Bioturbated ‘pipe rock’ of the basal Cambrian sandstones of NW Scotland. Credit: British Geological Survey photograph P531881
About 530 Ma ago most of the basic body plans of today’s living organisms can be detected as fossils, i.e. preserved hard parts. Yet studies of trace fossils (ichnofossils) – marks left in sediments by active soft bodied creatures suggest that many modern phyla arose before the start of the Cambrian (~539 Ma), as early as 545 Ma. So the term ‘Cambrian explosion’ seems to be a bit of a misnomer on two counts: it lasted around 15 Ma and began before the Cambrian. Preceding it was the Ediacaran Period that began around 100 Ma earlier in the Neoproterozoic Era. Traces of its eponymous fauna of large soft-bodied organisms are found on all continents, but apparently none of them made it into the Phanerozoic fossil record. Another characteristic of the Ediacaran is that its sedimentary rocks – and those of earlier times – show no signs of burrowing: they are not bioturbated. That may be why the Ediacaran pancake-, bun-, bag- and pen-like lifeforms are so remarkably well preserved. But a lack of burrowing did not extend to the beginning of Cambrian times. The most likely reason why it was absent during the early Ediacaran Period is that sea-floor sediments then were devoid of oxygen so eukaryote animals could not live in them. But the presence of these large organisms showed that seawater must have been oxygenated. Now clear signs of burrowing have emerged from study of Ediacaran rocks exposed in the Yangtze Gorge of Hubei,southern China ( Zhe Chen & Yarong Liu 2025. Advent of three-dimensional sediment exploration reveals Ediacaran-Cambrian ecosystem transition. Science Advances, v. 11, article eadx9449; DOI: 10.1126/sciadv.adx9449).
Tadpole-like trace fossils from the Ediacaran Dengying Formation in the Yangtze Gorge: 5 cm scale bars. The ‘heads’ show tiny depressions suggesting that there maker probed into the sediments as well as foraging horizontally. Credit: Zhe Chen & Yarong Liu; Figs 3B and 3D
Zhe Chen and Yarong Liu of the Nanjing Institute of Geology and Palaeontology and Chinese Academy of Sciences in China examined carbonates of the upper Ediacaran Dengying Formation. This overlies the Doushantuo Formation (550 to 635 Ma), known for tiny fossils of possibly the oldest deuterostome Saccorhytus coronaries; a potential candidate for the ancestor of modern bilaterian phyla. In the Yangtze Gorge locality sediments at this level show only traces of browsing of bacterial mats on the sediment surface; i.e. 2-D feeders. The basal Dengying sediments host clear signs that organisms could then penetrate into the sediments. These 3-D feeders , would have had access to buried organic remains, hitherto unexploited by living organisms. Such animal-sediment interactions would have disturbed and diminished the living microbial mats that held the sediment surface in place, and thus began to dismantle the substrate for the typical Edicaran fauna. Similar 3-D feeders occur throughout the 11 Ma represented by the Dengying Formation to the start of the Cambrian. This beginning of bioturbation heralded a period during which the Ediacaran fauna steadily waned. It also released nutrients into deep water, and opened up new ecological niches for more advanced animals on the seabed. Dissolved oxygen could only slowly enter the sediments since atmospheric and oceanic O2 levels were low. But by the earliest Cambrian it had risen to about 5 to 10% by volume to support many other kinds of burrowing animals that could penetrate more deeply, as witnessed by the abundant sandstones that occur at the base of the Cambrian in Britain.
A symposium hosted by the Royal Society in 1965 aimed at resurrecting Alfred Wegener’s hypothesis of continental drift. During the half century since Wegener made his proposal in 1915, it had been studiously ignored by most geologists. The majority had bumbled along with the fixist ideology of their Victorian predecessors. The symposium launched what can only be regarded as a revolution in the Earth Sciences. In the three years following the symposium, the basic elements of plate tectonics had emerged from a flurry of papers, mainly centred on geophysical evidence. Geology itself became part of this cause célèbre through young scientists eager to make a name for themselves. The geological history of Britain, together with that of the eastern North America, became beneficiaries only four years after the Royal Society meeting (Dewey, J. 1969. Evolution of the Appalachian/Caledonian Orogen. Nature222, 124–129; DOI: 10.1038/222124a0).
In Britain John Dewey, like a few other geologists, saw plate theory as key to understanding the many peculiarities revealed by geological structure, igneous activity and stratigraphy of the early Palaeozoic. These included very different Cambrian and Ordovician fossil assemblages in Scotland and Wales, now only a few hundred kilometres apart. The Cambro-Ordovician of NW Scotland was bounded to the SE by a belt of highly deformed and metamorphosed Proterozoic to Ordovician sediments and volcanics forming the Scottish Highlands. That was terminated to the SE by a gigantic fault zone containing slivers of possible oceanic lithosphere. The contorted and ‘shuffled’ Ordovician and Silurian sediments of the Southern Uplands of Scotland. The oldest strata seemed to have ocean-floor affinities, being deposited on another sliver of ophiolites. A few tens of km south of that there was a very different Lower Palaeozoic stratigraphy in the Lake District of northern England. It included volcanic rocks with affinities to those of modern island arcs. A gap covered by only mildly deformed later Palaeozoic shelf and terrestrial sediments, dotted by inliers of Proterozoic sediments and volcanics separated the Lake District from yet another Lower Palaeozoic assembly of arc volcanics and marine sediments in Wales. Intervening in Anglesey was another Proterozoic block of deformed sediments that also included ophiolites.
Dewey’s tectonic assessment from this geological hodge-podge, which had made Britain irresistible to geologists through the 19th and early 20th centuries, was that it had resulted from blocks of crust (terranes), once separated by thousands of kilometres, being driven into each other. Britain was thus formed by the evolution and eventual destruction of an early Palaeozoic ocean, Iapetus: a product of plate tectonics. Scotland had a fundamentally different history from England and Wales; the unification of several terranes having taken over 150 Ma of diverse tectonic processes. Dewey concluded that the line of final convergence lay at a now dead, major subduction zone – the Iapetus Suture – roughly beneath the Solway Firth. During the 56 years since Dewey’s seminal paper on the Caledonian-Appalachian Orogeny details and modifications have been added at a rate of around one to two publications per year. The latest seeks to date when and where the accretion of 6 or 7 terranes was finally completed (Waldron, J.W.F. et al. 2025. Is Britain divided by an Acadian suture?Geology, v. 53, p. 847–852; DOI: 10.1130/G53431.1).
Kernel density plots – smoothed versions of histograms – of detrital zircon ages in Silurian and Devonian sandstones from Wales. The bracketed words are stratigraphic epochs. Credit: Waldron et al. 2025, Fig 3A
John Waldron and colleagues from the University of Alberta and Acadia University in Canada and the British Geological Survey addressed this issue by extracting zircons from four late Silurian and early Devonian sandstones in North and South Wales. These sediments had been deposited between 433 and 393 Ma ago at the southernmost edge of the British Caledonide terrane assemblage towards the end of terrane assembly. The team dated roughly 250 zircons from each sandstone using the 207Pb/206Pb and 206Pb/238U methods. Each produced a range of ages, presumed to be those of igneous rocks from whose magma the zircon grains had crystallised. These data are expressed as plots of probable frequency against age. Each pattern of ages is assumed to be a ‘fingerprint’ for the continental crust from which the zircons were eroded and transported to their resting place in their host sediment. In this case, the researchers were hoping to see signs of continental crust from the other side of the Caledonian orogen; i.e. from the Precambrian basement of the Laurentia continent.
The three late-Silurian sediments showed distinct zircon-age peaks around 600 Ma and a spread of smaller peaks extending to 2.2 Ga. This tallied with a sediment source in Africa, from which the southernmost Caledonian terrane was said to have split and moved northwards. The Devonian sediment lacked signs of such an African ‘heritage’ but had a prominent age peak at about 1.0 Ga, absent from the Welsh Silurian sediments. Not only is this a sign of different sediment provenance but closely follows the known age of a widespread magmatic pulse in the Laurentian continent. So, sediment transport from the opposite side of the Iapetus Ocean across the entire Caledonian orogenic belt was only possible after the end of the Silurian Period at around 410 Ma. There must have been an intervening barrier to sediment movement from Laurentia before that, such as deep ocean water further north. Previous studies from more northern Caledonian terranes show that Laurentian zircons arrived in the Southern Uplands of Scotland and the English Lake District around 432 Ma in the mid-Silurian. Waldron et al. suggest, on these grounds that the suture marking the final closure of the Iapetus Ocean lies between the English Lake District and Anglesey, rather than beneath the Solway. They hint that the late-Silurian to early Devonian granite magmatism that permeated the northern parts of the Caledonian-Appalachian orogen formed above northward subduction of the last relics of Iapetus, which presaged widespread crustal thickening known as the Acadian orogeny in North America.
Readers interested in this episode of Earth history should download Waldron et al.’s paper for its excellent graphics, which cannot be reproduced adequately here.
During the Industrial Revolution not only did the emission of greenhouse gases by burning fossil fuels start to increase exponentially, but so too did the movement of rock and sediment to get at those fuels and other commodities demanded by industrial capital. In the 21st century about 57 billion tons of geological materials are deliberately moved each year. Global population followed the same trend, resulting in increasing expansion of agriculture to produce food. Stripped of its natural cover on every continent soil began to erode at exponential rates too. The magnitude of human intervention in natural geological cycles has become stupendous, soil erosion now shifting on a global scale about 75 billion tons of sediment, more than three times the estimated natural rate of surface erosion. Industrial capital together with society as a whole also creates and dumps rapidly growing amounts of solid waste of non-geological provenance. The Geological Society of America’s journal Geology recently published two research papers that document how capital is transforming the Earth.
Dust Bowl conditions on the Minnesota prairies during the 1930s.
One of the studies is based on sediment records in the catchment of a tributary of the upper Mississippi River. The area is surrounded by prairie given over mainly to wheat production since the mid 19th century. The deep soil of the once seemingly limitless grassland developed by the prairie ecosystem is ideal for cereal production. In the first third of the 20th century the area experienced a burst of erosion of the fertile soil that resulted from the replacement of the deep root systems of prairie grasses by shallow rooted wheat. The soil had formed from the glacial till deposited by the Laurentide ice sheet than blanketed North America as far south as New York and Chicago. Having moved debris across almost 2000 km of low ground, the till is dominated by clay- and silt-sized particles. Once exposed its sediments moved easily in the wind. Minnesota was badly affected by the ‘Dust Bowl’ conditions of the 1930s, to the extent that whole towns were buried by up to 4.5 metres of aeolian sediment. For the first time the magnitude of soil erosion compared with natural rates has been assessed precisely by dating layers of alluvium deposited in river terraces of one of the Mississippi’s tributaries (Penprase, S.B. et al. 2025. Plow versus Ice Age: Erosion rate variability from glacial–interglacial climate change is an order of magnitude lower than agricultural erosion in the Upper Mississippi River Valley, USA. Geology, v. 53, p. 535-539; DOI: 10.1130/G52585.1).
Shanti Penprase of the University of Minnesota and her colleagues were able to date the last time sediment layers at different depths in terraces were exposed to sunlight and cosmic rays, by analysing optically stimulated luminescence (OSL) and cosmogenic 10Be content of quartz grains from the alluvium. The data span the period since the Last Glacial Maximum 20 thousand years ago during which the ecosystem evolved from bare tundra through re-vegetation to pre-settlement prairie. They show that post-glacial natural erosion had proceeded at around 0.05 mm yr-1 from a maximum of 0.07 when the Laurentide Ice Sheet was at its maximum extent. Other studies have revealed that after the area was largely given over to cereal production in the 19th century erosion rates leapt to as high as 3.5 mm yr-1 with a median rate of 0.6 mm yr-1, 10 to 12 times that of post-glacial times. It was the plough and single-crop farming introduced by non-indigenous settlers that accelerated erosion. Surprisingly, advances in prairie agriculture since the Dust Bowl have not resulted in any decrease in soil erosion rates, although wind erosion is now insignificant. The US Department of Agriculture considers the loss of one millimetre per year to be ‘tolerable’: 14 times higher than the highest natural rate in glacial times.
The other paper has a different focus: how human activities may form solid rock. The world over, a convenient means of disposing of unwanted material in coastal areas is simply to dump waste in the sea. That has been happening for centuries, but as for all other forms of anthropogenic waste disposal the volumes have increased at an exponential rate. The coast of County Durham in Britain began to experience marine waste disposal when deep mines were driven into Carboniferous Coal Measures hidden by the barren Permian strata that rest unconformably upon them. Many mines extended eastwards beneath the North Sea, so it was convenient to dump 1.5 million tons of waste rock annually at the seaside. The 1971 gangster film Get Carter starring Michael Caine includes a sequence showing ‘spoil’ pouring onto the beach below Blackhall colliery, burying the corpse of Carter’s rival. The nightmarish, 20 km stretch of grossly polluted beach between Sunderland and Hartlepool also provided a backdrop for Alien 3. Historically, tidal and wave action concentrated the low-density coal in the waste at the high-water mark, to create a free resource for locals in the form of ‘sea coal’ as portrayed in Tom Scott Robson’s 1966 documentary Low Water. Closure of the entire Duham coalfield in the 1980s and ‘90s halted this pollution and the coast is somewhat restored – at a coast of around £10 million.
‘Anthropoclastic’ conglomerate formed from iron-smelting slag dumped on the West Cumbrian coast. It incorporates artefacts as young as the 1980s, showing that it was lithified rapidly. Credit: Owen et al, Supplementary Figure 2
On the West Cumbrian coast of Britain another industry dumped millions of tons of waste into the sea. In the case it was semi-molten ‘slag’ from iron-smelting blast furnaces poured continuously for 130 years until steel-making ended in the 1980s. Coastal erosion has broken up and spread an estimated 27 million cubic metres of slag along a 2 km stretch of beach. Astonishingly this debris has turned into a stratum of anthropogenic conglomerate sufficiently well-bonded to resist storms (Owen, A., MacDonald, J.M. & Brown, D.J 2025. Evidence for a rapid anthropoclastic rock cycle. Geology, v. 53, p. 581–586; DOI: 10.1130/G52895.1). The conglomerate is said by the authors to be a product of ‘anthropoclastic’ processes. Its cementation involves minerals such as goethite, calcite and brucite. Because the conglomerate contains car tyres, metal trouser zips, aluminium ring-pulls from beer cans and even coins lithification has been extremely rapid. One ring-pull has a design that was not used in cans until 1989, so lithification continued in the last 35 years.
Furnace slag ‘floats’ on top of smelted iron and incorporates quartz, clays and other mineral grains in iron ore into anhydrous calcium- and magnesium-rich aluminosilicates. This purification is achieved deliberately by including limestone as a fluxing agent in the furnace feed. The high temperature reactions are similar to those that produce aluminosilicates when cement is manufactured. Like them, slag breaks down in the presence of water to recrystallis in hydrated form to bond the conglomerate. This is much the same manner as concrete ‘sets’ over a few days and weeks to bind together aggregate. There is vastly more ‘anthropoclastic’ rock in concrete buildings and other modern infrastructure. Another example is tarmac that coats millions of kilometres of highway.
False-colour electron microscope image of a shocked grain of zircon recovered from the Stac Fada Member. The red and pink material is a high-pressure polymorph of zircon, arranged in shock lamellae. Zircon is rendered in cyan, some of which is in granulated form. Credit: Kirkland et al. 2025, Fig 2C
Judging by its content of shards and spherules made of murky green glass, one of the lowest units in the Torridonian continental sediments of NW Scotland had long been regarded as simply red sandstone that contained volcanic debris. This Stac Fada Member was thus celebrated as the only sign of a volcanic contribution to a vast thickness (up to 2.5 km) of Neoproterozoic lake and fluviatile sediments. Current flow indicators suggested that the Torridonian was laid down by large alluvial fans derived by erosion of much older crystalline basement far to what is today the west. That is, the Archaean core of the ancient continent of Laurentia, now the other side of the North Atlantic. In 2002 more sophisticated sedimentological and geochemical analysis of the Stac Fada Member revealed a surprise: it contains anomalously elevated platinum-group elements, quartz grains that show signs of shock and otherworldly chromium isotope concentrations. The 10 m thick bed is made from ejecta, perhaps from a nearby impact crater to the WNW concluded from brittle fractures that may have been produced by the impact. Some idea of its age was suggested by Ar-Ar dating of feldspar crystals (~1200 Ma) believed to have formed authigenically in the hot debris. Being the only decent impactite known in Britain, it continues to attract attention.
A group of geoscientists from Western Australia, NASA and the UK, independent of the original discoverers, have now added new insights ( Kirkland, C.L. and 12 others 2025. A one-billion-year old Scottish meteorite impact. Geology, v. 53, early online publication; DOI: 10.1130/G53121.1). They dated shocked zircon grains using U-Pb analyses at 990 ± 22 Ma; some 200 Ma younger than the previously dated, authigenic feldspars. Detrital feldspar grains in the Stac Fada Member yield Rb-Sr radiometric ages of 1735 and 1675, that are compatible with Palaeoproterozoic granites in the underlying Lewisian Gneiss Complex.
Photomicrograph of Bicellum brazieiri: scale bar = 10μm; arrows point to dark spots that may be cell nuclei (credit: Charles Wellman, Sheffield University)
In a separate publication (Kirkland, C.L et al 2025. 1 billion years ago, a meteorite struck Scotland and influenced life on Earth. The Conversation, 29 April 2025) three of the authors take things a little further, as their title suggests. In this Conversation piece they ponder, perhaps unwarily, on the spatial and temporal association of the indubitable impact with remarkably well-preserved spherical fossils found in Torridonian lake-bed sediments (Bicellum brasieri, reported in Earth-logs in May 2021), which are the earliest-known holozoan animal ancestors. The Torridonian phosphatic concretions in which these important fossils were found at a different locality are roughly 40 Ma younger than the Stac Fada impactite. The authors of the Conversation article appeal to the residual thermal effect of the impact as a possible driver for the appearance of these holozoan organisms. Whether a residual thermal anomaly would last long enough for them to evolve to this biological status would depend on the magnitude of the impact, of which we know nothing. Eukaryote fossils are known from at least 650 Ma older sedimentary rocks in northern China and perhaps as far back as 2.2 Ga in a soil that formed in the Palaeoproterozoic of South Africa. Both the Torridonian organism and impactite were found in a small area of fascinating geology that has been studied continuously in minute detail since Victorian times, and visited by most living British geologists during their undergraduate days. Ideas will change as curiosity draws geologists and palaeobiologists to less-well studied sites of Proterozoic antiquity, quite possibly in northern China.
A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook
Fig Interpreted 2D seismic section across the Nadir crater and central uplift beneath the Guinea Terrace. (Credit: Nicholson, et al. 2022. Fig 2c)
In 2022 four geoscientists from Heriot-Watt University in Edinburgh, Scotland and the Universities of Arizona and Texas (Austin), USA were geologically interpreting seismic-reflection data beneath the seafloor off Guinea and Guinea-Bissau, West Africa. Individual sedimentary strata that cover the upper continental crust show up as many reflectors. They are calibrated to rock cores from exploratory well that had revealed up to 8 km of sedimentary cover deposited continuously since the Upper Jurassic. The team’s objective was to collect information on tectonic structures that had formed when South America separated from Africa during the Cretaceous. The geophysical data were from commercial reconnaissance surveys aimed at locating petroleum fields beneath part of the West African continental shelf known as the Guinea Terrace. One of the seismic sections revealed a ~9 km wide basin-like depression at the level of the Cretaceous-Palaeogene boundary, which is underlain by a prominent upward bulge in reflectors corresponding to the mid-Cretaceous, plus a large number of nearby faults (Nicholson, U., and 3 others 2022. The Nadir Crater offshore West Africa: a candidate Cretaceous-Paleogene impact structure. Science Advances, v. 8, article eabn3096; DOI: 10.1126/sciadv.abn3096). Elsewhere on the Guinea Terrace the strata were featureless by comparison.
The Nadir crater showed many of the signs to be expected from an asteroid impact. That it drew attention stemmed partly from being of roughly the same age as the much larger 66 Ma Chicxulub impact off the Yucatan Peninsula of Mexico: the likely culprit for the K-Pg mass-extinction event. Perhaps both impactors stemmed from the break-up of a large, near-Earth asteroid because of gravitational forces resulting from a previous close encounter with either the Earth or another planet. The crater lies at the centre of a 23 km wide zone of faults that only affect Cretaceous and older strata; i.e. they formed just before the K-Pg event. The seismic data also show signs of widespread liquefaction of nearby Cretaceous sedimentary strata and that the crater had been filled by sediments shortly after it formed. Yet the data were too fuzzy for an astronomical catastrophe to be absolutely certain: similar structures can form from the rise of bodies of rock salt, which is less dense than sediments and will dissolve on reaching the seabed. The owners of the seismic data donated a much larger collection from a grid of survey lines. Processing of such seismic grids turns the collection of individual two-dimensional sections into a 3D regional data set showing the complete shape of subsurface structures. Seismic data of this kind enables more detailed structural and lithological interpretation of both cross section and plan views. They enable sedimentary layers to be ‘peeled’ back to examine the crater at all depths, in much the same manner as CT and MRI scans reveal the inner anatomy of the human body.
Map of faults around the Nadir crater at a level in the 3D seismic data that was about 200 m below the sea bed at the time of the impact. (Credit: Nicholson, et al. 2024, Fig 6)
Uisdean Nicholson and a larger team have now published their findings from the 3D seismic data that show the structure in unique detail (Nicholson, U., and 6 others 2024. 3D anatomy of the Cretaceous–Paleogene age Nadir Crater. Communications Earth & Environment v. 5, article number 547; DOI: 10.1038/s43247-024-01700-4). Nadir crater was affected by spiral-shaped thrust faults that suggest it was formed by an oblique impact from the northeast by an object around 450 m across, probably travelling at 20 km s-1 at 20 to 40° to the surface. Seconds after excavation uplift of deeper sediments was a response to removal of the load on the crust. The energy was sufficient to vaporise both sediment and impactor within a few seconds, the to drive drive seawater outwards in a tsunami about half a kilometre high, which in about 30 seconds exposed the incandescent crater floor. In the succeeding minutes hours and days liquefied sea water sloshed in and out of the crater, repeated tsunami resurgence forming gullies on its flanks and transporting sediment mixed with glass (suevite) flowed to refill the crater.
Time line for the Nadir impact, derived from detail shown by 3D seismic data. (Credit: Nicholson, et al. 2024, Fig 7)
There is no means of assigning any of the K-Pg extinctions to the Nadir crater, just that it happened at roughly the same time as Chicxulub. But it is the first impact crater to reveal the processes involved through complete coverage by high-resolution 3D seismic data. The majority of the roughly 200 craters are on the continental surface, and were thus ravaged to some extent by later erosion. Yet of the influx of hypervelocity objects through time at least 70% must have struck the oceans, but only 15 to 20 are known. That may reflect the fact that much deeper water could have buffered even giant impacts from affecting the oceanic crust beneath the abyssal plains, whose average depth is about 4 km. Only a small proportion of the continental shelves deemed to contain petroleum reserves have been explored seismically. Chicxulub itself has been drilled, but only two seismic reflection sections have crossed its centre since its discovery, although earlier 3D data from petroleum exploration cover its outermost northern parts. More detail is available for Nadir and its lower energy did not smash its structural results, unlike Chicxulub. So, despite Nadir’s smaller size, fortuitously it gives more clues to how such marine craters formed. It looks to be an irresistible target for drilling.
The greatest mass extinction in Earth’s history at around 252 Ma ago snuffed out 81% of marine animal species, 70% of vertebrates and many invertebrates that lived on land. It is not known how many land plants were removed, but the complete absence of coals from the first 10 Ma of the Early Triassic suggests that luxuriant forests that characterised low-lying humid area in the Permian disappeared. A clear sign of the sudden dearth of plant life is that Early Triassic river sediments were no longer deposited by meandering rivers but by braided channels. Meanders of large river channels typify land surfaces with abundant vegetation whose root systems bind alluvium. Where vegetation cover is sparse, there is little to constrain river flow and alluvial erosion, and wide braided river courses develop (see: End-Permian devastation of land plants; September 2000. You can follow 21st century developments regarding the P-Tr extinction using the Palaeobiology index).
The most likely culprit was the Siberian Trap flood basalts effusion whose lavas emitted huge amounts of CO2 and even more through underground burning of older coal deposits (see:Coal and the end-Permian mass extinction; March 2011) which triggered severe global warming. That, however, is a broad-brush approach to what was undoubtedly a very complex event. Of about 20 volcanism-driven global warming events during the Phanerozoic only a few coincide with mass extinctions. Of those none comes close the devastation of ‘The Great Dying’, which begs the question, ‘Were there other factors at play 252 Ma ago?’ That there must have been is highlighted by the terrestrial extinctions having begun significantly earlier than did those in marine ecosystems, and they preceded direct evidence for climatic warming. Also temperature records – obtained from shifts in oxygen isotopes held in fossils – for that episode are widely spaced in time and tell palaeoclimatologists next to nothing about the details of the variation of air- and sea-surface temperature (SST) variations.
Modelled sea-surface temperatures in the tropics in the early stages of Siberian Trap eruptions with atmospheric CO¬2 at 857 ppm – twice today’s level. (Credit: Sun et al., Fig. 1A)
Earth at the end of the Permian was very different from its current wide dispersal of continents and multiple oceans and seas. Then it was dominated by Pangaea, a single supercontinent that stretched almost from pole to pole, and a surrounding vast ocean known as Panthalassa. Geoscientists from China, Germany, Britain and Austria used this simple palaeogeography and the available Early Triassic greenhouse-gas and palaeo-temperature data as input to a climate prediction model (HadCM3BL) (Yadong Sun and 7 others 2024. Mega El Niño instigated the end-Permian mass extinction. Science 385, p. 1189–1195; DOI: 10.1126/science.ado2030 – contact yadong.sun@cug.edu.cn for PDF).. The computer model was developed by the Hadley Centre of the UK Met Office to assess possible global outcomes of modern anthropogenic global warming. It assesses heat transport by atmospheric flow and ocean currents and their interactions. The researchers ran it for various levels of atmospheric CO2 concentrations over the estimate 100 ka duration of the P-Tr mass extinction.
The pole-to-pole continental configuration of Pangaea lends itself to equatorial El Niño and El Niña type climatic events that occur today along the Pacific coast of the Americas, known as the El Niño-Southern Oscillation. In the first, warm surface water builds-up in the eastern tropical Pacific Ocean. It then then drifts westwards to allow cold surface water to flow northwards along the Pacific shore of South America to result in El Niña. Today, this climatic ‘teleconnection’ not only affects the Americas but also winds, temperature and precipitation across the whole planet. The simpler topography at the end of the Permian seems likely to have made such global cycles even more dominant.
Sun et al’s simulations used stepwise increases in the atmospheric concentration of CO2 from an estimated 412 parts per million (ppm) before the eruption of the Siberian Traps (similar to those today) to a maximum of 4000 ppm during the late-stage magmatism that set buried coals ablaze. As levels reached 857 ppm SSTs peaked at 2 °C above the mean level during El Niño events and the cycles doubled in length. Further increase in emissions led to greater anomalies that lasted longer, rising to 4°C above the mean at 4000 ppm. The El Niña cooler parts of the cycle steadily became equally anomalous and long lasting. This amplification of the 252 Ma equivalent of the El Niño-Southern Oscillation would have added to the environmental stress of an ever increasing global mean surface temperature. The severity is clear from an animation of mean surface temperature change during a Triassic ENSO event.
Animation of monthly average surface temperatures across the Earth during an ENSO event at the height of the P-Tr mass extinction. (Credit: Alex Farnsworth, University of Bristol, UK)
The results from the modelling suggest increasing weather chaos across the Triassic Earth, with the interior of Pangaea locked in permanent drought. Its high latitude parts would undergo extreme heating and then cooling from 40°C to -40°C during the El Niño- El Niña cycles. The authors suggest that conditions on the continents became inimical for terrestrial life, which would be unable to survive even if they migrated long distances. That can explain why terrestrial extinctions at the P-Tr boundary preceded those in the global ocean. The marine biota probably succumbed to anoxia (See: Chemical conditions for the end-Permian mass extinction; November 2008)
There is a timely warning here. The El Niño-Southern Oscillation is becoming stronger, although each El Niño is a mere 2 years long at most, compared with up to 8 years at the height of the P-Tr extinction event. But it lay behind the record 2023-2024 summer temperatures in both northern and southern hemispheres, the North American heatwave of June 2024 being 15°C higher than normal. Many areas are now experiencing unprecedentedly severe annual wildfires. There also finds a parallel with conditions on the fringes of Early Triassic Pangaea. During the early part of the warming charcoal is common in the relics of the coastal swamps of tropical Pangaea, suggesting extensive and repeated wildfires. Then charcoal suddenly vanishes from the sedimentary record: all that could burn had burnt to leave the supercontinent deforested.
Curiously, two weeks after my previous post about Stonehenge, a wider geochemical study of the Devonian sandstones and a number of Neolithic megaliths in Orkney seems to have ruled out the Stonehenge Altar Stone having been transported from there (Bevins, R.E. et al. 2024. Was the Stonehenge Altar Stone from Orkney? Investigating the mineralogy and geochemistry of Orcadian Old Red sandstones and Neolithic circle monuments. Journal of Archaeological Science: Reports, v. 58, article 104738; DOI: 10.1016/j.jasrep.2024.104738). Since two of the authors of Clarke et al. (2024) were involved in the newly published study, it is puzzling at first sight why no mention was made in that paper of the newer results. The fact that the topic is, arguably, the most famous prehistoric site in the world may have generated a visceral need for getting an academic scoop, only for it to be dampened a fortnight later. In other words, was there too much of a rush?
The manuscript for Clarke et al. (2024) was received by Nature in December 2023 and accepted for publication on 3 June 2024; a six-month turnaround and plenty of time for peer review. On the other hand, Bevins et al. (2024) was received by the Journal of Archaeological Science on 23 July 2024, accepted a month later and then hit the website a week after that: near light speed in academic publishing. And it does not refer to the earlier paper at all, despite two of its authors’ having contributed to it. Clarke et al. (2024) was ‘in press’ before Bevins et al. (2024) had even hit the editor’s desk. The work that culminated in both papers was done in the UK, Australia, Canada and Sweden, with some potential for poor communication within the two teams. Whatever, the first paper dangled the carrot that Orkney might have been the Altar Stone’s source, on the basis of geochemical evidence that the grains that make up the sandstone could not have been derived from Wales but were from the crystalline basement of NE Scotland. The second shows that this ‘most popular’ Scottish source may be ruled out. To Orcadians and the archaeologists who worked there, long in the shade of vast outpourings from Salisbury Plain, this might come as a great disappointment.
Cyclical sediments of the Devonian Stromness Flagstones. (Credit Mike Norton, Wikimedia)
The latest paper examines 13 samples from 8 outcrops of the Middle Devonian Stromness Flagstones strata in the south of the main island of Orkney close to the Ring of Brodgar and the Stones of Stenness, and the individual monoliths in each. On the main island, however, there is a 500 m sequence of Stromness Flagstones in which can be seen 50 cycles of sedimentation. Each cycle contains sandstone beds of various thicknesses and textures. They are fluviatile, lacustrine or aeolian in origin. So the Neolithic builders of Orkney had a wide choice, depending on where they erected monumental structures. Almost certainly they chose monolithic stones where they were most easy to find: close to the coast where exposure can be 100 %. The Ring of Brodgar and the Stones of Stenness are not on the coast, so the enormous stones would have to be dragged there. There is an ancient pile of stones (Vestra Fiold) about 20 km to the NW where some of the mmegaliths may have been extracted, but ancient Orcadians would have been spoilt for choice if they had their hearts set on erecting monoliths!
In a nutshell, the geological case made by Bevins et al. (2024) for rejecting Orkney as the source for the Stonehenge Altar Stone (AS) is as follows: 1. Grains of the mineral baryte (BaSO4) present in the AS are only found in two of the Orkney rock samples. 2. All the Orcadian sandstone samples contain lots of grains of K-feldspar (KAlSi3O8) – common in the basement rocks of northern Scotland – but the AS contains very little. 3. A particular clay mineral (tosudite) is plentiful in the AS, but was not detected in the rock samples from Orkney. Does that rule out a source in Orkney altogether? Well, no: only the outcrops and megalith samples involved in the study are rejected.
To definitely negate an Orcadian source would require a monumental geochemical and mineralogical study across Orkney; covering every sedimentary cycle. Searching the rest of the Old Red Sandstone elsewhere in NE Scotland – and there is a lot of it – would be even more likely to be fruitless. Tracking down the source for the basaltic bluestones at Stonehenge was easy by comparison, because they crystallised from a particular magma over a narrow time span and underwent a specific degree of later metamorphism. They were easily matched visually and under the microscope with outcrops in West Wales in the 1920s and later by geochemical features common to both.
But all that does not detract from the greater importance of the earlier paper (Clarke et al., 2024), which enhanced the idea of Neolithic cultural coherence and cooperation across the whole of Britain. The building of Stonehenge drew people from the far north of Scotland together with those of what are now Wales and England. Since then it hasn’t always been such an amicable relationship …
In 1976 three scientists from Columbia and Brown (USA) and Cambridge (UK) Universities published a paper that revolutionised the study of ancient climates (Hays J.D., Imbrie J. and Shackleton N.J. 1976. Variations in the Earth’s Orbit: Pacemaker of the Ice Ages. Science, v. 194, p. 1121-1132; DOI: 10.1126/science.194.4270.1121). Using variations in oxygen isotopes from foraminifera through two cores of sediments beneath the floor of the southern Indian Ocean they verified Milutin Milankovich’s hypothesis of astronomical controls over Earth’s climate. This centred on changes in Earth’s orbital parameters induced by gravitational effects from the motions of other planets: its orbit’s eccentricity, and the tilt and precession of its rotational axis. Analysis of the frequency of isotopic variations in the resulting time series yielded Milankovich’s predictions of ~100, 41 and 21 ka periodicities respectively. The time spanned by the cores was that of the last 500 ka of the Pleistocene and thus the last 5 glacial-interglacial cycles. Subsequently, the same astronomical climate forcing has been detected for various climate-induced changes in the earlier sedimentary record, including the glacial cycles of the Carboniferous and Neoproterozoic, Jurassic climate changes due to oceanic methane emissions and many other types of cyclicity during the Phanerozoic.
One hemisphere of Mars captured by ESA’s Mars Express. Credit: ESA / DLR / FU Berlin /
As well as time series based on isotopic and other geochemical changes in marine cores, other variables such as thickness of turbidite beds or cyclical repetitions of short rock sequences such as the ‘cyclothems’ of Carboniferous age (repetitions of a limestone, sandstone, soil, coal sequence) have also been subject to frequency analysis. Sedimentary features that have not been tried are gaps or hiatuses in stratigraphic sequences where strata are missing from a deep-sea sequence. These signify erosion of sediment due to vigorous bottom currents in sequences otherwise dominated by continuous deposition under low-energy conditions. Three geoscientists from the University of Sydney, Australia and the Sorbonne University, France, have subjected records of gaps in Cenozoic sedimentation from 293 deep-sea drill cores to time-series analysis to discover what such ‘big data’ might reveal as regards climate fluctuations on the order of millions of years (Dutkiewicz, A., Boulila, S. & Müller, R.D. 2024. Deep-sea hiatus record reveals orbital pacing by 2.4 Myr eccentricity grand cycles. Nature Communications, v. 15, article 1998; DOI: 10.1038/s41467-024-46171-5).
In theory gravitational interrelationships between all the orbiting planets should have an effect on the orbital parameters of each other, and thus the amount of received solar radiation and changes in global climate. As well as the Milankovich effect, longer astronomical ‘grand cycles’ may therefore have been reflected somehow in Earth’s climatic history (Laskar, J. et al. 2004. A long-term numerical solution for the insolation quantities of the Earth. Astronomy & Astrophysics, v. 428, p. 261-285; DOI: 10.1051/0004-6361:20041335). Based on Laskar et al.’s calculations Adriana Dutkiewicz and colleagues sought evidence for two predicted ‘grand cycles’ that result from orbital interactions between Earth and Mars. These are a 2.4 Ma period in the eccentricity of Earth’s orbit and one of 1.2 Ma in the tilt of its axis.
The authors were able to detect cyclicity in the hiatus time series that is close to the 2.4 Ma Mars-induced waxing and waning of solar heating. Warming would increase mixing of ocean water through cyclones and hurricanes. That would then induce more energetic deep ocean currents and more erosion on the deep ocean floor: more gaps in sedimentation. Cooler conditions would ‘calm’ deep ocean currents so that deposition would outweigh evidence of erosion. The 1.2 Ma axial tilt cyclicity is not apparent in the data. Interestingly, the ~2.4 Ma cyclicity underwent a significant deviation at the Palaeocene-Eocene Boundary’ (56Ma), seemingly predicted by Laskar et al’s astronomical solutions as a chaotic orbital transition between 56 and 53 Ma. Dutkiewicz et al. also chart the relations between the sedimentary-hiatus time series and major tectonic, oceanographic, and climatic changes during the Cenozoic Era, and found that terrestrial processes did disrupt the Mars-related orbital eccentricity cycles.
The findings suggest that long-term astronomical climate forcing needs to be borne in mind for better understanding the future response of the ocean to global warming. Also, if Mars had such an influence so must have Venus, which is more massive and closer. That remains to be investigated, and also the effects of the giant planets. In the very distant past there behaviour may have resulted in unimaginable astronomical changes. According to the bizarrely named Nice Model a back and forth shuffling of the Giant Planets was probably responsible for the Late Heavy Bombardment 4.1 to 3.8 billion years (Ga) ago. Such errant behaviour may even have triggered the flinging of some of the Sun’s original planetary complement out of the solar system and changed the outward order of the existing eight. Fortunately, the present planetary set-up seems to be stable …
The International Commission on Stratigraphy (ICS) issues guidance for the division of geological history that has evolved from the science’s original approach: that was based solely on what could be seen in the field. That included: variations in lithology and the law of superposition; unconformities that mark interruptions through deformation, erosion and renewed deposition; the fossil content of sediments and the law of faunal succession; and more modern means of division, such as geomagnetic changes detected in rock over time. That ‘traditional’ approach to relative time is now termed chronostratigraphy, which has evolved since the 19th century from the local to the global scale as geological research widened its approach. Subsequent development of various kinds of dating has made it possible to suggest the actual, absolute time in the past when various stratigraphic boundaries formed – geochronology. Understandably, both are limited by the incompleteness of the geological record – and the whims of individual geologists. For decades the ICS has been developing a combination of both approaches that directly correlates stratigraphic units and boundaries with accurate geochronological ages. This is revised periodically, the ICS having a detailed protocol for making changes. You can view the Cenozoic section of the latest version of the International Chronostratigraphic Chart and the two systems of units below. If you are prepared to travel to a lot of very remote places you can see a monument – in some cases an actual Golden Spike – marking the agreed stratigraphic boundary at the ICS-designated type section for 80 of the 93 lower boundaries of every Stage/Age in the Phanerozoic Eon. Each is a sonorously named Global Boundary Stratotype Section and Point or GSSP (see: The Time Lords of Geology, April 2013). There are delegates to various subcommissions and working groups of the ICS from every continent, they are very busy and subject to a mass of regulations …
Chronostratigraphic Chart for the Cenozoic Era showing the 5 tiers of stratigraphic time division. The little golden spikes mark where a Global Boundary Stratotype Section and Point monument has been erected at the boundary’s type section.
On 11 May 2011, the Geological Society of London hosted a conference, co-sponsored by the British Geological Survey, to discuss evidence for the dawn of a new geological Epoch: the Anthropocene, supposedly marking the impact of humans on Earth processes. There has been ‘lively debate’ about whether or not such a designation should be adopted. An Epoch is at the 4th tier of the chronostratigraphic/geochronologic systems of division, such as the Holocene, Pleistocene, Pliocene and Miocene, let alone a whole host of such entities throughout the Phanerozoic, all of which represent many orders of magnitude longer spans of time and a vast range of geological events. No currently agreed Epoch lasted less than 11.7 thousand years (the Holocene) and all the others spanned 1 Ma to tens of Ma (averaged at 14.2 Ma). Indeed, even geological Ages (the 5th tier) span a range from hundreds of thousands to millions of years (averaged at 6 Ma). Use ‘Anthropocene’ in Search Earth-logs to read posts that I have written on this proposal since 2011, which outline the various arguments for and against it.
In the third week of May 2019 the 34-member Anthropocene Working Group (AWG) of the ICS convened to decide on when the Anthropocene actually started. The year 1952 was proposed – the date when long-lived radioactive plutonium first appears in sediments before the 1962 International Nuclear Test-Ban Treaty. Incidentally, the AWG proposed a GSSP for the base of the Anthropocene in a sediment core through sediments in the bed of Crawford Lake an hour’s drive west of Toronto, Canada. After 1952 there are also clear signs that plastics, aluminium, artificial fertilisers, concrete and lead from petrol began to increase in sediments. The AWG accepted this start date (the Anthropocene ‘golden spike’) by a 29 to 5 vote, and passed it into the vertical ICS chain of decision making. This procedure reached a climax on Monday 4 March 2024, at a meeting of the international Subcommission on Quaternary Stratigraphy (SQS): part of the ICS. After a month-long voting period, the SQS announced a 12 to 4 decision to reject the proposal to formally declare the Anthropocene as a new Epoch. Normally, there can be no appeals for a losing vote taken at this level, although a similar proposal may be resubmitted for consideration after a 10 year ‘cooling off’ period. Despite the decisive vote, however, the chair of the SQS, palaeontologist Jan Zalasiewicz of the University of Leicester, UK, and one of the group’s vice-chairs, stratigrapher Martin Head of Brock University, Canada have called for it to be annulled, alleging procedural irregularities with the lengthy voting procedure.
Had the vote gone the other way, it would marked the end of the Holocene, the Epoch when humans moved from foraging to the spread of agriculture, then the ages of metals and ultimately civilisation and written history. Even the Quaternary Period seemed under threat: the 2.5 Ma through which the genus Homo emerged from the hominin line and evolvd. Yet a pro-Anthropocene vote would have faced two more, perhaps even more difficult hurdles: a ratification vote by the full ICS, and a final one in August 2024 at a forum of the International Union of Geological Sciences (IUGS), the overarching body that represents all aspects of geology.
There can be little doubt that the variety and growth of human interferences in the natural world since the Industrial Revolution poses frightening threats to civilisation and economy. But what they constitute is really a cultural or anthropological issue, rather than one suited to geological debate. The term Anthropocene has become a matter of propaganda for all manner of environmental groups, with which I personally have no problem. My guess is that there will be a compromise. There seems no harm either way in designating the Anthropocene informally as a geological Event. It would be in suitably awesome company with the Permian and Cretaceous mass extinctions, the Great Oxygenation Event at the start of the Proterozoic, the Snowball Earth events and the Palaeocene–Eocene Thermal Maximum. And it would require neither special pleading nor annoying the majority of geologists. But I believe it needs another name. The assault on the outer Earth has not been inflicted by the vast majority of humans, but by a tiny minority who wield power for profit and relentless growth in production. The ‘Plutocracene’ might be more fitting. Other suggestions are welcome …
Both physical and chemical weathering reflects climatic controls. Erosion is effectively climate in continuous action on the Earth’s solid surface through water, air and bodies of ice moving under the influence of gravity. These two major processes on the land surface are immensely complicated. Being the surface part of the rock cycle, they interact with biological processes in the continents’ web of climate-controlled ecosystems. It is self-evident that climate exerts a powerful influence on all terrestrial landforms. But at any place on the Earth’s surface climate changes on a whole spectrum of rates and time scales as reflected by palaeoclimatology. With little room for doubt, so too do weathering and erosion. Yet other forces are at play in the development of landforms. ‘Wearing-down’ of elevated areas removes part of the load that the lithosphere bears, so that the surface rises in deeply eroded terrains. Solids removed as sediments depress the lithosphere where they are deposited in great sedimentary basins. In both cases the lithosphere rises and falls to maintain isostatic balance. On the grandest of scales, plate tectonics operates continuously as well. Its lateral motions force up mountain belts and volcanic chains, and drag apart the lithosphere, events that in themselves change climate at regional levels. Tectonics thereby creates ‘blips’ in long term global climate change. So evidence for links between landform evolution and palaeoclimate is notoriously difficult to pin down, let alone analyse.
The evidence for climate change over the last few million years is astonishingly detailed; so much so that it is possible to detect major global events that took as little as a few decades, such as the Younger Dryas, especially using data from ice cores. The record from ocean-floor sediments is good for changes over hundreds to thousands of years. The triumph of palaeoclimatology is that the last 2.5 Ma of Earth’s history has been proved to have been largely paced by variations in the Earth’s orbit and in the angle of tilt and wobbles of its rotational axis: a topic that Earth-logs has tracked since the start of the 21st century. The record also hints at processes influencing global climate that stem from various processes in the Earth system itself, at irregular but roughly millennial scales. The same cannot be said for the geological record of erosion, for a variety of reasons, foremost being that erosion and sediment transport are rarely continuous in any one place and it is more difficult to date the sedimentary products of erosion than ice cores and laminations in ocean-floor sediments. Nonetheless, a team from the US, Germany, the Netherlands , France and Argentina have tackled this thorny issue on the eastern side of the Andes in Argentina (Fisher, G.B. and 11 others 2023. Milankovitch-paced erosion in the southern Central Andes. Nature Communications, v. 14, 424-439; DOI: 10.1038/s41467-023-36022-0.
Burch Fisher (University of Texas at Austin, USA) and colleagues studied sediments derived from a catchment that drains the Puna Plateau that together with the Altiplano forms the axis of the Central Andes. In the late 19th century the upper reaches of the Rio Iruya were rerouted, which has resulted in its cutting a 100 m deep canyon through Pliocene to Early Pleistocene (6.0 to 1.8 Ma) sediments. The section includes six volcanic ash beds (dated precisely using the zircon U-Pb method) and records nine palaeomagnetic reversals, which together helped to calibrate more closely spaced dating. Their detailed survey used the decay of radioactive isotopes of beryllium and aluminium (10Be and 26Al) in quartz grains that form in the mineral when exposed at the surface to cosmic-ray bombardment. Such cosmogenic radionuclide dating thus records the last time different sediment levels were at the surface, presumably when the sediment was buried, and thus the variation in the rate of sediment supply from erosion of the Rio Iruya catchment since 6 Ma ago.
Measured concentrations (low to high values downwards) of cosmogenic 10Be (turquoise) and 26Al (red) in samples from the Rio Iruya sediment sequence. The higher the value, the longer the layer had resided at the surface; i.e. the slower the erosion rate. (Credit: Fisher et al. Fig 4)
The data from 10Be suggest that erosion rates were consistently high from 6 to 4 Ma, but four times during the later Pliocene and the earliest Pleistocene they slowed dramatically. Each of these episodes occupies downturns in solar warming forced by the 400 ka cycle of orbital eccentricity. The 26Al record confirms this trend. The most likely reason for the slowing of erosion is long-term reductions in rainfall, which Fisher et al have modelled based on Milankovich cycles. However the modelled fluctuations are subtle, suggesting that in the Central Andes at least erosion rates were highly sensitive to climatic fluctuations. Yet the last 400 ka cycle in the record shows no apparent correlation with climate change. Despite that, astronomical forcing while early Pleistocene oscillations between cooling and warming ramped up does seem to have affected erosion rates based on the cosmogenic dating. The authors attribute this loss of the 400 ka pattern to a kind of swamping effect of dramatically increased erosion rates as the regional climate became more erratic. Whether or not data of this kind will emerge for the more climatically drastic 100 ka cyclicity of the last million years remains to be seen … Anyone who has walked over terrains covered in glacial tills and glaciofluvial gravel beds nearer to the former Late Pleistocene ice sheets can judge the difficulty of such a task.
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