Oman, provide such opportunities at the much lower cost of air travel, Land Cruiser hire and camping. Indeed what we know of the structure of the oceanic lithosphere – pillow lavas, sheeted dyke complexes, gabbro cumulates and serpentinised ultramafic mantle – has come from such bodies thrust onto continental crust at ancient plate margins. So, why the celebration in this case? They are the first samples of mantle from young oceanic lithosphere; the rocks of ophiolites may not have formed at mid-ocean ridges. These should give clues to the long-term magmatism that has created the vast abyssal basins that the mantle eventually reabsorbs by subduction. Then, of course, there is the link to biogenic processes at constructive margins that underpinned the return to the active hydrothermal venting at ‘Lost City’.

Flash: Huge rockslide imminent in Swiss village of Brienz

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The rockslide above Brienz in eastern Switzerland marked by a white surface bare of vegetation. Credit CHRISTOPH NÄNNI, TIEFBAUAMT GR, SWITZERLAND via the BBC

On 9 May 2023 the authorities of the Albula/Alvra municipality in the Swiss canton of Graubünden informed people living in the small village of Brienz that they must evacuate the area by 18 May as the threat of rock falls from the mountain beneath which they live had triggered a red alert. By 13 May all 130 dwellings had been abandoned.

The danger is posed by an estimated 5 million tons of rock associated with a developing landslide that is now estimated to be moving at around 32 m per year. The village itself had long been creeping down slope at a few centimetres each year, but recently its church spire had begun to tilt and buildings became riven by cracks. Seemingly, engineering attempts to mitigate the hazards have been unsuccessful, and large boulders have already tumbled into the vicinity of Brienz.

Being situated beneath a crumbling scree slope devoid of vegetation that had been developing since the last glaciation, the geological risk to the village comes as no surprise to its population and local authority. The local geology has a thick limestone resting on the thinly bedded Flysch – a metamorphosed sequence of fine-grained turbidites – from which groundwater escapes very slowly, thereby becoming lubricated. A curved (listric) failure zone has developed beneath the exposed mountainside, hence the danger. Acceleration on the listric surface began about 20 years ago.

At least the people of Brienz have been moved to safety, unlike 144 school children and adults in the mining village of Aberfan in South Wales. On 21 October 1966 they were crushed to death by coal-mining waste that suddenly flowed from waste tips on the steep valley side above the village. In that case no warning was given by the National Coal Board authorities who allowed  the tipping witout a thought for its geological consequences.

See also: Petley, D. 2023. The very large incipient rockslide at Brienz in Switzerland. The Landslide Blog (10 May 2023)

Origin of the genus Homo: a Paranthropus link?

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Reconstruction of a Paranthropus head (Credit: Jerry Humphrey, Pinterest)

Paranthropoids had large, broad teeth and pronounced cheekbones plus a bone crest on the top of their skulls that were the attachments for powerful jaw muscles, much as in modern gorillas. Unlike gorillas they were definitely bipedal and were more similar to australopithecines. They have been called ‘robust’ australopithecines but they were not significantly taller or heavier. The first to be unearthed at Olduvai, Tanzania in 1959 (Paranthropus boisei) was dubbed ‘Nutcracker Man’ by its finder, and many have implied that paranthropoids’ teeth and powerful jaws were signs of a vegetarian diet that needed a lot of chewing. Yet their teeth do not show the microscopic pitting associated with living primates that eat hard plant parts and nuts, or the heavy wear that results from eating grasses. They probably ate soft plants, such as semi-aquatic succulents or tubers, but meat-eating that causes little dental wear cannot be ruled out. Some specimens are associated with long bones of other animals whose ends are worn, suggesting that they may have used them as tools for digging. Plant remains found at paranthropoid sites suggests that they inhabited woodland, together with coexisting australopithecines. They were around in the form of three successive species from 2.9 to 1.2 Ma, outlasting australopithecines. The later paranthropoids coexisted with Homo habilis and H. erectus: they were clearly just as successfully adapted to their surroundings as were early humans.

In early 2023 evidence was published that associated Oldowan stone tools with remains of Paranthropus, together with deliberately defleshed and cut bones (see also): though association is not proof of a direct link. Interestingly, the hand of a P. robustus found in the Swartkrans cave system in South Africa is consistent with a human-like precision grip, i.e. it had an opposable thumb. Swarkrans also yielded the earliest evidence for the deliberate use of fire about 1.5 Ma ago, associated with remains of both P. robustus and H. erectus. All this suggests that a case could be made for paranthropoids’ being human ancestors – supporting evidence has just been published (Braga, J. et al. 2023. Hominin fossils from Kromdraai and Drimolen inform Paranthropus robustus craniofacial ontogeny. Science Advances, v. 9, article eade7165; DOI: 10.1126/sciadv.ade7165).

Fossil-bearing breccias beneath the floor of the Kromdraai cave in the Cradle of Humankind World Heritage Site 45 km NW of Johannesburg, South Africa yielded the first near-complete P. robustus skull in 1938, another being found in cave breccias at the nearby Drimolen quarry. These deposits also contained remains of four infants assigned to the species, whose teeth and cranial parts were at different stages of juvenile development (ontogeny). José Braga of the University of Toulouse, France and co-workers from South Africa and the USA compared this growth sequence with those teased out from immature specimens of Australopithecus africanus and early Homo.Their tentative conclusion is that Paranthropus robustus is more closely related to early humans than to australopithecines of the same stratigraphic age.

Skull of a probable adult female P. robustus (left) with that of H. habilis (centre) and A. africanus (right). Credits: all from Wikipedia pages

So, it now seems possible that paranthropoids are not ‘robust’ australopithecines in an acceptable, taxonomic sense. Their closer resemblance in early development to early humans, together with their association with early stone tools used for defleshing prey animals, together with evidence for possible their use of fire, further strengthens their candidacy for an ancestral link to humans. The best preserved skulls of Homo habilis and a female P. robustus (males of that species show the distinctive saggital crest) do show close similarities, that of a roughly contemporary A. africanus having distinctly wider cheeks than both. All three species were in life probably of much the same weight and stature (30 to 40 kg and 110 to 130 cm) but H. habilis had a significantly larger brain volume (500 to 900 cm3) than the other two (each ~450 cm3). However, this isn’t proof that the genus Homo evolved from a paranthropoid ancestor. That would require genetic evidence, unlikely to be extracted from specimens because DNA seems to degrade more quickly under the conditions of the tropics than at high latitudes. Debate on ultimate human origins will probably be endless. Perhaps it would make more sense simply to accept that early humans weren’t the only ‘smart kids on the palaeoanthropological block’, one of which left no issue after 1.2 Ma ago.

See also: Handwerk, B. 2023. Who made the first stone tool kits? Smithsonian Magazine, 8 February 2023, article 180981606

Music based on earthquake waves

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Many readers will have heard the vibration signal of an earthquake, as recorded by a seismometer, and replayed through a speaker: listen to some examples here. They are eerily like the sounds of falling, multi-storey buildings. Scary, especially if you think of the horrors of the devastation in SE Turkiye and NE Syria caused by the 6 February 2023 magnitude 7.8 event on the East Anatolian Fault system

Since P-waves are very like sound waves, audibly converting the one to the other is relatively simple. However, earthquakes are rarely single events, each major one being preceded by foreshocks and followed by aftershocks, both recurring over weeks or months. Highly active areas are characterised by earthquake swarms that can go on continuously, as happens with sea-floor spreading at mid-ocean ridges. In the case of Yellowstone National Park there are continual quakes, but there the seismicity results from magma rising and falling above a superplume. Most of such swarm-quakes are diminutive, so playing the speeded-up signal through a loudspeaker just sounds like a low, tremulous hiss.

Domenico Vicinanza a physicist at the Anglia Ruskin University in Cambridge UK specialises in creating music from complex scientific data, including those from CERN’s Large Hadron Collider in Geneva, to help interpret them. He has recently turned his hand to the Yellowstone earthquake swarm, converting the amplitudes and frequencies of its real-time seismograph to notes in a musical score: listen to the results here. They are surprisingly soothing, perhaps in the manner of the song of the humpback whale used by some to help with their chronic insomnia.

See also:  Davis, N. Rock concert: Yellowstone seismic activity to be performed on live flute, The Guardian; 8 May 2023

Extraction of ancient human DNA from artefacts

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The Denisova cave in southern Siberia is now famous for the evidence that it has provided for Neanderthals and Denisovans and their interbreeding based on DNA recovered from their bones, even a tiny finger bone of the latter. Indeed we would not know of the former existence of Denisovans without such a clue. Scientists at the Max Planck Institute for Evolutionary Anthropology in Leipzig, responsible for both breakthroughs, also pioneered the extraction of hominin DNA from soil in the cave. Now they have refined the intricate extraction of genetic material to such an extent that detailed hominin DNA sequences can be analysed from ornaments worn by ancient people, in much the same manner as applied in forensic studies of crime scenes (Essel, E. and 22 others 2023. Ancient human DNA recovered from a Palaeolithic pendant. Nature, early release 3 May 2023; DOI: 10.1038/s41586-023-06035-2).

Elk-tooth pendant found at Denisova cave, before cleaning and DNA extraction (top) and after the ‘washing’ procedure (bottom). Credit: Essel et al., Fig 1.

Russian archaeologists who continue to work at Denisova cave found a pierced pendant made from the tooth of a Siberian elk or wapiti during the 2019 field season. It was sent to Leipzig, where the palaeogenetics team had been trying to extract the DNA of whoever had worn personal artefacts found in French and Bulgarian caves. Their efforts had been unsuccessful, but such an object from Denisova clearly spurred them on. When someone wears next to the skin objects made of porous materials their sweat and the DNA that it carries seeps into the pores. If the materials decay very slowly, as do bone and especially teeth, genetic material can, in principle be extracted. But crushing up important ancient objects is not an option: for such rarities the extraction has to be non-destructive. It can only be done by ‘washing’ it in reagents that do not themselves break down DNA. Elena Essel and her many colleagues experimented with many ‘brews’ of reagents and repeated immersion at steadily rising temperature (up to 90°C). This releases genetic material in a stepwise fashion, allowing separation of contaminants in the host sediment from that which had penetrated into the tooth’s pores from whoever made the pendant and the wearer, and the animal from which it came

 Analysis of the recovered material yielded elk mtDNA, which was compared with that from four other ancient elks of known ages. This suggested that the elk had lived between 19 and 25 ka ago, thereby indirectly dating the time when the pendant was made and worn. A surprisingly large amount human DNA showed that the wearer was a female who was genetically allied with ancient anatomically modern humans who lived further east in Siberia at about that time.

Obviously this astonishing result opens up a wide vista for archaeology, though not from Palaeolithic burials, which are extremely rare. But artefacts of various kinds are much more common that actual human remains. Because the technique is non-destructive museums may be more willing to make objects in their collections available for analysis. Maybe the approach will be restricted to porous bone or tooth ornaments worn for long periods by individuals. Yet stone tools that were handled continually could be a more important target, depending on the rock from which they were made and its porosity.

See also: Lesté-Lasserre, C.. DNA from 25,000-year-old tooth pendant reveals woman who wore it. New Scientist, 3 May 2023.

Hydrogen and how the Earth formed

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A third piece with hydrogen as its focus in a couple of months? Well, from a galactic perspective there’s a lot of it about. Modern cosmology suggests that only 4.6% of the energy in the universe consists of elemental atoms made of protons, neutrons and electrons, dwarfed by dark energy and dark matter that are something of mystery. But of the more familiar energy equivalent, tangible matter (as in E=mc2), 74% of the universe is hydrogen, 24% is helium and the other 92 elements amount to just 2%. That tiny proportion of heavier elements was created by nucleosynthesis within stars from the two products of the Big Bang (H and He). Nuclear fusion reactions formed those with atomic numbers (protons in their nuclei) up to that of iron (26), whereas the heavier elements were created through neutron- and proton capture when the largest stars destroyed themselves cataclysmically as supernovae. Yet the planet whose surface we inhabit contains only minute amounts of helium and elemental hydrogen. Of course water at and beneath the surface, in the form of atmospheric vapour and locked within minerals retains some of the cosmically available hydrogen. But current estimates suggest that hydrogen accounts for a mere 0.03% of Earth’s mass. Despite the fact that some forms of radioactive decay generate alpha particles that become helium it forms a vanishingly small proportion of terrestrial mass.

The solar system formed around 4.6 billion years ago by a complex gravitational accretion of the gas and dust of an interstellar cloud: mainly H and He. Its dynamic collapse resulted in gravitational potential energy being transformed into heat: in the case of the Sun, sufficient to set off self-sustaining nuclear fusion. As a body grows in this way so does its gravity and thus the speed needed for matter to escape from its pull (escape velocity). As temperature increases so does the speed at which atoms of each element vibrate; the lower the atomic mass the faster the vibration and the greater the chance of escape. So the ‘blend’ of elements that an astronomical body retains during its early evolution depends on its gravity and its surface temperature. The Sun is so massive that very little has escaped its pull, despite a surface temperature of about 5 to 6 thousand degrees Celsius. Its composition is thus close to the cosmic average. Those of the giant planets Jupiter, Saturn, Uranus and Neptune are not far short because of their large gravities and low surface temperatures. Even today, the smaller Inner Planets are unable to cling on to elemental hydrogen and helium and nearly all that is left of the matter from which they formed is the 2% of heavier cosmic elements locked into solids, liquids and gases.

Processes in the early solar system were far more complicated than they are today. In the mainly gaseous disc, from which the solar system evolved, gravity dragged matter towards its centre. That eventually ignited nuclear fusion of hydrogen to form our star. More remote from its gravitational pull vortices aggregated dust into bodies known as planetesimals that in turn accreted to larger protoplanets. Solar gravity dragged gas from the inner solar system leaving rocky protoplanets, whereas gas was able to be attracted to the surface of what became the gas giants where their gravity outweighed that of the far-off Sun. This was complicated by a sort of Milankovich Effect on steroids in which protoplanets continuously changed their orbits and underwent collisions. The best known of these was between the protoEarth and a Mars-sized body that formed the Earth-Moon system, both bodies having deep magma oceans as a result of the huge energy focussed on them by the collision. What may have happened to the protoplanet that became Earth before the Moon-forming collision has been addressed by three geoscientists at the University of California Los Angeles and the Carnegie Institution for Science Washington DC, USA (Young, E.D. et al. 2023. Earth shaped by primordial H2 atmospheres. Nature, v. 616, p. 306–311; DOI: 10.1038/s41586-023-05823-0 [PDF request to: eyoung@epss.ucla.edu]).

A thick hydrogen-rich atmosphere’s interacting chemically with a protoplanet (left). A possible later stage (right) where iron oxide in the magma ocean of the Early Hadean after Moon formation oxidises a hydrogen atmosphere to form surface water (Credit: Sean Raymond 2023, Fig 1)

The focus of the work of Edward Young, Anat Shahar and Hilke Schlichting is directed at the possibility that the Earth-forming protoplanets originally retained thick hydrogen atmospheres. They use thermodynamic modelling of the equilibrium between hydrogen and silicate magma oceans that had resulted from the energy of their accretion. The authors’ main assumption is that insufficient time had elapsed during accretion for the protoplanets to cool and crystallise: a distinct possibility because loss of accretionary heat by thermal radiation would have been ‘blanketed’ by actively accreting dust and gas in orbit around the growing protoplanets. Effectively, the equilibrium would have been chemical in nature: reactions between highly reducing hydrogen and oxidised silicate melts or even vaporised rock evaporated from the very hot surface. The authors suggest that protoplanets bigger than Mars (0.2 to 0.3 times that of Earth) could retain a hydrogen-rich atmosphere long enough for the chemical reactions to come to a balance, despite high temperatures. There would have been no shortage of hydrogen at this early stage in solar system evolution: perhaps as much as 0.2% percent the mass of the Earth surrounding a protoplanet about half its present size.

Two outcomes may have emerged. Reaction between hydrogen and anhydrous silicates could produce H2O in amounts up to three times that currently in the Earth’s oceans, some locked in the magma ocean, some in the dense atmosphere. A by-product would have been iron oxide, giving the current mantle its oxidising properties known from the geochemistry of basaltic magmas.  Hydrogen might also have dissolved in molten iron alloys, thereby contributing to the nascent core. That second outcome would help explain why the modern core is less dense than expected for iron-nickel alloy, both solid and liquid. In fact densities calculated by geophysicists from the speeds of seismic waves that have travelled through the core are 5 to 10% percent lower than expected for the alloy. So the core must contain substantial amounts of elements with low atomic numbers.

Several other possibilities have been suggested to account for Earth’s abundance of water. Two popular ideas are comets arriving in the ‘settled’ times of the Hadean or by original accretion of hydrous chondrite meteorites, whose hydrogen isotope proportions match those of ocean water. Hydrogen as the light element needed in the core is but one possibility along with oxygen, sulfur and other ‘light’ elements. Also, the oxidising potential of the modern mantle may have resulted from several billion years of wet lithosphere being subducted. To paraphrase Sean Raymond (below), ‘other hypotheses are available’!

See also: Raymond, S.N. 2023. Earth’s molten youth had long-lasting consequences. Nature (News & Views), v. 616, p. 251-252; DOI: 10.1038/d41586-023-00979-1 [PDF request to: rayray.sean@gmail.com]

The Moon may have water resources in its soil

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Apart from signs of water ice in permanently shadowed areas of some polar craters, the Moon’s surface has generally been considered to be very dry. Rocks returned by the various Apollo missions contain minute traces of water by comparison with similar rocks on Earth. They consist only of anhydrous minerals such as feldspars, pyroxenes and olivines. But much of the lunar surface is coated by regolith: a jumble of rock fragments and dust ejected from a vast number of impact craters over billions of years. It is estimated to be between 3 and 12 m deep. Much of the finer grained regolith is made up of silicate-glass spherules created by the most powerful impacts.

The lunar regolith at Tranquillity Base bearing an astronaut’s bootprint (Credit: Buzz Aldrin, NASA Apollo 11, Photo ID AS11-40-5877)

The scientific and economic (i.e. mining) impetus for the establishment of long term human habitation on the lunar surface hangs on the possibility of extracting water from the Moon itself. It is needed for human consumption and as a source through electrolysis of both oxygen and hydrogen for breathing and also for rocket fuel. The stupendous cost, in both monetary and energy terms, of shifting mass from Earth to the Moon clearly demands self-sufficiency in water for a lunar base occupied for more than a few weeks.

Remote sensing that focussed on the ability of water molecules and hydroxyl (OH–) ions to absorb solar radiation with a wavelength of 2.8 to 3.0 micrometres was deployed by the Indian lunar orbiter Chandrayaan-1 that collected data for several months in 2008-9. The results suggested that OH– and H2O were detectable over a large proportion of the lunar surface at concentrations estimated at between 10 parts per million (ppm) up to about 0.1%. Where did these hydroxyl ions and water molecules come from and what had locked them up? There are several possibilities for their origin: volcanic activity that tapped the Moon’s mantle (magma could not have formed had some water not been present at great depths); impacts of icy bodies such as comets; even the solar wind that carries protons, i.e. hydrogen atoms stripped of their electrons. Conceivably, protons could react with oxygen in silicate material at the surface to produce both OH– and H2O to be locked within solid particles. To assess the possibilities a group of researchers at Chinese and British institutions have examined in detail the 1.7 kg of lunar-surface materials collected and returned to Earth by the 2020 Chinese Chang’e 5 lunar sample return mission (He, H. and 27 others 2023. A solar wind-derived water reservoir on the Moon hosted by impact glass beads. Nature Geoscience, online article; DOI: 10.1038/s41561-023-01159-6)

He et al. focussed on glass spherules formed by impact melting of lunar basalts, whose bulk composition they retain. The glass ‘beads’ contain up to 0.2 % water, mainly concentrated in their outermost parts. This alone suggests that the water and hydroxyl ions were formed by spherules being bathed in the solar wind rather than being of volcanic or cometary origin and trapped in the glass. An abnormally low proportion of deuterium (2H) relative to the more abundant 1H isotope of hydrogen in the spherules is consistent with that hypothesis. Indeed, the high temperatures involved in impact melting would be expected to have driven out any ‘indigenous’ water in the source rocks. The water and OH– ions seem to have built up over time, diffusing into the glass from their surfaces rather than gradually escaping from within.

An awful lot of regolith coats the lunar surface, as many of the images taken by the Apollo astronauts amply show. So how much water might be available from the lunar regolith? The Chinese-British team reckon between 3.0 × 108 to 3.0 × 1011 metric tons. But how much can feasibly be extracted at a lunar base camp? The data suggest that a cubic metre (~2 t) of regolith could yield enough to fill 4 shot glasses (~0.13 litres). Using a solar furnace and a condenser – the one in full sunlight the other in the shade – is not, as they say, ‘rocket science’. But for a minimum 3 litres per day intake of fluids per person, a team of 4 astronauts would need to shift and process roughly 100 m3 of regolith every day. Over a year, this would produce a substantial pit. But that assumes all the regolith contains some water, yet the data are derived from the surface alone …See also:Glass beads on moon’s surface may hold billions of tonnes of water, scientists say. The Guardian, 27 March 2023.