Human impact on surface geological processes

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

‘Badlands’ formed by accelerated soil erosion.

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

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

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

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

Photosynthesis, arsenic and a window on the Archaean world

At the very base of the biological pyramid life is far simpler than that which we can see.  It takes the form of single cells that lack a nucleus and propagate only by cloning: the prokaryotes as opposed to eukaryote life such as ourselves. It is almost certain that the first viable life on Earth was prokaryotic, though which of its two fundamental divisions – Archaea or Bacteria – came first is still debated. At present, most prokaryotes metabolise other organisms’ waste or dead remains: they are heterotrophs (from the Greek for ‘other nutrition’). But there are others that are primary producers getting their nutrition by themselves, exploiting the inorganic world in a variety of ways: the autotrophs. Biogeochemical evidence from the earliest sedimentary rocks suggests that, in the Archaean prokaryotic autotrophs were dominant, mainly exploiting chemical reactions to gain energy necessary for building carbohydrates. Some reduced sulfate ions to those of sulphide, others combined hydrogen with carbon dioxide to generate methane as a by-product. Sunlight being an abundant energy resource in near-surface water, a whole range of prokaryotes exploit its potential through photosynthesis. Under reducing conditions some photosynthesisers convert sulfur to sulfuric acid , yet others combine photosynthesis with chemo-autotrophy. Dissolved material capable of donating electrons – i.e. reducing agents – are exploited in photosynthesis: hydrogen, ferrous iron (Fe2+), reduced sulfur, nitrite, or some organic molecules. Without one group, which uses photosynthesis to convert CO2 and water to carbohydrates and oxygen, eukaryotes would never have arisen, for they depend on free oxygen. A transformation 2400 Ma ago marked a point in Earth history when oxygen first entered the atmosphere and shallow water (see: Massive event in the Precambrian carbon cycle; January, 2012), known as Great Oxygenation Event (GOE). It has been shown that the most likely sources of that excess oxygen were extensive bacterial mats in shallow water made of photosynthesising blue-green bacteria that produced the distinctive carbonate structures known as stromatolites. These had formed in Archaean sedimentary basins for 1.9 billion years. It has been generally assumed that blue-green bacteria had formed them too, before the oxygen that they produced overcame the reducing conditions that had generally prevailed before the GOE. But that may not have been the case …

Microbial mats made by purple sulfur bacteria in highly toxic spring water flowing into a salt-lake in northern Chile. (credit: Visscher et al. 2020; Fig 1c)

Prokaryotes are a versatile group and new types keep turning up as researchers explore all kinds of strange and extreme environments, for instance: hot springs; groundwater from kilometres below the surface and highly toxic waters. A recent surprise arose from the study of anoxic springs laden with dissolved salts, sulfide ions and arsenic that feed parts of hypersaline lakes in northern Chile (Visscher, P.T. and 14 others 2020. Modern arsenotrophic microbial mats provide an analogue for life in the anoxic ArcheanCommunications Earth & Environment, v. 1, article 24; DOI: 10.1038/s43247-020-00025-2). This is a decidedly extreme environment for life, as we know it, made more challenging by its high altitude exposure to high UV radiation. The springs’ beds are covered with bright-purple microbial mats. Interestingly the water’s arsenic concentration varies from high in winter to low in summer, suggesting that some process removes it, along with sulfur, according to light levels: almost certainly the growth and dormancy of mat-forming bacteria. Arsenic is an electron donor capable of participating in photosynthesis that doesn’t produce oxygen. The microbial mats do produce no oxygen whatever – uniquely for the modern Earth – but they do form carbonate crusts that look like stromatolites. The mats contain purple sulfur bacteria (PSBs) that are anaerobic photosynthesisers, which use sulfur, hydrogen and Fe2+ as electron donors. The seasonal changes in arsenic concentration match similar shifts in sulfur, suggesting that arsenic is also being used by the PSBs. Indeed they can, as the aio gene, which encodes for such an eventuality, is present in the genome of PSBs.

Pieter Visscher and his multinational co-authors argue for prokaryotes similar to modern PSBs having played a role in creating the stromatolites found in Archaean sedimentary rocks. Oxygen-poor, the Archaean atmosphere would have contained no ozone so that high-energy UV would have bathed the Earth’s surface and its oceans to a considerable depth. Moreover, arsenic is today removed from most surface water by adsorption on iron hydroxides, a product of modern oxidising conditions (see: Arsenic hazard on a global scale; May 2020): it would have been more abundant before the GOE. So the Atacama springs may be an appropriate micro-analogue for Archaean conditions, a hypothesis that the authors address with reference to the geochemistry of sedimentary rocks in Western Australia deposited in a late-Archaean evaporating lake. Stromatolites in the Tumbiana Formation show, according to the authors, definite evidence for sulfur and arsenic cycling similar to that in that Atacama springs. They also suggest that photosynthesising blue-green bacteria (cyanobacteria) may not have viable under such Archaean conditions while microbes with similar metabolism to PSBs probably were. The eventual appearance and rise of oxygen once cyanobacteria did evolve, perhaps in the late-Archaean, left PSBs and most other anaerobic microbes, to which oxygen spells death, as a minority faction trapped in what are became ‘extreme’ environments when long before they ‘ruled the roost’. It raises the question, ‘What if cyanobacteria had not evolved?’. A trite answer would be, ‘I would not be writing this and nor would you be reading it!’. But it is a question that can be properly applied to the issue of alien life beyond Earth, perhaps on Mars. Currently, attempts are being made to detect oxygen in the atmospheres of exoplanets orbiting other stars, as a ‘sure sign’ that life evolved and thrived there too. That may be a fruitless venture, because life happily thrived during Earth’s Archaean Eon until its closing episodes without producing a whiff of oxygen.

See also: Living in an anoxic world: Microbes using arsenic are a link to early life. (Science Daily, 22 September 2020)

End-Triassic mass extinction: evidence for oxygen depletion on the ocean floor

For British geologists of my generation the Triassic didn’t raise our spirits to any great extent. There’s quite a lot of it on the British Geological Survey 10-miles-to-the-inch geological map (South Sheet) but it is mainly muds, sandstones or pebble beds, generally red and largely bereft of fossils. For the Triassic’s 50 Ma duration following the end-Permian extinction at 252 Ma Britain was pretty much a desert in the middle of the Pangaea supercontinent. Far beyond our travel grants’ reach, the Triassic is a riot, as in the Dolomites of Northern Italy. Apart from a day trip to look at the Bunter Pebble Beds in a quarry near Birmingham and several weeks testing the load-bearing strength of the Keuper mudstones in the West Midlands (not far off zero) in a soil-mechanics lab, we did glimpse the then evocatively named Tea Green Marl (all these stratigraphic names have vanished). Conveniently they outcrop by the River Severn estuary, below its once-famous suspension bridge and close-by the M5 motorway. Despite the Tea Green Marl containing a bone bed with marine reptiles, time didn’t permit us to fossick, and, anyway, there was a nearby pub … The formation was said to mark a marine transgression leading on to the ‘far more interesting Jurassic’ – the reason we were in the area. We were never given even a hint that the end of the Triassic was marked by one of the ‘Big Five’ mass extinctions: such whopping events were not part of the geoscientific canon in the 1960s.

Pangaea just before the start of Atlantic opening at the end of the Triassic, showing the estimated extend of the CAMP large igneous province. The pink triangles show the sites investigated by He and colleagues.

At 201.3 Ma ago around 34 % of marine genera disappeared, comparable with the effect of the K-Pg extinction that ended the Mesozoic Era. Extinction of Triassic terrestrial animals is less quantifiable. Early dinosaurs made it through to diversify hugely during the succeeding Jurassic and Cretaceous Periods. Probably because nothing famous ceased to be or made its first appearance, the Tr-J mass extinction hasn’t captured public attention in the same way as those with the K-Pg or the P-Tr acronyms.  But it did dramatically alter the course of biological evolution. The extinctions coincided with a major eruption of flood basalts known as the Central Atlantic Magmatic Province (CAMP), whose relics occur on either side of the eponymous ocean, which began to open definitively at about the same time. So, chances are, volcanic emissions are implicated in the extinction event, somehow (see: Is end-Triassic mass extinction linked to CAMP flood basalts? June 2013). Tianchen He  of Leeds University, UK and the China University of Geosciences and British and Italian colleagues have studied three Tr-J marine sections on either side of Pangaea: in Sicily, Northern Ireland and British Columbia (He, T. and 12 others 2020. An enormous sulfur isotope excursion indicates marine anoxia during the end-Triassic mass extinction. Science Advances, v. 6, article eabb6704; DOI: 10.1126/sciadv.abb6704). Their objective was to test the hypothesis that CAMP resulted in an episode of oceanic anoxia that caused the many submarine organisms to become extinct. Since eukaryote life depends on oxygen, a deficit would put marine animals of the time under great stress. Such events in the later Mesozoic account for global occurrences of hydrocarbon-rich, black marine shales – petroleum source rocks – in which hypoxia thwarted complete decay of dead organisms over long periods. However there is scant evidence for such rocks having formed ~201 Ma ago. Such as there is dates to about 150 ka younger than the Tr-J boundary in an Italian shallow marine basin. The issue of evidence is compounded by the fact that there are no ocean-floor sediments as old as that, thanks to their complete subduction as Pangaea broke apart in later times and its continental fragments drifted to their present configuration.

But there is an indirect way of detecting deep-ocean anoxia, in the inevitable absence of any Triassic and early Jurassic oceanic crust. It emerges from what happens to the stable isotopes of sulfur when there are abundant bacteria that use the reduction of sulfate (SO42-) to sulfide (S2-) ions. Such microorganisms thrive in anoxic conditions and produce abundant hydrogen sulfide, which in turn leads to the precipitation of dissolved iron as minute grains of pyrite (FeS2). This biogenic process selectively excludes 34S from the precipitated pyrite. As a result, at times of widespread marine reducing conditions seawater as a whole becomes enriched in 34S relative to sulfur’s other isotopes. The enrichment is actually expressed in the unreacted sulfate ions, and they may be precipitated as calcium sulfate or gypsum (CaSO4) in marine sediments deposited anywhere: He et al. focussed on such fractionation. They discovered large ‘spikes’ in the relative enrichment of 34S at the Tr-J boundary in shallow-marine sedimentary sequences exposed at the three sites. Moreover, they were able to estimate that the conditions on the now vanished bed of the Triassic ocean that gave rise to the spikes lasted for about 50 thousand years. The lack of dissolved oxygen resulted in a five-fold increase in pyrite burial in the now subducted ocean-floor sediments of that time. The authors suggest that the oxygen depletion stemmed from extreme global warming, which, in turn, encouraged methane production by other ocean-floor bacteria and, in a roundabout way, other chemical reactions that consumed free dissolved oxygen. Quite a saga of a network of interactions in the whole Earth system that may hold a dreadful warning for the modern Earth and ourselves.

‘Mud, mud, glorious mud’

Earth is a water world, which is one reason why we are here. But when it comes to sedimentary rocks, mud is Number 1. Earth’s oceans and seas hide vast amounts of mud that have accumulated on their floors since Pangaea began to split apart about 200 Ma ago during the Early Jurassic. Half the sedimentary record on the continents since 4 billion years ago is made of mudstones. They are the ultimate products of the weathering of crystalline igneous rocks, whose main minerals – feldspars, pyroxenes, amphiboles, olivines and micas, with the exception of quartz – are all prone to breakdown by the action of the weakly acidic properties of rainwater and the CO2 dissolved in it. Aside from more resistant quartz grains, the main solid products of weathering are clay minerals (hydrated aluminosilicates) and iron oxides and hydroxides. Except for silicon, aluminium and ferric iron, most metals end up in solution and ultimately the oceans.  As well as being a natural product of weathering, mud is today generated by several large industries, and humans have been dabbling in natural muds since the invention of pottery some 25 thousand years ago.  On 21 August 2020 the journal Science devoted 18 pages to a Special Issue on mud, with seven reviews (Malakoff, D. 2020. Mud. Science, v. 369, p. 894-895; DOI: 10.1126/science.369.6506.894).

Mud carnival in Brazil (Credit:

The rate at which mud accumulates as sediment depends on the rate at which erosion takes place, as well as on weathering. Once arable farming had spread widely, deforestation and tilling the soil sparked an increase in soil erosion and therefore in the transportation and deposition of muddy sediment. The spurt becomes noticeable in the sedimentary record of river deltas, such as that of the Nile, about 5000 years ago. But human influences have also had negative effects, particularly through dams. Harnessing stream flow to power mills and forges generally required dams and leats. During medieval times water power exploded in Europe and has since spread exponentially through every continent except Antarctica, with a similar growth in the capacity of reservoirs. As well as damming drainage these efforts also capture mud and other sediments. A study of drainage basins in north-east USA, along which mill dams quickly spread following European colonisation in the 17th century, revealed their major effects on valley geomorphology and hydrology (see: Watermills and meanders; March 2008). Up to 5 metres of sediment build-up changed stream flow to an extent that this now almost vanished industry has stoked-up the chances of major flooding downstream and a host of other environmental changes. The authors of the study are acknowledged in one Mud article (Voosen, P. 2020. A muddy legacy. Science, v. 369, p. 898-901; DOI: 10.1126/science.369.6506.898) because they have since demonstrated that the effects in Pennsylania are reversible if the ‘legacy’ sediment is removed. The same cannot be expected for truly vast reservoirs once they eventually fill with muds to become useless. While big dams continue to function, alluvium downstream is being starved of fresh mud that over millennia made it highly and continuously productive for arable farming, as in the case of Egypt, the lower Colorado River delta and the lower Yangtze flood plain below China’s Three Gorges Dam.

Mud poses extreme risk when set in motion. Unlike sand, clay deposits saturated with water are thixotropic – when static they appear solid and stable but as soon as they begin to move en masse they behave as a viscous fluid. Once mudflows slow they solidify again, burying and trapping whatever and whomever they have carried off. This is a major threat from the storage of industrially created muds in tailings ponds, exemplified by a disaster at a Brazilian mine in 2019, first at the site itself and then as the mud entered a river system and eventually reached the sea. Warren Cornwall explains how these failures happen and may be prevented (Cornwall, W. 2020. A dam big problem.  Science, v. 369, p. 906-909; DOI: 10.1126/science.369.6506.906). Another article in the Mud special issue considers waste from aluminium plants (Service, R.F. 2020. Red alert. Science, v. 369, p. 910-911; DOI: 10.1126/science.369.6506.910). The main ore for aluminium is bauxite, which is the product of extreme chemical weathering in the tropics. The metal is smelted from aluminium hydroxides formed when silica is leached out of clay minerals, but this has to be separated from clay minerals and iron oxides that form a high proportion of commercial bauxites, and which are disposed of in tailings dams. The retaining dam of such a waste pond in Hungary gave way in 2010, the thixotropic red clay burying a town downstream to kill 10 people. This mud was highly alkaline and inflicted severe burns on 150 survivors. Service also points out a more positive aspect of clay-rich mud: it can absorb CO2 bubbled through it to form various, non-toxic carbonates and help draw down the greenhouse gas.

Muddy sediments are chemically complex, partly because their very low permeability hinders oxygenated water from entering them: they maintain highly reducing conditions. Because of this, oxidising bacteria are excluded, so that much of the organic matter deposited in the muds remains as carbonaceous particles. They store carbon extracted from the atmosphere by surface plankton whose remains sink to the ocean floor. Consequently, many mudrocks are potential source rocks for petroleum. Although they do not support oxygen-demanding animals, they are colonised by bacteria of many different kinds. Some – methanogens – break down organic molecules to produce methane. The metabolism of others depends on sulfate ions in the trapped water, which they reduce to sulfide ions and thus hydrogen sulfide gas: most muds stink. Some of the H2S reacts with metal ions, to precipitate sulfide minerals, the most common being pyrite (FeS2). In fact a significant proportion of the world’s copper, zinc and lead resources reside in sulfide-rich mudstones: essential to the economies of Zambia and the Democratic Republic of Congo. But there are some strange features of mud-loving bacteria that are only just emerging. The latest is the discovery of bacteria that build chains up to 5 cm long that conduct electricity (Pennisi, E. 2020. The mud is electric. Science, v. 369, p. 902-905; DOI: 10.1126/science.369.6506.902). The bacterial ‘nanowires’ sprout from minute pyrite grains, and transfer electrons released by oxidation of organic compounds, effectively to catalyse sulfide-producing reduction reactions. NB Oxygen is not necessary for oxidation as its chemistry involves the loss of electrons, while reduction involves a gain of electrons, expressed by the acronym OILRIG (oxidation is loss, reduction is gain). It seems such electrical bacteria are part of a hitherto unsuspected chemical ecosystem that helps hold the mud together as well as participating in a host of geochemical cycles. They may spur an entirely new field of nano-technology, extending, bizarrely, to an ability to generate electricity from moisture in the air.

If you wish to read these reviews in full, you might try using their DOIs at Sci Hub.

How marine animal life survived (just) Snowball Earth events

A Cryogenian glacial diamictite containing boulders of many different provenances from the Garvellach Islands off the west coast of Scotland. (Credit: Steve Drury)

Glacial conditions during the latter part of the Neoproterozoic Era extended to tropical latitudes, probably as far as the Equator, thereby giving rise to the concept of Snowball Earth events. They left evidence in the form of sedimentary strata known as diamictites, whose large range of particle size from clay to boulders has a range of environmental explanations, the most widely assumed being glacial conditions. Many of those from the Cryogenian Period are littered with dropstones that puncture bedding, which suggest that they were deposited from floating ice similar to that forming present-day Antarctic ice shelves or extensions of onshore glaciers. Oceans on which vast shelves of glacial ice floated would have posed major threats to marine life by cutting off photosynthesis and reducing the oxygen content of seawater. That marine life was severely set back is signalled by a series of perturbations in the carbon-isotope composition of seawater. Its relative proportion of 13C to 12C (δ13C) fell sharply during the two main Snowball events and at other times between 850 to 550 Ma. The Cryogenian was a time of repeated major stress to Precambrian life, which may well have speeded up evolution, sediments of the succeeding Ediacaran Period famously containing the first large, abundant and diverse eukaryote fossils.

For eukaryotes to survive each prolonged cryogenic stress required that oxygen was indeed present in the oceans. But evidence for oxygenated marine habitats during Snowball Earth events has been elusive since these global phenomena were discovered. Geoscientists from Australia, Canada, China and the US have applied novel geochemical approaches to occasional iron-rich strata within Cryogenian diamictite sequences from Namibia, Australia and the south-western US in an attempt to resolve the paradox (Lechte, M.A. and 8 others 2019. Subglacial meltwater supported aerobic marine habitats during Snowball Earth. Proceedings of the National Academy of Sciences, 2019; 201909165 DOI: 10.1073/pnas.1909165116). Iron isotopes in iron-rich minerals, specifically the proportion of 56Fe relative to that of 54Fe (δ56Fe), help to assess the redox conditions when they formed. This is backed up by cerium geochemistry and the manganese to iron ratio in ironstones.

In the geological settings that the researchers chose to study there are sedimentological features that reveal where ice shelves were in direct contact with the sea bed, i.e. where  they were ‘grounded’. Grounding is signified by a much greater proportion of large fragments in diamictites, many of which are striated through being dragged over underlying rock. Far beyond the grounding line diamictites tend to be mainly fine grained with only a few dropstones. The redox indicators show clear changes from the grounding lines through nearby environments to those of deep water beneath the ice. Each of them shows evidence of greater oxidation of seawater at the grounding line and a falling off further into deep water. The explanation given by the authors is fresh meltwater flowing through sub-glacial channels at the base of the grounded ice fed by melting at the glacier surface, as occurs today during summer on the Greenland ice cap and close to the edge of Antarctica. Since cold water is able to dissolve gas efficiently the sub-glacial channels were also transporting atmospheric oxygen to enrich the near shore sub-glacial environment of the sea bed. In iron-rich water this may have sustained bacterial chemo-autotrophic life to set up a fringing food chain that, together with oxygen, sustained eukaryotic heterotrophs. In such a case, photosynthesis would have been impossible, yet unnecessary. Moreover, bacteria that use the oxidation of dissolved iron as an energy source would have caused Fe-3 oxides to precipitate, thereby forming the ironstones on which the study centred. Interestingly, the hypothesis resembles the recently discovered ecosystems beneath Antarctic ice shelves.

Small and probably unconnected ecosystems of this kind would have been conducive to accelerated evolution among isolated eukaryote communities. That is a prerequisite for the sudden appearance of the rich Ediacaran faunas that colonised sea floors globally once the Cryogenian ended. Perhaps these ironstone-bearing diamictite occurrences where the biological action seems to have taken place might, one day, reveal evidence of the precursors to the largely bag-like Ediacaran animals

What followed the K-Pg extinction event?

A study of boron isotopes in the tests of foraminifera that lived deep in the oceans and near their surface just after the K-Pg boundary event has revealed that ocean water suddenly became more acidic (Henehan, M.J. and 13 others 2019. Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact. Proceedings of the National Academy of Sciences. Online; DOI: 10.1073/pnas.1905989116). Because the data came from marine sediment sequences exposed in Europe and North America  and from ocean-floor cores beneath the Atlantic and Pacific Oceans, the acidification was global in scope. The sharp fall in pH, almost certainly due to massive release of sulphuric and carbonic acids from thick anhydrite  and limestone beds beneath the Chicxulub impact site was instrumental in the collapse of marine ecosystems. A rebound to higher, more alkaline pH values (overshooting those of the preceding Late Cretaceous) was equally rapid. That is ascribed to the post-extinction dearth of marine organisms that take up calcium in their shells so that dissolved Ca became more abundant. Within less than 100 ka of the Chicxulub impact ocean pH had returned to its pre-impact levels. Since Deccan flood-basalt volcanism was active until long after, Henehan et al. consider that its influence on ocean acidification was minimal and that The Chicxulub impact ‘was key in driving end-Cretaceous mass extinction’.

Records of marine fossils are both more abundant and continuous than are those of land-based organisms. That animal extinctions on the continents were dramatic has been clear for over a century. Entire classes, notably the dinosaurs (except for birds), as well as orders, families, genera and species disappear from the fossil record. The event more than decimated plant taxa too. How and at what pace the vacated ecological niches were reoccupied during the evolutionary radiation among what became modern fauna and flora remain poorly understood. For the first million years of post-impact time fossils of terrestrial and freshwater organisms are very rare. Well-dated sedimentary sequences are patchily distributed, and fossils preserved in them as rare as proverbial hen’s teeth, apart from a few, better endowed strata separated by thick, unproductive sediments. A Lower Palaeocene site near Denver in Colorado, USA extends for 27 km. At first sight it does not impress palaeontologists, but it carries concretions that yield rich hauls of tiny vertebrate fossils. Dating using U-Pb dating of interleaved volcanic ash layers, stratigraphy based on normal and reversed polarity of remanent magnetism, and plant pollen variations. The 250 m thick sedimentary unit can be divided into 150 levels that represent the first million years flowing the Chicxulub impact (Lyson, T.R. and 15 others 2019. Paleogene mass extinction -Exceptional continental record of biotic recovery after the Cretaceous. Science, online first release; DOI: 10.1126/science.aay2268.

Reconstruction of the 35 kg early Palaeocene mammal Taeniolabis (credit: Wikipedia)

The levels contain abundant remains of early Cenozoic mammals, particularly skulls that are vitally important in taxonomy and size estimation. During the last few hundred thousand years of the Cretaceous, mammals about the size of a modern racoon (~8 kg) were abundant. The oldest Palaeocene holds nothing bigger than a 600 g rat, and few of them. Then, remarkably, the numbers, diversity and mean body mass of mammals grow; raccoon-size back within 100 ka then, in a series of steps, beasts around 25, 35 and 45 kg emerged successively during the next 600 ka. Clearly, the local food chain had to support this growth in size as well as numbers. Pollen records reveal a terrain first dominated by ferns – not especially nutritious – then after 200 ka by palms and finally legumes (pulses) appear. The diversification of animals and plants changed in lockstep. Studies of fossil-leaf shapes (toothed = cooler; smooth = warmer) indicated a similarly triple-stepwise amelioration in climate from cool, post-impact to hot by 65 Ma ago. This climatic warming may have been connected to successive pulses of Deccan volcanism that drove up atmospheric CO2 levels. Geologically, that is pretty quick. In the context of a possible, equally rapid mass extinction as a result of anthropogenic factors, such a pace of recovery is hardly reassuring…

Ancient oil migration

In order for petroleum deposits to form, the first requirement is a source of abundant hydrocarbons, most usually from a mudstone that was deposited under highly reducing conditions. In such an environment dead organic matter can accumulate without complete decay and oxidation to form a source rock or black shale. The next step comes from burial and heating until the dead matter matures to release liquid and gaseous hydrocarbons. In turn these fluids, along with heated water, must leave the impermeable source rock and migrate through more porous and permeable strata, such as sandstone or limestone reservoir rocks. Either they reach the surface to escape or become trapped in some kind of geological structure. In migrating, the hydrocarbons induce reducing condition in the rocks through which they flow, often bleaching them as the colouring agents based on insoluble iron-3 compounds are reduced to iron-2 that dissolves and is carried out of the system along with the hydrocarbons.

Throughout the Precambrian, the Earth was lacking in free or dissolved oxygen, even after the Great Oxidation Event at around 2.4 to 2.1 billion years ago; ideal conditions for the formation of black-shale source rocks. And indeed there are huge volumes of them going back to the Palaeoarchaean Era (>3.25 Ga). The Earth’s heat flow having be greater then, due to less decay of radioactive heat-producing elements in the mantle, petroleum must have been generated in volumes at least as large as that released during the Phanerozoic. Yet there are few oilfields of Precambrian age, and geologists usually don’t bother looking for oil in very ancient rocks, largely because the older a rock sequence is the more likely it has been deeply buried and heated above the temperature at which oil breaks down into hydrocarbon gases (~130°C), which in turn are destroyed above about 250°C. Moreover, many such ancient rocks have generally been deformed by many phases of brittle tectonic processes that formed zones of fracturing that give lines of easy escape for pressurised fluids.

Interleaved chert (white) and ironstone of the Palaeoproterozoic Gunflint Iron Formation of Ontario, Canada and Minnesota, USA.

So, looking for telltale signs of oil formation and migration in Precambrian strata is pretty much a matter of academic curiosity. Solid, bituminous hydrocarbons granules and veins are not uncommon in Precambrian sediments, although their relationships do not rule out later introduction into ancient rocks. Birger Rasmussen of the University of Western Australia has been tracking down such signs for over 30 years, his best known discovery – in 2005 – being in Archaean rocks (3.2 to 2.6 Ga) of the Pilbara craton in Western Australia. Recently, he and Janet Muhling of the same institution reported stunning evidence of migration in the Palaeoproterozoic Era (Rasmussen, B. & Muhling, J.R. 2019. Evidence for widespread oil migration in the 1.88 Ga Gunflint Formation, Ontario, Canada. Geology, v. 47, p. 899-903; DOI: 10.1130/G46469.1). The sedimentary unit is a banded iron formation containing interleaved cherts (famous for their content of some of the oldest incontrovertible microfossils), a granular variant of which is pervaded by solid bitumen in both granules and former pore spaces. This is interpreted as the result of oil migration during the actual cementation of the ironstone by silica; i.e. during diagenesis below the seabed rather than through solid sedimentary rock. Bitumen also fills later fractures. Rasmussen and Muhling consider the most likely scenario for this undoubted Palaeoproterozoic reservoir to have formed. They conclude that it coincided with the tectonic burial of the BIF basin beneath an exotic thrust block about 20 Ma after its formation. This generated petroleum from older source rocks, remote from the site of BIF deposition, that migrated away and up-dip from the thrust belt following the unconsolidated BIF formation.

Geochemical background to the Ediacaran explosion

The first clear and abundant signs of multicelled organisms appear in the geological record during the 635 to 541 Ma Ediacaran Period of the Neoproterozoic, named from the Ediacara Hills of South Australia where they were first discovered in the late 19th century. But it wasn’t until 1956, when schoolchildren fossicking in Charnwood Forest north of Leicester in Britain found similar body impressions in rocks that were clearly Precambrian age that it was realised the organism predated the Cambrian Explosion of life. Subsequently they have turned-up on all continents that preserve rocks of that age (see: Larging the Ediacaran, March 2011). The oldest of them, in the form of small discs, date back to about 610 Ma, while suspected embryos of multicelled eukaryotes are as old as the very start of the Edicaran (see; Precambrian bonanza for palaeoembryologists, August 2006).

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

The Ediacaran fauna appeared soon after the Marinoan Snowball Earth glaciogenic sediments that lies at the top of the preceding Cryogenian Period (650-635 Ma), which began with far longer Sturtian glaciation (715-680 Ma). A lesser climatic event – the 580 Ma old Gaskiers glaciation – just preceded the full blooming of the Ediacaran fauna. Geologists have to go back 400 million years to find an earlier glacial epoch at the outset of the Palaeoproterozoic. Each of those Snowball Earth events was broadly associated with increased availability of molecular oxygen in seawater and the atmosphere. Of course, eukaryote life depends on oxygen. So, is there a connection between prolonged, severe climatic events and leaps in the history of life? It does look that way, but begs the question of how Snowball Earth events were themselves triggered. Continue reading “Geochemical background to the Ediacaran explosion”

A major Precambrian impact in Scotland

The northwest of Scotland has been a magnet to geologists for more than a century. It is easily accessed, has magnificent scenery and some of the world’s most complex geology. The oldest and structurally most tortuous rocks in Europe – the Lewisian Gneiss Complex – which span crustal depths from its top to bottom, dominate much of the coast. These are unconformably overlain by a sequence of mainly terrestrial sediments of Meso- to Neoproterozoic age – the Torridonian Supergroup – laid down by river systems at the edge of the former continent of  Laurentia. They form a series of relic hills resting on a rugged landscape carved into the much older Lewisian. In turn they are capped by a sequence of Cambrian to Lower Ordovician shallow-marine sediments. A more continuous range of hills no more than 20 km eastward of the coast hosts the famous Moine Thrust Belt in which the entire stratigraphy of the region was mangled between 450 and 430 million years ago when the elongated microcontinent of Avalonia collided with and accreted to Laurentia.  Exposures are the best in Britain and, because of the superb geology, probably every geologist who graduated in that country visited the area, along with many international geotourists. The more complex parts of this relatively small area have been mapped and repeatedly examined at scales larger than 1:10,000; its geology is probably the best described on Earth. Yet, it continues to throw up dramatic conclusions. However, the structurally and sedimentologically simple Torridonian was thought to have been done and dusted decades ago, with a few oddities that remained unresolved until recently.

NW Scotland geol
Grossly simplified geological map of NW Scotland (credit: British Geological Survey)

Continue reading “A major Precambrian impact in Scotland”

The effect of surface processes on tectonics

Active sedimentation in the Indus and Upper Ganges plains (green vegetated) derived from rapid erosion of the Himalaya (credit: Google Earth)

The Proterozoic Eon of the Precambrian is subdivided into the Palaeo-, Meso- and Neoproterozoic Eras that are, respectively, 900, 600 and 450 Ma long. The degree to which geoscientists are sufficiently interested in rocks within such time spans is roughly proportional to the number of publications whose title includes their name. Searching the ISI Web of Knowledge using this parameter yields 2000, 840 and 2700 hits in the last two complete decades, that is 2.2, 1.4 and 6.0 hits per million years, respectively. Clearly there is less interest in the early part of the Proterozoic. Perhaps that is due to there being smaller areas over which they are exposed, or maybe simply because what those rocks show is inherently less interesting than those of the Neoproterozoic. The Neoproterozoic is stuffed with fascinating topics: the appearance of large-bodied life forms; three Snowball Earth episodes; and a great deal of tectonic activity, including the Pan-African orogeny. The time that precedes it isn’t so gripping: it is widely known as the ‘boring billion’ – coined by the late Martin Brazier – from about 1.75 to 0.75 Ga. The Palaeoproterozoic draws attention by encompassing the ‘Great Oxygenation Event’ around 2.4 Ga, the massive deposition of banded iron formations up to 1.8 Ga, its own Snowball Earth, emergence of the eukaryotes and several orogenies. The Mesoproterozoic witnesses one orogeny, the formation of a supercontinent (Rodinia) and even has its own petroleum potential (93 billion barrels in place in Australia’s Beetaloo Basin. So it does have its high points, but not a lot. Although data are more scanty than for the Phanerozoic Eon, during the Mesoproterozoic the Earth’s magnetic field was much steadier than in later times. That suggests that motions in the core were in a ‘steady state’, and possibly in the mantle as well. The latter is borne out by the lower pace of tectonics in the Mesoproterozoic. Continue reading “The effect of surface processes on tectonics”