Changing Atlantic Ocean currents may threaten Gulf Stream warming of Europe

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Climate during the last Ice Age was continually erratic. Generally fine-grained muds cored from the floor of the North Atlantic Ocean show repeated occurrences of layers containing gravelly debris. These have been ascribed to periods when ice sheets on Greenland and Scandinavia calved icebergs at an exceptionally fast rate, to release coarse debris as they melted while drifting to lower latitudes. These ‘iceberg armadas’ (known as Heinrich events) left their unmistakable signs as far south as Portugal. Their timing correlates with short-lived (1 to 2 ka) warming-cooling episodes (Dansgaard-Oeschger events) recorded in Greenland ice cores that involved variations in air temperature of up to 15°C. The process that resulted in these sudden climate shifts seems to have been changing ocean circulation brought about by vast amounts of fresh water flooding into the Arctic and North Atlantic Oceans. This lowered seawater density to the extent that its upper parts could not sink when cooled. It is this thermohaline circulation that drags warmer surface water northwards, known as the Atlantic Meridional Overturning Circulation (AMOC), part of which is the Gulf Stream. When it fails or slows the result is plummeting temperatures at high latitudes. The last major AMOC shutdown was after 8 ka of warming that followed the last glacial maximum. Between 12.9 and 11.7 ka major glaciers grew again north of about 50°N in the period known as the Younger Dryas, almost certainly in the aftermath of a flood to the Arctic Ocean of glacial meltwater from the Canadian Shield. Around 8.2 thousand years ago human re-colonisation of Northern Europe was set back by a similar but lesser cooling event.

The Atlantic Meridional Overturning Circulation (AMOC). Red – warm surface currents; cyan – cold deep-water flow. (Credit: Stefano Crivellari)

Three researchers at Utrecht University, the Netherlands have issued an early warning that the AMOC may have reached a critical condition (Van Westen, R.M., Kliphuis, M & Dijkstra, H.A. 2024. Physics-based early warning signal shows that AMOC is on tipping course. Science Advances, v. 10, article adl1189; DOI: 10.1126/sciadv.adk1189). Previous modelling of AMOC has suggested that only rapid, massive decreases in the salinity of North Atlantic surface water near the Arctic Circle could shut down the Gulf Stream in the manner of Younger Dryas and Dansgaard-Oeschger events. René van Westen and colleagues have simulated the effects of steady, long-term addition of fresh water from melting of the Greenland ice sheet. They ran a sophisticated Earth System model for six months on the Netherlands’ Snellius super computer. Their model used a slowly increasing influx of glacial meltwater to the Atlantic at high northern latitudes.

The various feedbacks in the model eventually shut down the AMOC, predicted to result in cooling of NW Europe by 10 to 15 °C in a matter of a few decades. Yet to achieve that required the model to simulate more than 2000 years of change. It took 1760 years for a persistent AMOC transport of 10 to 15 million m3 s-1 to drop over a century or so and reach near-zero. That collapse involved around 80 times more melting of Greenland’s ice sheet than at present. Yet their modelling does not take into account global warming: including that factor would have exceeded their budgeted supercomputer time by a long way. Melting of the Greenland ice sheet is, however, accelerating dramatically

Van Westen et al. have shown the possibility that steadily increasing ice-sheet melting can, theoretically, ’flip’  the huge current system associated with the Atlantic Ocean, and with it regional climate patterns. The tangible fear today is of a more than 1.5°C increase in global surface temperature, yet a warming-induced failure of AMOC may cause local annual temperatures to fall by up to ten times that. Rather than the currently heralded disappearance of sea-ice from the Arctic Ocean, it may spread in winter to as far south as the North Sea. The only way of forecasting in detail what may actually happen – and where – is ever-more sophisticated and costly modelling of ocean currents and ice melting in a warming world. Uncertain as it stands, the work by van Westen and colleagues may well be ignored: perhaps as a ‘thing we dinnae care to speak aboot’.

See also: Le Page, M. 2024. Atlantic current shutdown is a real danger, suggests simulation. New Scientist, 9 February 2024; Watts, J. 2024. Atlantic Ocean circulation nearing ‘devastating’ tipping point, study finds. The Guardian, 9 February 2024.

Earliest evidence for rope making: a sophisticated tool

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Even at my age, if I rummage through pockets of various bits of outdoor clothing there’s a good chance I’ll find a handy length of string that I have scavenged at some time. It’s a just-in-case thing, which I learned from my father and grandad. One can hardly imagine a hunter-gatherer not having string or lengths of sinew for that very reason. Cordage has many other uses than merely securing something: bags, mats, nets, snares, fabric, baskets, huts made of sticks and fronds, and even watercraft. Yet archaeological evidence for twine is exceedingly rare. The oldest known string – made of bark fibres – was found wrapped around a stone tool at a 52 to 41 ka Neanderthal site in the Rhône valley 120 km north-west of Marseille. Rope is somewhat more difficult to make as it requires twisting together several lengths of simpler cordage. Once that skill is cracked a rope maker is on the verge of engineering!

The reassembled rope-making tool from Hohle Fels Cave (Credit: Conard & Rots, Fig 2)

In 2015 archaeologists unearthed several pieces of worked mammoth ivory from the Hohle Fels Cave in SW Germany. They were dated to between 40 to 35 ka and associated with Aurignacian stone tools made by modern humans. Fifteen pieces could be fitted together to yield a 20 cm long ‘baton’. First believed to be some kind of ritual object, the fact that 4 circular holes had been bored through the ‘baton’ suggested it must have had some practical use, perhaps for straightening wooden shafts. Then it became clear that each hole was surrounded by spirals of carefully carved, V-shaped notches. Nicholas Conard and Veerle Rots of the University of Tubingen realised that the object may have been used for making rope using a technique known from the Egyptian pharaonic period into medieval times (Conard, N.J. & Rots, V. 2024. Rope making in the Aurignacian of Central Europe more than 35,000 years ago. Science Advances, v. 10, article adh5217; DOI: 10.1126/sciadv.adh5217).

Frame from a movie showing how the tool may have been used to make ropes. The three ‘feeders’ twist foliage clockwise whereas the fourth pulls and imparts an anticlockwise twist to the three stands. (Credit: Conard & Rots, Supplementary material, Fig S15)

After a little practice, four people were able to make sturdy rope using a replica of the tool. Three twisted together fibrous materials, such as the stems and leaves of bulrushes (Typha), and pushed the rough cordage through the intact holes. A fourth person pulled the cordage through and counter-twisted the three strands into rope about 1.5 cm thick – thicker rope would also have required a tool with more holes and more operators. The spiral grooves maintained the initial clockwise rotation of each strand of cordage, so that when all three were twisted together in an anticlockwise sense the counter rotation held the rope together firmly. Remarkably, the small team were able to produce 5 m of rope in 10 minutes. Other common kinds of fibrous plant material, such as linden and willow were used successfully. Incidentally, the tool squeezed edible starch from the foliage of bulrushes. But it seems that this particular rope-making took only performed well for coarse materials. Making rope from finer firbres, such as animal sinew, nettle, flax and hemp would probably have required another design with smaller holes.

A movie of the manufacturing process can be downloaded.

An astronomical background to flood basalt events and mass extinctions?

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Michael Rampino and Ken Caldeira of New York University and the Carnegie Institute have for at least three decades been at the forefront of studies into mass extinctions and their possible causes, including flood-basalt volcanism, extraterrestrial impacts and climate change. As early as 1993 the duo reported an ubiquitous 26-million year cycle in plate tectonic and volcanic activity. In Rampino’s 2017 book Cataclysms: A New Geology for the Twenty-First Century the notion of a process similar to Milutin Milankovich’s prediction of Earth’s orbital characteristics underpinning climate cyclicity figured in his thinking (see Shock and Er … wait a minute, Earth-logs, October 2017). Rampino postulated then that this longer-term geological cyclicity could be linked to gravitational changes during the Solar System’s progress around the Milky Way galaxy. He was by no means the first to turn to galactic forces, Johann Steiner having made a similar suggestion in 1966. The notion stems from the Solar System’s wobbling path as it orbits the centre of the Milky Way galaxy about every 250 Ma, which may result in its passage through a vast layered variation in several physical properties aligned at right angles to galactic orbital motions. This grand astronomical theory is ‘a story that will run and run’; and it has. It is possible that the galaxy has corralled dark matter in a disc within the galactic plane, which Rampino and Caldeira latched onto that notion a year after it appeared in Physical Review Letters in 2014.

Map of the Milky Way galaxy as it would appear from a viewpoint above the galactic centre. The Solar System is located in the Orion Spur (green dot) (Credit: Wikipedia)

As I commented in my brief review of Rampino’s book: “As for Rampino’s galactic hypothesis, the statistics are decidedly dodgy, but chasing down more forensics is definitely on the cards.” Indeed they have been chased in a recent review by the pair and their colleague Sedelia Rodriguez (Rampino, M.R., Caldeira, K. & Rodriguez, S. 2023. Cycles of ∼32.5 My and ∼26.2 My in correlated episodes of continental flood basalts (CFBs), hyper-thermal climate pulses, anoxic oceans, and mass extinctions over the last 260 My: Connections between geological and astronomical cycles. Earth-Science Reviews, v. 246 ; DOI: 10.1016/j.earscirev.2023.104548; reprint available on request from Rampino). They base their amplified case on much more than radiometric dates of continental flood basalt (CFB) events matched against the stratigraphic record of biotic diversity. Among the proxies are published measurements of mercury and osmium isotope anomalies in oceanic sediments that are best explained by sudden increases in basaltic magma eruption; signs of deep ocean anoxia; new dating of marine and non-marine extinctions in the fossil record, and episodes of sudden extreme climatic heating.

Statistical analysis of the ages of anoxic events and marine extinctions has yielded cycles of 32.5 and 26.2 Ma, those for CFBs having a 32.8 Ma periodicity. A note of caution, however: their data only cover the last 266 Ma – about one orbit of the solar system around the galactic centre. The authors attribute their interpretation of the cycles “to the Earth’s tectonic-volcanic rhythms, but the similarities with known Milankovitch Earth orbital periods and their amplitude modulations, and with known Galactic cycles, suggest that, contrary to conventional wisdom, the geological events and cycles may be paced by astronomical factors”.

Whether or not a detailed record of appropriate proxies can be extended back beyond the Late Permian, remains to be seen. The main fly-in-the-ointment is the tendency of CFB provinces to form high ground so that they are readily eroded away. Pre-Mesozoic signs of their former presence lie in basaltic dyke swarms that cut through older  crystalline continental crust. The marine sedimentary record is somewhat better preserved. A search for distinctive anomalies in osmium isotopes and mercury concentrations, which are useful proxies for global productivity of basaltic magmas, will be costly. Moreover, dating will depend to a large degree on the traditional palaeontology of strata, which in Palaeozoic rocks is more difficult to calibrate precisely by absolute radiometric dating.

Darwin’s ‘warm little pond’: a new discovery

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There may still be a few people around today who, like Aristotle did, reckon that frogs form from May dew and that maggots and rats spring into life spontaneously from refuse. But the idea that life emerged somehow from the non-living is, to most of us, the only viable theory. Yet the question, ‘How?’, is still being pondered on. Readers may find Chapter 13 of Stepping Stones useful. There I tried to summarise in some detail most of the modern lines of research. But the issue boils down to means of inorganically creating the basic chemical building blocks from which life’s vast and complex array of molecules might have been assembled. Living materials are dominated by five cosmically common elements: carbon, hydrogen, oxygen, nitrogen and phosphorus – CHONP for short. Organic chemists can readily synthesise countless organic compounds from CHONP. And astronomers have discovered that life is not needed to assemble the basic ingredients: amino acids, carbon-ring compounds and all kinds of simpler CHONP molecules occur in meteorites, comets and even interstellar molecular clouds. So an easy way out is to assume that such ingredients ended up on the early Earth simply because it grew through accretion of older materials from the surrounding galaxy. Somehow, perhaps, their mixing in air, water and sediments together with a kind of chaotic shuffling did the job, in the way that an infinity of caged monkeys with access to typewriters might eventually create the entire works of William Shakespeare.  But, aside from the statistical and behavioural idiocy of that notion, there is a real snag: the vaporisation of the proto-Earth’s outer parts by a Moon-forming planetary collision shortly after initial accretion.

In 1871 Charles Darwin suggested to his friend Joseph Hooker that:

          ‘… if (and Oh, what a big if) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would never have been the case before living creatures were formed’.

Followed up in the 1920s by theorists Alexander Oparin and J.B.S. Haldane, a similar hypothesis was tested practically by Harold Urey and Stanley Miller at the University of Chicago. They devised a Heath-Robinson simulation of an early atmosphere and ocean seeded with simple CHONP (plus a little sulfur) chemicals, simmered it and passed electrical discharges through it for a week. The resulting dark red ‘soup’ contained 10 of the 20 amino acids from which a vast array of proteins can be built. A repeat in 1995 also yielded two of the four nucleobases at the heart of DNA – adenine and guanine.  But simply having such chemicals around is unlikely to result in life, unless they are continually in close contact: a vessel or bag in which such chemicals can interact. The best candidates for such a containing membrane are fatty acids of a form known as amphiphiles. One end of an amphiphile chain has an affinity for water molecules, whereas the other repels them. This duality enables layers of them, when assembled in water, spontaneously to curl up to make three dimensional membranes looking like bubbles. In the last year they too have been created in vitro (Purvis, G. et al. 2024. Generation of long-chain fatty acids by hydrogen-driven bicarbonate reduction in ancient alkaline hydrothermal vents. Nature Communications (Earth & Environment), v. 5, article 30; DOI: 10.1038/s43247-023-01196-4).

Cell-like membranes formed by fatty acid amphiphiles

Graham Purvis and colleagues from Newcastle University, UK allowed three very simple ingredients – hydrogen and bicarbonate ions dissolved in water and the iron oxide magnetite (Fe3O4) – to interact. Such a simple, inorganic mixture commonly occurs in hydrothermal vents and hot springs. Bicarbonate ions (HCO3–) form when CO2 dissolves in water, the hydrogen and magnetite being generated during the breakdown of iron silicates (olivines) when  ultramafic igneous rocks react with water:

3Fe2SiO4 + 2H2O → 2 Fe3O4 + 3SiO­2 +3H2

Various simulations of hydrothermal fluids had previously been tried without yielding amphiphile molecules. Purvis et al. simplified their setup to a bicarbonate solution in water that contained dissolved hydrogen – a simplification of the fluids emitted by hydrothermal vents – at 16 times atmospheric pressure and a temperature of 90°C. This was passed over magnetite. Under alkaline conditions their reaction cell yielded a range of chain-like hydrocarbon molecules. Among them was a mixture of fatty acids up to 18 carbon atoms in length. The experiment did not incorporate P, but its generation of amphiphiles that can create cell-like structures are but a step away from forming the main structural components of cell membranes, phospholipids.

When emergence of bag-forming membranes took place is, of course, hard to tell. But in the oldest geological formations ultramafic lava flows are far more common than they are today. In the Hadean and Eoarchaean, even if actual mantle rocks had not been obducted as at modern plate boundaries, at the surface there would have been abundant source materials for the vital amphiphiles to be generated through interaction with water and gases: perhaps in ‘hot little ponds’. To form living, self-replicating cells requires such frothy membranes to have captured and held amino acids and nucleobases. Such proto-cells could become organic reaction chambers where chemical building blocks continually interacted, eventually to evolve the complex forms upon which living cells depend.

Why did the largest ever primate disappear?

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Chinese apothecary shops sell an assortment of fossils. They include shells of brachiopods that when ground up and dissolved in water allegedly treat rheumatism, skin diseases, and eye disorders. Traditional apothecaries also supply  ‘dragons’ teeth’, said by Dr Subhuti Dharmananda, Director of the Institute for Traditional Medicine in Portland, Oregon to treat epilepsy, madness, manic running about, binding qi (‘vital spirit’) below the heart, inability to catch one’s breath, and various kinds of spasms, as well as making the body light, enabling one to communicate with the spirit light, and lengthening one’s life. Presumably have done a roaring trade in ‘dragons’ teeth’ since they were first mentioned in a Chinese pharmacopoeia (the Shennong Bencao Jing) from the First Century of the Common Era. In 1935 the anthropologist Gustav von Koenigswald came across two ‘dragons’ teeth’ in a Hong Kong shop. They were unusually large molars and he realised they were from a primate, but far bigger (20  × 22 mm) than any from living or fossil monkeys, apes or humans.

Eventually, in 1952 (he had been interned by Japanese forces occupying Java), von Koenigswald formally described the teeth and others that he had found. Their affinities and size prompted him to call the former bearer the ‘Huge Ape’ (Gigantopithecus). By 1956 Chinese palaeontologists had tracked down the cave site in Guangxi province where the teeth had been sourced, and a local farmer soon unearthed a complete lower jawbone (mandible) that was indeed gigantic. More teeth and mandibles have since been found at several sites in Southern and Southeast Asia, with an age range from about 2.0 to 0.3 Ma. Anatomical differences between teeth and mandibles suggest that there may have been 4 different species. Using mandibles as a very rough guide to overall size it has been estimated that Gigantopithecus may have been up to 3 m tall weighing almost 600kg.

Above: Size comparison of G. blacki with a 1.8 m tall human male; NB G.blacki probably walked on all fours, as do living orangutans when they rarely descend from the forest canopy. (Credit: Frido Welker) Below: Mandible of Gigantopithecus blacki from India (Credit: Prof. Wei Wang, Photo retouched by Theis Jensen)

Plaque on some teeth contain evidence for fruit, tubers and roots, but not grasses, which suggest suggest that Gigantopithecus had a vegetarian diet based on forest plants. Mandibles also showed affinities with living and fossil orangutans (pongines). Analysis of proteins preserved in tooth enamel confirm this relationship (Welker, F. and 17 others 2019. Enamel proteome shows that Gigantopithecus was an early diverging pongine. Nature, v.576, p. 262–265; DOI: 10.1038/s41586-019-1728-8). It was one of the few members of the southeast Asian megafauna to go extinct at the genus level during the Pleistocene. Its close relative Pongo the orangutan survives as three species in Borneo and Sumatra. Detailed analysis of material from 22 southern Chinese caves that have yielded Gigantopithecus teeth has helped resolve that enigma (Zhang, Y. and 20 others 2024. The demise of the giant ape Gigantopithecus blacki. Nature, v. 625; DOI: 10.1038/s41586-023-06900-0).

At the time Gigantopithecus first appeared in the geological record of China (~2.2 Ma), it ranged over much of south-western China. The early Pleistocene ecosystem there was one of diverse forests sufficiently productive to support large numbers of this enormous primate and also the much smaller orangutan Pongo weidenreichi.  By 295 to 215 ka, the age of the last known Gigantopithecus fossils, its range had shrunk dramatically. The teeth show marked increases in size and complexity by this time, which suggests adaptation of diet to a changing ecosystem. That is confirmed by pollen analysis of cave sediments which reveal a dramatic decrease in forest cover and increases in fern and non-arboreal flora at the time of extinction. One physical sign of environmental stress suffered by individual late G. blacki is banding in their teeth defined by large fluctuations of barium and strontium concentrations relative to calcium. The bands suggest that each individual had to change its diet repeatedly over its lifetime. Closely related orangutans, on the other hand survived into the later Pleistocene of China, having adapted to the changed ecosystem, as did early humans in the area. It thus seems likely that Gigantopithecus was an extreme specialist as regards diet, and was unable to adapt to changes brought on by the climate becoming more seasonal. Today’s orangutans in Indonesia face a similar plight, but that is because they have become restricted to forest ‘islands’ in the midst of vast areas of oil palm plantations. Their original range seems to have been much the same as that of Gigantopithecus, i.e. across south-eastern Asia, but Pongo seems to have gone extinct outside of Indonesia (by 57 ka in China) during the last global cooling and when forest cover became drastically restricted.

When giant worms roamed the seas!

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At the start of the Cambrian Period animal life began to diversify from that of the Ediacaran world. For the first time sediments on the seafloor were explored for sustenance, leading to a variety of burrows that disrupted fine depositional layers. The basal Cambrian sandstones found in Britain and elsewhere are pervasively bioturbated: good evidence for the start of a ‘Worm world’ that marks the Precambrian-Phanerozoic boundary. That is probably a misnomer for the shallow seabed of that time, as fossils of burrowers with a variety of hard parts turn up in the oldest Cambrian sequences. Also appearing for the first time are tooth-like microfossils that took on such a range of bizarre shapes that they have long been used for correlating sedimentary strata in the absence of larger creatures. Some of these conodonts have been attributed to early vertebrates akin to modern lampreys and hag fish, but others may have been the grasping mouth-spines of a group of predatory worms which also survive to the present: chaetognaths. Apart from these oral spines chaetognaths lack hard parts, so anatomical details of ancient ones are only found in sites of exquisite preservation or lagerstätten. In such rare, tranquil places soft tissues such as muscles may be preserved by phosphatisation during decay.

Reconstruction of Timorebestia koprii showing its musculature, nerve system and mouthparts, It probably propelled itself by fluttering its outer and rear flaps, much like a modern flatfish. Credit: Park et al., Fig 4

One of the earliest Phanerozoic lagerstätten (Sirius Passet) occurs in northern Greenland. It is curiously named after the Sirius Dog Sled Patrol, an elite pair of naval troops with a sledge and 12 dogs that enforces Danish sovereignty over the Greenlandic shore of the Arctic Ocean. The Sirius Passet fauna includes a monstrous chaetognath over 30 cm long (Park, T.-Y. S. and 12 others 2024. A giant stem-group chaetognath. Science Advances, v. 10 article eadi6678; DOI: 10.1126/sciadv.adi6678). It is called Timorebestia koprii (Timorebestia is Latin for ‘terror beast’) and was related to the living, but tiny, arrow worms that prey on zooplankton in modern oceans. This description and moniker may seem to be somewhat hyperbolic, but Timorobestia outranks in size any Early Cambrian predatory arthropods. It was probably high in the Early Cambrian trophic pyramid, but was soon relegated by the later Cambrian rise of trilobites and then of cephalopods and eventually jawed vertebrate fishes in the Silurian. One specimen contained shells of a swimming arthropod whose protective spines did not deter the ‘terrible’ chaetognath from swimming them down.

See also: ‘Giant’ predator worms more than half a billion years old discovered in North Greenland. Science Daily, 3 January 2024.

 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