Magmatism and water in the mantle

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That Earth has always been such an active planet is largely due to water having continually being shifted into the mantle by subduction of oceanic lithosphere. Emplaced at temperatures around 1200°C the basaltic crust and ultramafic rocks of the lithospheric mantle become hydrothermally altered by interaction with the ocean, so that they contain a range of hydrous minerals. The mantle is estimated to contain between a quarter and four times the present volume of all ocean water. The vast bulk of the mantle is not undergoing partial melting at any one time. Most magmatic activity is linked to plate tectonics through linear belts such as those along oceanic rift systems and above subduction zones, with a small proportion at ocean islands above isolated mantle plumes and similarly, though more sparsely above hot spots below continents. Such plume-related magmatism has at intervals in the past been vastly bigger than now, forming flood-basalt provinces and ocean-floor plateaus, such as the Deccan Traps and the Ontong Java Plateau; some linked to extinction events. The particular role of water in mantle melting is its reduction in the temperature at which melting begins at depth: ‘dry’ mantle does not melt but remains solid, albeit ductile. Two recent studies have provided important insights into previously unsuspected roles that water can play deep in the mantle.

Four different modes of subduction. Credit: Li et al. Fig 6

Jianfeng Yang of the Chinese Academy of Science and colleagues from China and University of Padua, Italy provide evidence that ancient subducted slabs that gather at the mantle transition zone (MTZ) may trigger ocean-island  and ocean-plateau volcanism  (Yang, J. et al. 2026.  Subduction legacies in the mantle transition zone modulate intraplate oceanic volcanism. Nature Communications, in press; DOI: 10.1038/s41467-026-73403-7). In fact there is a multiplicity of modes wherein subducted slabs interact with the MTZ, some are retained within it while, in one way or another, others eventually pass through it to the deeper mantle.  Long-dead relics of subduction zones trapped there form ‘reservoirs’ of water 410 to 660 km below the surface at concentrations far higher (1 to 3 %) than does pristine mantle (less than 0.1%). It is stored as OH ions in dense mafic minerals, such as ringwoodite a high-pressure form of olivine (Mg2SiO4) containing up to 2.6 % of OH ions, and bridgmanite (MgSiO3), which forms once subducted slabs pass into the mantle transition zone. If that transformed lithosphere rises above about 410 km, such minerals transform back into anhydrous olivine, thereby liberating their water. At such depths, where temperature in the surrounding dry mantle is about 1800°C the emergence of water triggers a decrease in the temperature at which the ancient slab and also the surrounding mantle can melt. The authors cite evidence that such a process has contributed to the Azores oceanic plateau where the crust is 10 to 20 km thick. It is conceivable that a similar process of deep water ‘recycling’ may have been associated with continental flood basalts. Yang et al.’s new insight may also help unravel hitherto puzzling geochemical anomalies in other kinds of basaltic igneous rocks, such as those which well-up at mid ocean ridges to form modern oceanic crust.

Slabs that descend deeper into the mantle retain their dense mafic minerals and thus the water trapped within them. That water may eventually be involved in transformations at much higher pressures and temperatures, as deep as the core-mantle boundary. One possibility is their retention in mantle plumes that rise from the CMB to facilitate partial melting once they pass through the MTZ

See also: Subduction legacies shape intraplate ocean volcanoes. Scienmag, 20 May 2026

Teeth found in Chinese caves reveal a connection between Homo erectus and Denisovans

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Hominin fossils referred to as Homo erectus have been found in Africa, Central Europe and China. Those from Africa have also been attributed to H. ergaster and by some to ‘African H. erectus’– a point of lingering dispute and confusion. The African lineage spanned a long time period after appearing around 1.7 Ma ago, possibly to as late as 0.6 Ma. The confusion deepened with the discovery of similar, well-preserved remains at Dmanisi in Georgia that are actually older (1.77–1.85 Ma) than the African specimens. But they are so anatomically diverse that the five skulls might easily be assigned to five different hominin species had they been found at separate locations. Asia is again very odd in an H. erectus context. The species was first proposed in 1891 by Eugene Dubois from remains in sediments oof the Solo River near Trinil in Java – he originally suggested the name Pithecanthropus erectus.  Remarkably, the Solo sediments were dated at 53 to 27 ka in 2019, so did Homo erectus co-exist with anatomically modern humans (AMH) on Java? Similarly heavy-browed crania emerged from several sites in China. Curiously, the first complete Denisovan cranium, found near Harbin in China matched H. erectus in the eyebrow department. However, mtDNA from its dental plaque and bone proteomics tie in with those found in fragments from the eponymous Denisova Cave in Siberia and with a fragmentary mandible from Tibet. Without such evidence, and were itnot so young (146 ka), most palaeoanthropologists would probably have called the Harbin individual H. erectus, There are quite a few records of older Chinese hominin crania, dubbed H. erectus on anatomical grounds, including one dated at 1.7 Ma. There are a great many oddities and contradictions that need resolving.

Skulls of Homo erectus from Dmanisi , Georgia (credits; M.S. Ponce de Leon & P.E. Zollkofer, University of Zurich)
The cranium found near Harbin, China belonged to a Denisovan. Credit: Hebei Geo University

On 13 May 2026 a team led by palaeogeneticist Qiaomei Fu of the Chinese Academy of Sciences published data on proteins and amino acids yielded by enamel from six teeth of these ancient Chinese fossils, which emerged from three localities of  Middle Pleistocene age (580 to 400 ka) (Fu, Q. and 11 others 2026. Enamel proteins from six Homo erectus specimens across China. Nature, v.  10.1038/s41586-026-10478-8). One is from the Zhoukoudian Cave near Beijing, famous for ‘Peking Man’, the primary reference for the anatomy of Asian H. erectus. Older fossils are unlikely to yield meaningful data of this kind because of chemical degradation; the reason why DNA has so far proved elusive from these specimens. Tooth enamel is extremely durable and can protect proteins and amino acids. Since both are produced by genes on DNA they are proxies for variants of those long-vanished genes

The key protein in the supposed H. erectus teeth isameloblastin which is involved in the formation of tooth enamel. The ameloblastin of all six teeth shared two amino acid variants; one previously unknown in other hominin lineages and perhaps unique to H. erectus, the other has been identified in Denisovans. Fu and colleagues suggest that the original bearers of the teeth – presumed to be H. erectus – had interbred with Denisovans and passed on the second variant gene. In turn that had been passed on to Asian AMH with some of whom Denisovans had interbred; remarkably 21% of living people on the Philippine archipelago carry that gene. The authors go on further to suggest that their findings support the notion that H. erectus was the source by gene-flow for ‘super-archaic’ sections of DNA found in actual Denisovan DNA from one member of that group. That is certainly a possibility, but is not the only one.

Neither the proteomics nor the morphology of the teeth, nor the anatomy of the fossils that accompany them in any way prove that they are from actual 400 ka old Homo erectus individuals. That would require at the very least protein analyses from specimens that definitely pre-date the divergence of Denisovans from Neanderthals about 600 ka ago. Remarkably, proteins have been extracted from a ~1.8 Ma old tooth yielded by the Dmanisi H. erectus site in Georgia, but that failed to reveal anything useful in this context. Maybe future work on older Chinese hominin teeth could resolve the issue. Another hypothesis is that the bearers of the analysed teeth were a population of Denisovans who themselves developed genetic variations rather than inheriting them. Proteomics is at about the same stage in its development as human genomics was before 2010 in the run-up to discovering Neanderthal and Denisovan genomes. But in the case of H. erectus the problem began with biologists’ long record of trying to simplify the natural world, especially fossils, through ‘lumping’ rather than ‘splitting’.

Humans that science has designated as different species were capable of interbreeding over tens and hundreds of thousand years, probably repeatedly and maybe habitually. That fact makes it hard to defend the concept of their speciation. There were few environments where they could not thrive, yet their migrations spread small numbers over vast areas. Continually shifting, isolated populations would diverge genetically and physically, the more so the fewer individuals were banded together. Occasionally populations would meet: an opportunity for celebration, and more, for conscious beings facing the rigours of exploration with neither territory nor resources to defend.

See also: What Homo erectus teeth from three Chinese caves tell us about who we are. Anthropology.net, 13 May 2026; Curry, A. 2026. Ghost of long-extinct ancestor lives on in people today. Science, v.  392, p. 677-678; DOI:10.1126/science.zuwthcn; Timmer, J. 2026. Protein in Homo erectus teeth suggests Denisovans gave us some of their DNA. Arstechnica, 13 May 2026; Marshall, M. 2026. Ancient teeth hint at links between Denisovans and Homo erectus. New Scientist 13 May 2026

Surface temperature self-regulated by the Earth System during the Phanerozoic

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During the past 539 Ma (the Phanerozoic Eon) Earth’s geological history saw the explosion of rapidly evolving life in the oceans and on the land. The pace of that evolution swung up and down through a complex sequence of extinctions and adaptive radiations. They resulted from many intertwined inorganic changes: tectonics; impacts; igneous events; global climate change; atmosphere and sea-water composition. Although palaeoclimatic knowledge has become ever more detailed over the last few decades, its most important record, the varying temperature of the land surface and oceans, is lacking in precision. The timing of climatic events is not the issue, but the magnitude of changes in global mean surface temperature. The latter is largely down to the main tool in assessing past temperatures: the isotopic composition of oxygen (δ18O) in  marine fossils. In particular, the record for the Lower Palaeozoic has remained stubbornly odd. In the Cambrian and Ordovician Periods it implies that low-latitude seawater temperatures reached levels of 40 to 50 °C, that seem literally life threatening: phytoplankton at the base of modern marine ecosystems die at water temperatures above 35°C. Yet the fossil record is teeming throughout the Lower Palaeozoic at all latitudes. Some manner of imprecision in the oxygen-isotope method gives the impression of wild fluctuations and a dramatic overall cooling of the planet through the Phanerozoic: the temperature record as it stands seems implausible.

The carbonate-silicate cycle within the longer-term carbon cycle. Source: Wikimedia Commons

A group of palaeoclimatologists from China, the UK, Australia and the US have combined a variety of geochemical proxies, sedimentary records and climate modelling to correct the marine-carbonate δ18O record (Zheng, D. and 12 others 2026. Tight regulation of Earth’s long-term temperature over Phanerozoic time.  Nature Communications, in press 4 May 2026; DOI: 10.1038/s41467-026-72672-6). Their approach is based on a chemical index of alteration (CIA), i.e. a measure of the degree of chemical weathering of the source for sedimentary rocks. The CIA compares their content of immobile aluminium oxide (Al2O3) with calcium, sodium and potassium oxides that are more easily moved in solution. Analyses of recent river sediments show a positive correlation between CIA and local temperature, so CIA in ancient sedimentary rocks is a potential proxy for the ambient temperature of the region from which those sediments were derived. The CIA also depends on other factors, such as the intensity of physical erosion and transport. However, allowing for these factors in modern environments does not affect the correlation with ambient temperature: the method remains robust. The geochemical data from sedimentary rocks required to use CIA as an independent check on O-isotope derived temperature are available in abundance from all continents for most of the Phanerozoic.

The study by Zheng et al. suggests that throughout the Phanerozoic global mean temperature remained consistently within the 10 to 30°C range. Thus Palaeozoic ocean temperatures were comparable with those of the succeeding Mesozoic and Cenozoic Eras. The team concludes that various negative feedback processes inherent in the Earth System have been able to regulate its surface temperature through the Phanerozoic. The most important of these is climate-dependent silicate weathering in which acidic rain – produced by CO2 dissolved from the atmosphere – breaks down silicates to yield dissolved bicarbonate ions that combine with calcium and magnesium ions to precipitate carbonates. Such a process draws down the main greenhouse gas from the atmosphere. There are other aspects of the carbon cycle that also draw down atmospheric CO2 and reduce the greenhouse effect, such as burial of organic debris. Tectonics also shapes climate by modulating both silicate weathering and CO­2 emissions from volcanic activity.

It should be emphasised that anthropogenic global warming is proceeding at a far higher rate than natural negative feedback processes. We simply cannot rely on silicate weathering to reverse whatever climatic outcome results from what the current global economy does so very quickly. Yet the findings by Zheng et al. do seem likely to force a change in thinking about climate change on a geological timescale.

See also: Earth’s long-term temperature kept tight control. Scienmag; 4 May 2026

Were giant octopuses top predators during the Cretaceous?

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Octopuses defy common sense. They are invertebrate molluscs, so we don’t expect them to show well-developed intelligence, which they do. As well as tiptoeing around on their eight tentacles, they can move at high speed using a kind of backwards jet propulsion and some are even able to cross dry land.. Each of their tentacles has a sort of brain, as well their central one: distributed, semi-autonomous cognition in which their tentacles taste, touch, and move independently. Three hearts circulate their blue blood. Masters of swift camouflage using specialised skin cells that contain different coloured pigments, which behave like pixels in a TV screen. Octopuses can also rapidly manipulate their body texture and shape. They also seem to use such bizarre displays to communicate mood, at the very least.

Shape-shifting octopuses can squeeze through gaps far smaller than their own size to hide from both predators and their prey, which also makes them escapologists far outranking Harry Houdini. Their eyes look like those of goats, with horizontally linear pupils, although they evolved separately from the eyes of other animals.  Satan is said to have goat-eyes, hence the colloquial name for an octopus: devil fish. Mariners of old (and maybe some of the present day) reputedly feared giant octopuses to be capable of crushing ships and devouring the crew: the Kraken! Even small octopuses possess greater intelligence than a dog: some seem to enjoy playing, building dens, negotiating mazes and watching the antics of humans …

Since they are almost entirely soft flesh, the fossil record of octopuses is unsurprisingly meagre, apart from their jaws that use chitinous ‘beaks’ to munch their victims. The evolution of other cephalopods, for instance ammonites and squids, is better known from their external and internal skeletal remains and extends back to the Cambrian Period. So collecting and analysing fossil octopus jaws is the only option for palaeontologists. Shin Ikegami of Hokkaido University, together with Jörg Mutterlose of Ruhr University in Bochum Germany and eight other Japanese scientists, developed a new approach to supplement data on octopus jaws previously excavated from Cretaceous strata on Hokkaido and Vancouver Islands. (Ikegami, S. and 9 others 2026. Earliest octopuses were giant top predators in Cretaceous oceans. Science,  v. 392, p. 406-410; DOI: 10.1126/science.aea6285).

Cretaceous marine predators (at maximum estimated size) with a scuba diver for scale. Credit: After Ikegami et al. Fig. 4, and Jacobs 2026.

Ikegami et al. ground layer by layer through Japanese and Canadian sedimentary rocks to produce 3-D tomographic models of fossil beaks within them – quicker CT scanning proved inefficient in showing details. All the ‘beaks’ showed signs of wear from cracking the harder bones of their prey. Assuming that the same variation of beak size and body mass as in modern octopuses is relevant to those of Cretaceous age, the researchers came up with an astonishing result. Cretaceous octopuses reached huge sizes. They estimated one Nanaimoteuthis jeletzkyi to have been 19 metres long, roughly the size of an articulated truck. During the Cretaceous Period the top predator of the oceans had long been supposed to have been the formidable marine reptile Mosasaurus at around 15 m long.

We know what mosasaurs ate from fossilised stomach contents of two specimens: more or less anything, including other substantial marine reptiles, sharks, cephalopods and even other mosasaurs, some whole, some dismembered. As for Nanaimoteuthis and other giant Cretaceous octopuses, reconstructed from their fossilised beaks, there is little obvious evidence for what they ate, other than it would have had to have been in large amounts. Judging from the wear exhibited by their beaks at least a proportion of their diet was crunched up shells and bones of ammonites and fish. Modern octopus species, both small and moderately large, have other sorts of feeding strategies. Some eat planktonic animals, others drill holes in shells and suck out their innards rendered to the texture of a ‘smoothie’ by corrosive saliva.

It is not surprising that the media have made quite a fuss of these Cretaceous ‘krakens’, some suggesting that they preyed on formidable marine reptiles such as mosasaurs. That would definitely have made them ‘top’ marine predators. Yet such massive, moving mounds of nourishing flesh would have made them a worthwhile catch for a whole school of toothy reptiles and sharks. The modern sperm whale is known to devour giant squid at the great depths to which they can dive, as witnessed by numerous cephalopod beaks in their stomachs. So it is equally possible that the octopus beaks found in Cretaceous sediments of Hokkaido were excreted by marine reptiles

See also: Jacobs, P. 2026. Truck-size octopuses stalked Cretaceous seas. Science, v. 392, 23 April 2026; DOI: 10.1126/science.z8o79rn; Devlin, H. 2026. ‘Kraken-like’ giant octopuses 100m years ago crunched bones of prey. The Guardian, 23 April 2026.

Rapid human genetic changes after the last Ice Age

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Our hominin ancestors in Africa first fashioned tools about 3.5 Ma ago. Since then regular intake of animal protein through hunting, followed by the later discovery of fire and cooking, may progressively have encouraged the evolution of larger hominin brains. Both behavioural leaps would have reduced the length of the ‘working day’ needed to sustain hominin groups. That would have lengthened opportunities for cognitive reflection. social life and culture, and thus further evolution. They also expanded the opportunities for migration, beginning with Homo ergaster venturing beyond Africa at least 1.8 Ma ago. Hominins evolved to such an extent that several separate species occupied our home world at any one time until about 45 thousand years ago. After that only H. sapiens occupied Africa, Eurasia and Australasia.

Such protracted and meandering evolution and dispersal clearly involved episodic physiological and cultural changes, but all we have to go on are fragmentary fossil remains and artifacts of various kinds. DNA has yet to be extracted from hominin bones older than 400 ka (an early Neanderthal from northern Spain). Though H. sapiens first appeared in Morocco about 300 ka ago, DNA from our species dates back to only 45 ka (western and central Europe). What is today termed ‘ancient’ human DNA, is actually very young and restricted to climate zones where its decomposition has been slow. At present there is little point in analysing fossil material from tropical and subtropical latitudes; the DNA is degraded beyond recovery by even the most up-to-date techniques. Fascinating as discussion of human evolution is, in reality most is merely inferred from comparative anatomy and anthropological interpretation.

By 45 ka the heavy evolutionary lifting had been done, resulting in anatomically modern humans, but we have little, if any, chance of explaining in genetic terms how it was achieved. There has been much speculation about the conditions, particularly climatic ones, which may have driven the changes. During the last 2.6 Ma – the Quaternary Period – global climate has been the most changeable in the last 300 Ma. Ice ages have come and gone, first in 40 ka cycles and during the last million years every 100 ka. Much more rapid changes, such as millennial Dansgaard-Oeschger cycles, appeared during each glacial episode, the last being the Younger Dryas between 12.9 and 11.7 ka. For a long while ideas on the drivers of human evolution have been dominated by those concerning environmental stress. Unsurprisingly, genetic change has also been ascribed to such a Darwinian-ecological cause: adaptability to adversity. To test such a hypothesis requires genetic data, of course. But, except for the climatically more stable Holocene Epoch since 11.7 ka, ancient human genomes are in very short supply.

Renowned researcher into ancient human genetics David Reich of the Harvard Medical School in Boston, USA has collated more than 15 thousand ancient human genomes extracted from the remains of individuals who lived and died in Europe and parts of the Middle East during the last ten thousand years. These have been analysed statistically in the context of ‘directional selection’. This is a type of natural selection that occurs when one version of a gene – an allele – confers an extreme form of a trait. If it proves advantageous it rapidly gets passed on to more descendants than do less advantageous alleles, and thus rises in frequency across a population. This differs from other causes of gene frequency changes, such as human migration, population mixing, and random genetic fluctuations that occur in small populations. Well-known examples of directional selection are rapid changes among European Peppered moths, African cichlid fish, Alaskan Sockeye salmon and Big Cats which change over time in response to variations in their habitats. A human example is a genetic variant that maintains the ability to digest the sugar lactose in milk beyond infancy, which enables many modern Europeans to digest milk throughout their lives. The algorithm needed to separate signs of directional selection from other types of genetic change was developed by Ali Akbari, a computational geneticist also at Harvard Medical School. A recent paper by Akbari, Reich and colleagues in the US, Iran, Germany, and Austria (Akbari, A. and 15 others 2026. Ancient DNA reveals pervasive directional selection across West Eurasia. Nature, advance online publication; DOI: 10.1038/s41586-026-10358-1) seems set dramatically to change the research into recent human evolutionary genetics.

Akbari et al. discovered that directional selection has driven the spread or decline of hundreds of gene variants in human populations throughout Western Europe in the last ten millennia. In particular, selection accelerated with the adoption of farming rather than a hunter-gatherer lifestyle.  Among the gene variants are those connected with light skin, red hair, risk of celiac disease – linked to gluten in cereals –  susceptibility to gout, resistance to leprosy, baldness, rheumatoid arthritis and alcoholism. There are many more (see Figure 3 in the paper): the team identified 479 gene variants affected by directional selection, some that can be explained by changes in lifestyle, others less explicable and yet more that underlie complex traits such as mental illness and cognition. Some of the variants sprang up and were sustained in the population, others rose and then dwindled. The Neolithic began a period of fundamental life style changes in Europe, summed up as a shift from hunting and foraging to farming of cereals and livestock, as early as about 10 ka ago in what is now Türkiye. The pace of genetic changes of this kind reached a peak around the Bronze Age, perhaps because human activities in Europe became more complex then with the mass migration westwards of Yamnaya horse- and wagon-using people from the steppes to dominate Europe

The shift from small wandering bands to living in settlements was a drastic change from a lifestyle that had continued throughout all previous human history. So, it is hardly surprising that there was a major shift in humans’ genetic makeup. But such a change in human labour was not unique to Europe and is known to have occurred on all inhabited continents, with the exception of Australia, at different times during the Holocene. Other regional genetic databases can be analysed in much the same way, once sufficient ancient DNA is collected in Asia, Africa and the Americas. Yet not much is available. The authors comment: ‘A variant that now correlates to household income or years of schooling [remarkably, there are some!] had to have meant something different in the Stone Age. So these results do not mean that Europeans evolved to be smarter or healthier.’  Moreover, the research results in the paper seem likely to be amplified as the data set is so large and complex.

See also: Dutchen, S. et al. 2026. Massive Ancient-DNA Study Reveals Natural Selection Has Accelerated in Recent Human Evolution. Harvard Medical School: News & Events. 15 April 2026; Callaway, E. 2026. Landmark ancient-genome study shows surprise acceleration of human evolution. Nature, v. 652, News, 15 April 2026

What caused the Younger Dryas frigid spell: case closed?

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Around 20 thousand years ago, the Earth began to emerge from the grip of the Last Glacial Maximum (LGM). Huge ice sheets had locked up so much water that sea level was then about 125 m lower than it is today. At 12,870 years ago the warming and sea-level rise were reversed for 1,170 years in the Northern Hemisphere: an episode of near-full glacial conditions known as the Younger Dryas (YD). The adjectives ‘sudden’ or ‘abrupt’ grossly understate the pace of initial cooling – 3°, 6° and 15° C in North America, Europe and Greenland, respectively. Isotopic evidence from Greenland ice cores suggest that the cooling took place over three years or less. Such a degree of precision stems from the continuous annual layering in the Greenland ice cap. As far as humans were concerned, this would have been catastrophic for hunter gatherers following game northwards in Eurasia and North America as conditions ameliorated during the seven thousand years since the LGM. The archaeological record, or rather the lack of one, for what are now temperate zones suggests humans either retreated south or were blotted out.

There is no counterpart for the YD in the end stages of early glacial episodes. Some authors have suggested that it was the outcome of an appropriately catastrophic geological event, such as a large meteorite strike, as proposed in 2007 (See: Whizz-bang view of Younger Dryas; July 2007). This hypothesis gained traction in 2013, at least for its authors, with the discovery of anomalously high concentrations of the noble metal platinum (Pt) and other platinum Group metals, such as iridium (Ir) at or around the start of the YD in the GISP2 ice core. New research on this anomaly (Green, C.E. et al 2025. A possible volcanic origin for the Greenland ice core Pt anomaly near the Bølling-Allerød/Younger Dryas boundary. PLOS One, v. 20, article  e0331811; DOI: 10.1371/journal.pone.0331811) offers a different scenario. Charlotte Green of Royal Holloway, University of London and colleagues from universities in the UK, Germany and Austria examine the timing of this Pt spike and its detailed geochemistry.

The ‘killer’ observation is that the anomaly occurs in ice that formed 45 years after the onset of the Younger Dryas and has a spread of about 14 years. Whatever kind of event released the platinum, it definitely did not somehow trigger the onset of the YD. Moreover, the anomaly was significantly deficient in iridium compared with a wide range of meteorites and terrestrial igneous rocks. It also differed markedly in other elements, such as lutetium and hafnium, and in all three elements in melt rocks and ejecta sediments associated with five proven impact structures. The closest match is to volcanic gas condensates from a recent eruption of a submarine volcano near Tonga

Both the GISP2 and NGRIP cores through the Greenland ice also record a large, 12-year long spike in sulfate of volcanic origin spread across the very start of the YD. That roughly matches the age of an explosive eruption, which formed the circular Laacher See in the Eifel volcanic field in Germany. That eruption is thought to have blasted 6.3 km3 of highly alkaline magma into the atmosphere: about the magnitude of the 1991 Pinatubo eruption, but insufficient to yield the size and duration of the sulfate spike that coincides with the start of the YD. The sulfate anomaly suggests a far larger, currently unknown eruption at 12,870 years ago. The Pt and Ir data from the Laacher See event rule it out as a source for the younger Pt anomaly in the GISP2 ice core. One possibility is a nearby Icelandic subglacial fissure eruption at that time.

So, as regards what started the Younger Dryas, there is support for a very large, but so-far unknown volcanic event, and an as yet unresolved, perturbation in the Atlantic Meridional Overturning Circulation (AMOC) resulting from drainage of a huge glacial lake in northern North America (see: The Younger Dryas and the Flood; June 2006), but no support whatever for an impact event. Climatology of the distant past is always likely to be difficult to pin down. That is because, as now, it involves linkages between a large number of variables: not only physical ones, but issues of biogeochemistry, the inner Earth, the rest of the solar system and even cosmology. That is, it is as complex as human affairs and their history. Common sense, linear thinking and the like, simply will not do.

See also: Scientists solve 12,800-year-old climate mystery hidden in Greenland ice. Science Daily, 20 March 2026

How do subducted slabs accumulate at different mantle depths?

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Seismic tomography provides no evidence that slabs of oceanic lithosphere descend intact through the whole mantle to the core-mantle boundary. It might once have happened when they were capped by abundant high-density rocks, such as Precambrian banded-iron formations. A great many actively descending slabs have been shown to cease sinking, slide sideways and accumulate at depths around 660 and 1000 km. Until recently these discontinuities were been generally ascribed to transitions in the structure of the dominant mafic mineral olivine (Mg2SiO4) in mantle peridotite induced by increasing pressure and temperature. The resulting increases in mantle density supposedly form barriers to further slab descent. Pressure-induced mineral transitions in the slabs themselves that increase their density, such as pyroxene to garnet, may somehow be inhibited thereby leading to stagnation in slab descent. That may be true for the 660 km discontinuity, but for stagnation at 1000 km deep no such density-changing mineral transitions have shown up in high-P high-T mineralogical experiments. Some other process must therefore be responsible for slab descent to that depth. Recent work by geoscientists at several universities in China gives insights into what may be going on (Li, J., Li, K., Li, J. et al 2026. Dual slab stagnation depths controlled by grain-size-induced sporadic low-viscosity zones at around 1000 km depth. Nature Communications DOI: 10.1038/s41467-026-69987-9).

Four different modes of subduction at island arcs. Credit: Li et al. Fig 6

Jing Li and colleagues have focussed on the possibility that changes in the bulk viscosity of the mantle may play an important role. Their approach is twofold: experimental mineral physics and geodynamic modelling. Results suggest that recrystallization in the mantle when deeply penetrating slabs pass through it may patchily reduce the mantle’s grain size and thus its viscosity; the more so with larger volumes of subducted slab material. In turn, the resulting physical heterogeneity probably disrupts the steady downward passage of the slabs; fine-grained, less viscous zones ‘lubricating’ slab penetration, unchanged zones hindering it. The authors link such hypothetical micro-structural processes to modes of subduction that are currently active. They consider four modes of active subduction beneath island arcs with either a slow or a fast rate of trench retreat (see Figure). A slowly retreating trench system combined with low-viscosity patches at depth (Mode 1) results in penetration below 660 km and slab stagnation at 1000 km. Slow trench retreat with a homogenous lower mantle (Mode 2) gives rise to penetration and buckling of the descending slab between 660 and 1000 km. Fast trench retreat with a deeper low-viscosity zone (Mode 3), or with a homogeneous lower mantle (Mode 4) both result in slab stagnation at 660 km.

The models developed by Jing Li et al convincingly simulate various results of seismic tomography beneath island arcs. Interestingly, they suggest that the eventual assimilation of older slab materials into the deeper mantle (‘fossil’ slabs) may play a major role in mineral comminution and reduced mantle strength. That may leave behind low viscosity zones that later subduction may exploit. In fact, there are signs of possible fossil slabs in seismic tomograms more than 1000 km below the present Pacific Ocean floor in the form of zones of high P-wave velocity.

This work shows that plate tectonics is far from ‘done-and-dusted’, the mantle being far from uniform in its properties. Li et al’s results potentially open up new insights into whole-mantle convection, in which older tectonic events influence plate motions that are currently operating and the triggering of plumes rising from the deepest mantle. It also hints that such complex physical mixing of subducted material into the mantle may have resulted in the geochemical heterogeneities that increasingly emerge from analysis of magmas with ultimate origins in the mantle.

See also:Grain Size Creates Dual Slab Stagnation Zones at 1000 km. Scienmag 3 March 2026

Annual logs update

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Sorry for the lack of posts over the last few weeks.

I have finally updated all annual logs from 2022 to 2025.

You can download the new PDF compilations using the menu bar, if you wish.

Hopefully, normal service will be resumed soon, depending on the journals’ contents, of course …

Cheers

Steve Drury

Annual logs update

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Followers can now download newly posted annual logs for Human Evolution and Migrations covering the years 2022 to 2025. By downloading them you can get a clear idea of how palaeoanthropology has moved forward since the Covid pandemic.

Enjoy the experience if you have the time and inclination!

Steve Drury

Vanished continents of the Hadean Eon: the zircon key

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Over the last few decades improved analytical techniques have made it possible to analyse tiny mineral grains for a variety of trace elements and several isotopes. Zircons obtained directly from crushed granitic igneous rocks vary in chemistry according to the magmatic processes that generated them and their tectonic context. Elevated ratios between uranium and niobium (U/Nb) and scandium and ytterbium (Sc/Yb) are characteristic of zircons in intermediate granites. These contain 52 to 63 % SiO2 – between mafic and felsic magmas – which formed by melting of hydrated mafic crust in settings akin to modern continental arcs; i.e. in subduction zones. But such partial melting can also take place where the base of continental crust delaminates and ‘drips’ into the mantle. That process is part of what is known as stagnant lid tectonics, believed by many to have been important in the Palaeoarchaean and Hadean. Such a process would have involved nearly anhydrous conditions and thus different geochemical partitioning of elements in the magmas and minerals that crystallised from them. Exposures of crystalline continental crust become increasingly rare further back in geological time, and there are none older than 4.0 Ga – i.e. of Hadean age – with a granitic component. Consequently studying the generation of continental crust in the Hadean and the early Archaean is almost entirely dependent on ancient zircons that found their way into much younger sedimentary rocks. The most famous of these occur as detrital grains in the 3.6 Ga Jack Hills conglomerate of Western Australia. Others have been extracted from similar ~3.3 Ga sedimentary rocks in the Barberton Greenstone Belt of South Africa and Eswatini.

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)

John Valley of the University of Wisconsin-Madison, USA, and co-workers from the US, Germany, Australia and France have worked on a large number of zircons newly extracted from Jack Hills. They have radiometrically dated them, and analysed Nb, Sc, U and Yb trace elements and hafnium (Hf) and oxygen isotopes Together with data from earlier studies, including Barberton zircons, they have teased out some remarkable insights into  ‘continent-forming’ magmatism as far back in time as 4.4 billion years ago (Valley, J.W. and 11 others 2026. Contemporaneous mobile- and stagnant-lid tectonics on the Hadean Earth. Nature, Open access; DOI: 10.1038/s41586-025-10066-2). More than 70% of the >4.0 Ga Jack Hills zircons have elevated U/Nb and Sc/Yb ratios, which suggest that they formed in a setting akin to continental-arc subduction (CAS) zones, to produce now-vanished Hadean continental crust. The remainder seem to represent processes at mid-ocean ridge (MOR) and oceanic island (OI) settings. In contrast, the bulk of Barberton zircons of Hadean age show OI affinities, with only around 22% showing Nb–Sc–U–Yb signatures of probable CAS origins. From about 4.4 to 3.8 Ga two distinct forms of continental crust generation seem to have operated on Earth. In the erosional source region for the Barberton zircons their host granites seem to have formed during the Hadean and Eoarchaean by remelting of foundered lower crust, i.e. probably in a stagnant-lid-like tectonic setting. But at around 3.6 Ga they ‘flip’ to a subduction-like setting. The zircons yielded by Jack Hills conglomerates suggest substantially different conditions: alternating CAS and OI settings during the Hadean and a fall-off in crust generation during the Eoarchaean (4.0 to 3.8 Ga).

Plots of Sc/Yb and U/Nb against ages of zircons (vertical scale logarithmic). Black points are from Jack Hills, red from Barberton. The yellow field represents zircons formed in subduction zones; green suggests stagnant lid tectonics; grey the overlap between the two settings. Credit: Valley et al. Fig 3 a and b.

The mixed Hadean zircon signatures from Jack Hills possibly indicate that they were derived by erosion and transport from several distinct terranes that had been generated by two different processes: some kind of upper crustal recycling and stagnant lid tectonics. Meanwhile, that part of the Hadean Earth represented by the Barberton zircons may have been a long-lived regime of stagnant lid tectonics, replaced by dominant subduction at the end of the Eoarchaean.  Yet the data suggest that into the Palaeoarchaean (3.6 to 3.2 Ga) and perhaps later, lid tectonics continued to operate somewhere, but at no time after 4.4 Ga was the Earth entirely subject to lid tectonics. Likewise, the authors insist that subduction was not of the plate-tectonic style, referring to some form of recycling of hydrated upper crustal mafic and ultramafic rocks into the mantle to undergo partial melting. Plate tectonics as we know it probably developed later in the Archaean. The early Earth had much higher heat flow than in later times, and thus the lithosphere was more ductile rather than brittle. The essence of modern tectonics is a series of rigid plates that extend down to the asthenosphere. When they deform it is largely through brittle failure of the entire lithosphere.