Anthropocene more an Event than an Epoch.

The Vattenfall lignite mine in Germany; the Anthropocene personified

The issue of whether or not to assign the time span during which human activities have been significantly affecting the planet and its interwoven Earth Systems has been dragging on since the term ‘Anthropocene’ was first proposed more than two decades ago. A suggestion that may resolve matters, both amicably and with a degree of scientific sense, has emerged in a short letter to the major scientific journal Nature, written by six eminent scientists (Bauer, A.M. et al. 2021. Anthropocene: event or epoch? Nature, v. 597, p. 332; DOI: 10.1038/d41586-021-02448-z). The full text is below

The concept of the Anthropocene has inspired more than two decades of constructive scholarship and public discussion. Yet much of this work seems to us incompatible with the proposal to define the Anthropocene as an epoch or series in the geological timescale, with a precise start date and stratigraphic boundary in the mid-twentieth century. As geologists, archaeologists, environmental scientists and geographers, we have another approach to suggest: recognize the Anthropocene as an ongoing geological event.

The problems with demarcating the Anthropocene as a globally synchronous change in human–environment relations, occurring in 1950 or otherwise, have long been evident (P. J. Crutzen and E. F. Stoermer IGBP Newsletter 41, 17–18; 2000). As an ongoing geological event, it would be analogous to other major transformative events, such as the Great Oxidation Event (starting around 2.4 billion years ago) or the Great Ordovician Biodiversification Event (around 500 million years ago).

Unlike formally defined epochs or series, geological events can encompass spatial and temporal heterogeneity and the diverse processes — environmental and now social — that interact to produce global environmental changes. Defining the Anthropocene in this way would, in our view, better engage with how the term has been used and criticized across the scholarly world.”

AUTHORS: Andrew M. Bauer, Stanford University, Stanford, California, USA; Matthew Edgeworth, University of Leicester, Leicester, UK;  Lucy E. Edwards, Florence Bascom Geoscience Center, Reston, Virginia, USAErle C. Ellis, University of Maryland, Baltimore County, Maryland, USA ; Philip Gibbard, Scott Polar Research Institute, University of Cambridge, Cambridge, UK;  Dorothy J. Merritts, Franklin and Marshall College, Lancaster, Pennsylvania, USA.

I have been grousing about the attempt to assign Epoch/Series status to the Anthropocene for quite a while (you can follow the development of my personal opinions by entering ‘Anthropocene’ in the Search Earth-logs box). In general I believe that the proposal being debated is scientifically absurd, and a mere justification for getting a political banner to wave. What the six authors of this letter propose seems eminently sensible. I hope it is accepted by International Commission on Stratigraphy as a solution to the increasingly sterile discussions that continue to wash to and fro in our community. Then perhaps the focus can be on action rather than propaganda.

As things have stood since 21 May 2019, a proposal to accept the Anthropocene as a formal chrono-stratigraphic unit defined by a GSSP at its base around the middle of the 20th century is before the ICS and the International Union of Geological Sciences (IUGS) for ratification. It was accepted by 88% of the 34-strong Anthropocene Working Group of the ICS Subcommission on Quaternary Stratigraphy. But that proposal has yet to be ratified by either the ICS or IUGS. Interestingly, one of the main Anthropocene proponents was recently replaced as chair of the Working Group.

Opportunities for anatomically modern humans to have left Africa

Key ages of early H. sapiens, Neanderthals and Denisovans (credit: Delson, 2019; Fig. 1)

For almost 2 million years humans have migrated long distances, the earliest example of a move out of Africa being the Georgian Homo erectus specimens (see: First out of Africa? November 2003). As regards H. sapiens – anatomically modern humans (AMH) – the earliest fossils, found at Jebel Irhoud in Morocco, are about 300 ka old. By 260 ka they were present at several sites that span the African continent. The first sign of AMH having left Africa are fossils found at Mislaya in Israel and Apidima in Greece – dated to 177 and 210 to 170 ka respectively – and 125 ka-old tools tentatively attributed to AMH in the Arabian Peninsula (see: Arabia : staging post for human migrations?, September 2014). There is also genetically dated evidence of geneflow from Homo sapiens into Neanderthal DNA between 130 to 250 ka ago. The evidence for an early ‘Out of Africa’ migration by AMH is concrete but very sparse, a fuller story of our permanently colonising all habitable parts of the world only emerging for times after about 65 ka.

It is easy to appreciate that the main hindrance for palaeo-anthropological research into human migration centres on the issue of where to look for evidence, a great many discoveries owing more to luck than to a strategic approach. And, of course, once interesting sites are found researchers congregate there. There is a limited number of active palaeoanthropologists of whom only a proportion engage regularly in field exploration. And there is also an element of the old gold prospectors adage: ‘If you want to find elephants, go to elephant country’! But there are other issues connected with discoveries. When was it possible for AMH to make transcontinental journeys and what routes would have been feasible from time to time? Robert Beyer of the Cambridge University with scientists from New Zealand, Estonia and the UK have devised a rational approach to the questions of optimum times and routes for major migration (Beyer, R.M., et al. 2021. Climatic windows for human migration out of Africa in the past 300,000 yearsNature Communications, v.  12, article 4889; DOI: 10.1038/s41467-021-24779-1). Just two routes out of Africa have been considered feasible: by crossing the Strait of Bab el Mandab from Djibouti and southern Eritrea to the Yemen, and following the Nile northwards to access Eurasia via the Levant. The first depends to some extent on how wide the Strait was; depending on sea level fluctuations, it has varied from 4 to 20 km during the last 300 ka. Exit by way of both routes would also have depended on vegetation, game and drinking water supplies that varying amounts of rainfall would have supported.

Assessing the feasibility of crossing the southern Red Sea at different times is fairly easy. Sea level fluctuates according to the amount of water locked in the ice caps of Antarctica and Greenland and on the land glaciated during ice ages in northern North America and Scandinavia. Oxygen isotopes in Pleistocene sea-floor sediments and today’s ice caps reveal that variation. Being one of the world’s most important seaways the bathymetry of the Red Sea is known in considerable detail. At present the minimum sea distance needed to cross the Strait of Bab el Mandab is about 21 km. At the lowest sea levels during the Pleistocene the sea journey was reduced to slightly less than 5 km, which would not have required sophisticated boats or seafaring skills. There is evidence that AMH and earlier humans occupied the western shore of the Red Sea to use its rich marine resources, but none for boats or for habitation of the Yemeni coastline. However, calculations by Beyer et al. of sea level fluctuations during the last 300 ka show that for more than half that time the sea crossing was less than 7 km thanks to a shallow continental shelf and a very narrow stretch of deep water. Clearly the varying width of the Strait is not a useful guide to windows of opportunity for migration via that route. Except for warm interglacials and a few interstadials, people could have crossed at any time provided that the ecosystems on either side could sustain them.

Annual precipitation during each millennium of the Late Pleistocene for the two most likely out-of-Africa routes. The double green lines show the lower level of tolerance for hunter gatherers. The percentage of decades during which ANH could sustain themselves is colour-coded in blues. (Credit: Beyer et al. Fig 2)

Turning to climatic fluctuations, especially that of rainfall, Beyer et al. first estimated the lowest rainfall that hunter-gatherers can survive from the distribution of surviving groups according to annual precipitation and the biomass of grazing prey animals in their habitats. The lower limit is about 90 mm per year. Using the climate record for the Late-Pleistocene from proxies, such as oxygen isotopes, in global climate modelling produces a series of high-resolution ‘time-lapse snapshots’ of conditions in the geographic areas of interest – the Nile-Levant route and that from the Horn of Africa to Yemen. The results are expressed as the percentage of decades in each thousand-year interval that hunter-gatherers could sustain themselves under prevailing climatic conditions in the two regions. What seems clear from the figure (above) is that the southern, Bab el Mandab route had considerable potential for AMH migrants. The northern one looks as if it was more risky, as might be expected from today’s dominant aridity away from the Mediterranean and Gulf coasts. The northern route seems to have been just about feasible for these periods: 245-230; 220-210; 206-197; 132-94; 85-82; ~75 and ~72 ka. The climatic windows for possible migration via the southern route are: ~290; 275-240 (with optimums at ~273, ~269, ~246 and ~243); 230-210; 203-200; 182-145; 135-118; 112; 107; 70-30; 18-13 ka. The well documented beginning of major AMH migration into Eurasia was around 75 to 60 ka, which the southern route would most favour on climatic grounds. Yet before that there are many possibilities involving either route. Any AMH finds outside Africa before 250, and between 190-133 ka seem almost certain to have been via the southern route, based on arid conditions in the north. But, of course, there would have been other factors at play encouraging or deterring migration via either route. So perhaps not every climatic opportunity was exploited.

Beyer and colleagues have provided a basis for plenty of discussion and shifts in focus for future palaeo-anthropological work. One thing to bear in mind is that different humans may also have taken up the opportunities; for example, some Neanderthals are now suspected to have migrated back to Africa in the last 300 ka.

See also: Groucutt, H.S  and 22 others 2021. Multiple hominin dispersals into Southwest Asia over the past 400,000 years. Nature, ; DOI: 10.1038/s41586-021-03863-y

When Greenland was a warm place

On 14-15 August 2021 it rained for the first time since records began at the highest point on the Greenland ice cap. Summit Camp at 3.216 m is run by the US National Science Foundation, which set it up in 1989, and is famous for climate data gleaned from two deep ice cores there. This odd event came at a time when surface melting of the ice cap covered 870 thousand km2: over half of its total 1.7 million km2 extent: a sure sign of global warming. The average maximum temperature in August at Summit is -14°C, but since the mid 20th century the Arctic has been warming at about twice the global rate. Under naturally fluctuating climatic conditions during the Pleistocene, associated with glacial-interglacial cycles, Greenland may have been ice-free for extended periods, perhaps as long as a quarter of a million years around 1.1 Ma ago. If 75% of the up to 3 km thick ice on Greenland melted that would add 5 to 6 m to global sea level, perhaps as early as 2100 if current rates of climate warming persist.

The edge of the ice cap in NE Greenland (credit: Wikipedia)

The worst scenario is runaway warming on the scale of that which took place 56 Ma ago during the Palaeocene-Eocene Thermal Maximum (PETM) when global mean temperature rose by between 5 to 8°C at a rate comparable with what the planet is experiencing now as a result of growth in the world economy. In fact, the CO2 released during the PETM emerged at a rate that was only about tenth of modern anthropogenic emissions  Sediments that span the Palaeocene-Eocene boundary occur in NE Greenland, a study of which was recently published by scientists from Denmark, Greenland, the UK, Australia and Poland (Hovikoski, J. and 13 others 2021. Paleocene-Eocene volcanic segmentation of the Norwegian-Greenland seaway reorganized high-latitude ocean circulation. Communications Earth & Environment, v. 2, article 172; DOI: 10.1038/s43247-021-00249-w). The greenhouse world of NE Greenland that lay between 70 and 80°N then, as it still does, was an area alternating between shallow marine and terrestrial conditions, the latter characterised by coastal plain and floodplain sediments deposited in estuaries, deltas and lakes. They include coals derived from lush, wooded swamps, inhabited by hippo-like ungulates, primates and reptiles.

At that time the opening of the northern part of the North Atlantic had barely begun, with little chance for an equivalent of the Gulf Stream to have had a warming influence on the Arctic. Shortly after the PETM volcanism began in earnest, to form the flood basalts of the North Atlantic Igneous Province. Each lava flow is capped by red soil or bole: further evidence for a warm, humid climate and rapid chemical weathering. As well as lava build-up, tectonic forces resulted in uplift, effectively opening migration routes for animals and land plants to colonise the benign – for such high latitudes – conditions and perhaps escape the far hotter conditions further south.

The situation now is much different, with the potential for even more rapid melting of the Greenland ice cap to flood freshwater into the North Atlantic, as is currently beginning. Diluting surface seawater reduces its density and thus its tendency to sink, which is the main driving force that pulls warmer water towards high-latitudes in the form of the Gulf Stream. Slowing and even shutting down its influence may have an effect that contradicts the general tendency for global warming – a cooling trend at mid- to high latitudes with chaotic effects on atmospheric pressure systems, the jet stream and weather in general.

See also: Barham, M. et al. 2021. When Greenland was green: rapid global warming 55 million years ago shows us what the future may hold. The Conversation, 23 August 2021.

Geothermal heat from coalfields: a ‘green’ revolution?

Sooner rather than later all energy users will be forced to change their source of energy to ones that do not involve fossil fuels. On average, 84% of household energy consumption is for space heating and hot water. Reducing domestic greenhouse gas emissions must replace the current dominance of coal, natural gas and oil in keeping ourselves warm and clean. One widely suggested solution is the use of heat pumps to ‘concentrate’ the solar energy that is temporarily stored in air or in the soil and rock just beneath our homes and gardens: air- and ground-source heating systems. Heat pumps rely on overcoming the Second Law of Thermodynamics, as do refrigerators and air conditioners. The Law implies that hot things always cool down; that is, energy cannot move ‘of its own accord’ from a cooler source to a warmer destination. But reversing the natural flow of heat energy is possible. To achieve this involves work in some form. There are several kinds of heat pumps powered by electricity, the simplest using a vapour compression cycle, as in refrigeration, which ‘pumps’ heat out of a fridge interior or a room. So, warm air is emitted to the outside world. Reverse the flow and the pump becomes a device that captures heat from a cold source – either a gas or a fluid – and delivers it for a domestic or industrial use.

In the case of a ground-source heat pump, the energy delivered also includes geothermal heat flowing from deep in the Earth: ultimately from radioactive decay of unstable isotopes bound up in rocks and the minerals they contain. At the surface, this supply is far less than solar heating. Yet, because of the Second Law of Thermodynamics it flows from a much hotter source: the Earth’s mantle. The deeper down, the hotter it gets. Beneath most of the continental surface temperature increases at between 15 to 35°C per kilometre. Without surface air being circulated to the deepest parts of mines it would be impossible to work in them. I remember as a school student visiting the deepest level of Maltby Main coal mine in South Yorkshire. Because my school was co-educational, our guide shouted ahead to the miners that women were coming – these miners normally worked in just helmet, boots and a jock strap! Dozens of them rushed frantically to find their trousers. Maltby Main’s shafts reached almost a kilometre below the surface and without ventilation the air temperature would have been more than 50°C. As it was, it was well above 30 degrees

As well as methane (‘fire damp’), CO2 (‘choke damp’) and roof collapse, one of the main hazards of coal mining is flooding by groundwater. When operational, before the Conservative governments of the 1980s and early 90s set about destroying deep-coal mining in Britain, all mines had massive pumps to remove water from their deepest levels or ‘sumps’. It is hazardous stuff, being highly acidic and rich in dissolved metals – including arsenic in some areas – as well as being warm: a low-temperature hydrothermal fluid. In many cases such water seeps to the surface from the now flooded mine workings, and precipitates brightly coloured iron hydroxides. Most of the abandoned British coalfields are plagued by such minewater escaping to the surface. Untreated it contaminates surface water, soils and sediments, posing threats to vegetation, wild life and people. Yet, it has considerable potential as a heat source, especially for community heating systems based on large-scale heat pumps (Farr, G. et al. 2020. The temperature of Britain’s coalfields. Quarterly Journal of Engineering Geology and Hydrogeology, v. 54, article qjegh2020-109; DOI: 10.1144/qjegh2020-109).

Pollution of South Esk river near Edinburgh by water from old coal-mine workings (Credit: EdinburghLive, 11 June 2020)

For decades, geothermal energy has been touted as a near-ideal renewable, carbon-free resource, using natural hot-spring systems in volcanically active areas, such as Iceland and New Zealand, from areas of unusually high heat flow over highly radioactive granite intrusions or from very deep sedimentary aquifers as exploited in parts of the Paris Basin. But in Britain various optimistic projects have arisen and then faded away. All relied on pumping surface water down boreholes to depths that achieve high temperatures, returning it to the surface at around 60°C to the surface and then piping it to users: very expensive. About a quarter of the population live above the former coalfields of Britain. Around 2.2 million gigawatt hours of geothermal heat are currently stored in flooded mine workings, with the possibility of further expansion. The UK Coal Authority has about 40 minewater pumping stations aimed at reducing pollution, which remove 3,000 l s-1 at an average temperature of around 9-18°C. Expressed in terms of energy content this amounts to 63 MW if recovered. But this is just scratching the surface of the potential for large-scale district heating based on heat pumps, such as that planned at Seaham, County Durham to heat 1500 new homes. Community heating and wastewater treatment can be combined for all the former coal mining areas in Central Scotland, the North of England and Midlands and South Wales where population densities are still very high

See also: Lane, A.2021. How flooded coalmines could heat homes. BBC Future 7 July 2020. Taylor, M. 2021. Abandoned pits of former mining town fuel green revolution. The Guardian, 10 August 2021.

Apocalypse Soon: Will current global warming trigger a mass extinction?

Since the start of 2020 I doubt there has been much field research. But such a vast amount of data has been amassed over the years that there must be opportunities to keep the academic pot boiling. One way is to look for new correlations between different kinds of data. For instance matching the decades-old time series of extinctions with those of other parameters that have changed over geological time. At a time of growing concern about anthropogenic climate change a group based at the State Key Laboratory of Biogeology and Environmental Geology, at China University of Geosciences, Wuhan have checked the extinction rates of marine fossils over the last 450 Ma against variations in sea-surface temperature (Song, H. et al 2021. Thresholds of temperature change for mass extinctions. Nature Communications, v. 12, Article number 4694; DOI: 0.1038/s41467-021-25019-2).

Extinction data are usually presented in time ‘bins’ based on the number of disappearances of fossil genera in one or a number of geological Stages – the finest divisions of the stratigraphic column. The growing data set for sea-surface temperatures derived using oxygen isotopes from marine fossil shells is more continuous, being derived from many different layers of suitable sedimentary rock within a Stage. Clearly, the two kinds of data have to be expressed in a similar way to check for correlations. Haijun Song and co-workers converted both the extinction and temperature time series to 45 time ‘bins’, each around 10 Ma long. They express the binned climatic data in two ways: as the largest temperature change (°C) and the highest rate of temperature change (°C Ma-1) within each bin. That is, they expressed to some extent the greater continuity of seawater temperature data as well as matching them to those for extinctions.

Changes since the end of the Ordovician: red = extinction rate in time bins; green = the greatest magnitude of change in temperature in each bin; blue- the greatest rate of temperature change in each bin. Grey bars show mass extinctions (Credit: Song et al., Fig 1)

There are good correlations between the climatic and extinction data, particularly for mass extinctions. Bearing in mind that mass extinctions take place far more rapidly than can be expressed with 10 Ma time bins, the authors were concerned that bias could creep into the binned extinction data. They were able to discount this by examining both data sets in finer detail at the times of the ‘Big 5’ extinctions. Earlier research had identified warming episodes around the times of each mass extinction, often implicating greenhouse-gas emissions from Large Igneous Provinces. Yet there are other factors that may have influenced the 7 ‘lesser’ mass extinctions in the fossil record. The authors are sufficiently confident in the correlations they have revealed to suggest thresholds that seem to have launched major mass extinctions: greater than 5.2 °C and 10 °C Ma-1 for magnitudes and rates of sea-surface temperature change, respectively.

In the context of the modern climate, the data analysis predicts that a rise of 5.2 °C above the preindustrial mean global temperature spells extinctions of ‘Big Five’ magnitude. The rate of temperature increase since 1880 – 0.08 ° per decade – is hugely faster than that expressed by the data that span the last 450 Ma. This is more alarming than the stark Sixth Report of the Intergovernmental Panel on Climate Change IPCC released on 9 August 2021.

Subduction and continental collision in the Himalaya

The Indian subcontinent after it separated from Madagascar in the Late Cretaceous to move northwards to its destined collision with Eurasia and the formation of the Himalaya. (Credit: Frame from an animation ©Christopher Scotese)

During the Early Cretaceous (~140 Ma ago) India, Madagascar, Antarctica and Australia parted company with Africa after 400 Ma of unity as components of the Gondwana supercontinent. By 120 Ma Antarctica and Australia split from India and Madagascar, and the Indian Ocean began to form. India moved northwards , leaving Madagascar in its wake after about 70 Ma ago. By 50 Ma the subcontinent began to collide with Eurasia, its northward motion driving before it crustal materials that eventually formed the Himalaya. This highly complex process is wonderfully documented in an animation made in 2015 by Christopher Scotese, Emeritus Professor in the Department of Earth and Environmental Sciences, Northwestern University, USA. At the start of its journey India moved northwards at a slow rate of about 5 cm per year. After 80 Ma it speeded up dramatically to 15 cm per year, about twice as fast as any modern continental drift and a pace that lasted for over 30 Ma until collision began. How could that, in a geological sense, sudden and sustained acceleration have been induced? It would have required a change in the slab-pull force that is the primary driver of plate tectonics, suggesting an increase in the amount of subduction in the Tethys Ocean that formerly lay between India and Eurasia, probably at two, now hidden destructive plate margins.

A group of geoscientists from Canada, the US and Pakistan has documented that collision in terms of the record of metamorphism experienced beneath the Himalaya as slab after slab of once near-surface rocks were driven beneath the rising orogen (Soret, M. et al. 2021. How Himalayan collision stems from subduction. Geology, v. 49, p. 894-898; DOI: 10.1130/G48803.1). The Western Himalaya has trapped a deformed and tilted magmatic rock sequence of an island arc – the Kohistan Arc – between  the Eurasian plate and a zone of crustal thickening and shortening that was thrust southward over the ancient metamorphic basement of India itself. That crust was mantled by a variety of younger sediments deposited on the Tethyan continental shelf of the northern Indian plate which became involved in the process of crustal thickening. The Kohistan Arc probably formed above one of the destructive margins that consumed the oceanic lithosphere of the now vanished Tethys Ocean. Two distinct types of rock make up the slabs stacked-up by thrusting.

The uppermost, which also forms the highest part of the Western Himalaya in the form of Nanga Parbat (at 8,126 metres the world’s ninth highest mountain) comprises rocks thought to represent Tethyan oceanic lithosphere subducted perhaps at the second destructive margin. Their mineral assemblages, especially those of eclogites, indicate that they have been metamorphosed under pressures corresponding to depths of up to 100 km, but at low temperatures along a geothermal gradient of about 7°C km-1, i.e. in a low heat-flow environment. These ultra-high pressure (UHP) metamorphic rocks formed at the start of the India-Eurasia collision. The sequence of sedimentary slabs now overridden by the UHP slab were metamorphosed at around the same time, but under very different conditions. Their burial reached only about 35 km – the normal thickness of the continental crust – and a temperature of about 600°C on a 30°C km-1 geothermal gradient. Detailed mineralogy of the UHP slab reveals that as it was driven over the metasediments it evolved to the same geothermal conditions.

Matthew Soret and his colleagues explain how this marked metamorphic duality may have arisen in rocks that are now part of the same huge thrust complex. Their results are consistent with slicing together of oceanic lithosphere in a subduction zone to form a tectonic wedge of UHP mineral assemblages at the same time as continental shelf sediments were metamorphosed under more normal geothermal conditions. This was happening just as India came into contact with Eurasia. When crustal thickening began in earnest through the inter-slicing of the two assemblages, pressure on the UHP rocks fell rapidly as a result of their being thrust over the dominantly metasedimentary shelf sequence. It also moved into a zone of normal heat flow, first heating up equally quickly and then following a path of decreasing pressure and temperature as erosion pared away the newly thickened crust. Both assemblages now became part of the same metamorphic regime. In this way a subduction system evolved to become incorporated in an orogenic zone as two continents collided; a complex process that finds parallels in other orogens such as the Alps.

Signs of Milankovich Effect during Snowball Earth episodes

The idea that the Earth was like a giant snowball during the Neoproterozoic Era arose from the discovery of rocks of that age that could only have formed as a result of glaciation. However, unlike the Pleistocene ice ages, evidence for these much older glacial conditions occurs on all continents. In some locations remanent magnetism in sedimentary rocks of that age is almost horizontal; i.e. they had been deposited at low magnetic latitudes, equivalent to the tropics of the present day. Frigid as it then was, the Earth still received solar heating and magmatic activity would have been slowly adding CO2 to the atmosphere so that less heat escaped – a greenhouse effect must have been functioning. Moreover, an iced-over world would not have been supporting much photosynthetic life to draw down the greenhouse gas into solid carbohydrates and carbonates to be buried on the ocean floor. As far as we know the Solar System’s geometry during the Neoproterozoic was much as it is today. So changes in the gravitational fields induced by the orbiting Giant Planets would have been affecting the shape (eccentricity) of Earth’s orbit, the tilt (obliquity) of its rotational axis and the precession (wobble) of its rotation as they do at present through the Milankovich effect. These astronomical forcings vary the amount of solar energy reaching the Earth’s surface. It has been suggested that a Snowball Earth’s climate system would have been just as sensitive to astronomical forcing as it has been during the last 2 million years or more. Proof of that hypothesis has recently been achieved, at least for one of the Snowball events (Mitchell, R.N. and 8 others 2021. Orbital forcing of ice sheets during snowball Earth. Nature Communications, v. 12, article 4187; DOI: 10.1038/s41467-021-24439-4).

Another of the enigmas of the Neoproterozoic is that after and absence of more than a billion years banded iron formations (see: Banded iron formations (BIFs) reviewed, December 2017) began to form again. BIFs are part of the suite of sedimentary rocks that characterise Snowball Earth events, often alternating with glaciogenic sediments. Throughout each cold cycle – the Sturtian (717 to 663 Ma) and Marinoan (650 to 632 Ma) glacial periods – conditions of sediment deposition varied a great deal from place to place and over time. Some sort of cyclicity is hinted at, but the pace of alternations has proved impossible to check, partly because the dominant rocks (glacial conglomerates or diamictites) show little stratification and others that are bedded (various non-glacial sandstones) vary from place to place and give no sign of rates of deposition, having been deposited under high-energy conditions. BIFs, on the other hand are made up of enormous numbers of parallel layers on scales from millimetres to centimetres. Bundles of bands can be traced over large areas, and they may represent repeated cycles of deposition.

Typical banded iron formation

How BIFs formed is crucial. They were precipitated from water rich in dissolved iron in its reduced Fe2+ (ferrous) form, which originated from sea-floor hydrothermal vents. Precipitation occurred when the amount of oxygen in the water increased the chance of electrons being removed from iron ions to transform them from ferrous to ferric (Fe3+). Their combination with oxygen yields insoluble iron oxides. Cyclical changes in the availability of oxygen and the balance between reducing and oxidising conditions result in the banding. In fact several rhythms of alternation are witnessed by repeated packages at deci-, centi- and millimetre scales within each BIF deposit. Overall the packages suggest a constant rate of deposition: a ‘must-have’ for precise time-analysis of the cycles. BIFs contain both weakly magnetic hematite (Fe2O3) and strongly magnetic magnetite (Fe3O4), their ratio depending on varying geochemical conditions during deposition. Ross Mitchell of Curtin University, Western Australia and his Chinese, Australian and Dutch colleagues measured magnetic susceptibility at closely spaced intervals (1 and 0.25 m) in two section of BIFs from the Sturtian glaciation in the Flinders Ranges of South Australia. Visually both sections show clear signs of two high-frequency and three lower frequency kinds of cycles, expressed in thickness.

The tricky step was converting the magneto-stratigraphic data to a time series. High-precision zircon U-Pb dating of volcanic rocks in the sequence suggested a mean BIF deposition rate of 3.7 to 4.4 cm per thousand years. This allowed the thickness of individual bands and packages to be expressed in years, the prerequisite for time-series analysis of the BIF magneto-stratigraphic sequence. This involves a mathematical process known as the Fast-Fourier Transform, which expresses the actual data as a spectral curve. Peaks in the curve represent specific frequencies expressed as cycles per metre. The rate of deposition of the BIF allows each peak to be assigned a frequency in years, which can then be compared with the hypothetical spectrum associated with the Milankovich effect. One of the BIF sequences yielded peaks corresponding to 23, 97 and 106 ka. These match the effects of variation in precession (23 ka) and ‘short’ orbital eccentricity (97 and 106 ka) found in Cenozoic sea-floor sediments and ice cores. The other showed peaks at 405, 754 and 1.2 Ma corresponding to ‘long’ orbital eccentricity and long-term features of both obliquity and precession. Quite a result! But how does this bear on Snowball Earth events? Cyclical changes in solar heating would have affected the extent of ice sheets and sea ice at all latitudes, forcing episodes of expansion and contraction and thus changes in sediment supply to the sea floor. That helps explain the many observed variations in sedimentation other than that of BIFs. Rather than supporting a ‘hard’ Snowball model of total marine ice cover for millions of years, it suggests that such an extreme was relieved by period of extensive open water, much as affected the modern Arctic Ocean for the last 2 million years or so. There could have been global equivalents of ice ages and interglacials during the Sturtian and Marinoan. ‘Hard’ conditions would have shut down much of the oceans’ biological productivity, periodically to have been reprieved by more open conditions: a mechanism that would have promoted both extinctions and evolutionary radiations. Snowball events may have driven the takeover of prokaryote (bacteria) dominance by that of the multicelled eukaryotes that is signalled by the Ediacaran faunas that swiftly followed glacial epochs and the explosion of multicelled life during the Cambrian. As eukaryotes, we may well owe our existence to Snowball.

Global warming: Can mantle rocks reduce the greenhouse effect?

Three weeks ago I commented on a novel and progressive use for coal seams as stores for large quantities of hydrogen gas. That would be analogous to batteries for solar- and wind power plants by using electricity generated outside times of peak demand to electrolyse water to hydrogen and oxygen. There are other abundant rocks that naturally react with the atmosphere to permanently sequester carbon dioxide in alteration products, and form possible solutions to global warming. The most promising of these contain minerals that are inherently unstable under surface conditions; i.e. when they come into contact with rainwater that contains dissolved CO2. The most common are anhydrous minerals containing calcium and magnesium that occur in igneous rocks. Basalts contain the minerals plagioclase feldspar (CaAl2Si2O8), olivine ([Fe,Mg]2SiO4)] and pyroxene ([Fe,Mg]CaSi2O6)] that weather to yield the minerals calcium and magnesium carbonate. My piece Bury the beast in basalt, written in June 2016, mentions experiments in the basalts of Iceland and Washington State, USA to check their potential for drawing down atmospheric CO2. News of an even more promising prospect for CO2 sequestration in igneous rock emerged in the latest issue of Scientific American (Fox, D. 2021. Rare Mantle Rocks in Oman Could Sequester Massive Amounts of CO2. Scientific American, July 2021 issue).

Distribution of ophiolites around the Eastern Mediterranean and Black Seas. Most orogenic belts carry comparable volumes of ophiolites. (Credit: Gültekin Topuz, Istanbul Technical University)

The most abundant magnesium-rich material in our planet is the peridotite of the mantle, which consists of more than 60% olivine with lesser amounts of pyroxene and almost no feldspar. Being so rich in Mg and Fe, it is said to have an ultramafic composition and is extremely prone to weathering. The rock dunite is the ultimate ultramafic rock being made of more than 90% olivine. All ultramafic rocks are denser than 3,000 kg m-3, so might be expected to be rare in lower density continental crust (2,600 kg m-3). But they are present at the base of sections of oceanic lithosphere that plate tectonics has thrust up and onto the continents, known as ophiolite bodies. They often occur in orogenic belts at former destructive plate margins and are more common than one might expect. One of the largest and certainly the best-exposed occurs in the Semail Mountains of Oman, where scientists from the Lamont-Doherty Earth Observatory, New York State, USA, and other collaborators have been investigating its potential for absorbing CO2, since 2008.

Olivine-rich rocks react with naturally carbonated groundwater or hydrothermal fluids to form soft, often highly coloured material known as serpentine, well-known for the ease with which it can be carved and polished. As well as the mineral serpentinite [Mg3Si2O5(OH)4], the hydration reactions yield magnetite (Fe3O4), magnesium carbonate (magnesite) and silica (SiO2). If reaction takes place in the absence of oxygen gaseous hydrogen also forms. All these have been noted in the Oman ophiolite: fractures in serpentinites are filled with carbonates, and springs associated with them emit copious amounts of hydrogen and, in some cases, methane. Interestingly, the reactions – like those that involve anhydrous calcium-aluminium silicates when cement is wetted and then cures – release large amounts of heat. This makes the reactions self-sustaining once they begin in peridotite or dunite. However, at the Earth’s surface they are somewhat sluggish as the heat of reaction is lost to the air.  

Mantle rock in the Oman ophiolite, showing cores of fresh peridotite, brownish serpentinite and white carbonate veins (credit: Juerg Matter, Oman Drilling Project, Southampton University, UK)

The capacity for CO2 sequestration by ultramafic igneous rocks is high: a ton of olivine when completely hydrated takes in 0.62 tons of CO2. The Lamont-Doherty team has estimated that they speed up in crushed peridotite, for instance after milling during industrial-scale mining – peridotites are host rocks for platinum-group metals, nickel and chromium. (Kelemen, P.B. et al. 2020. Engineered carbon mineralization in ultramafic rocks for CO2 removal from air. Chemical Geology, v. 550, article 119628; DOI: 10.1016/j.chemgeo.2020.119628). Spreading mine waste over large areas of desert surfaces  would be one way of capturing CO2. However, using the age of emplacement of the Oman ophiolite (96-70 Ma) and the amount of carbonate found in fractured serpentinite there, the team estimates that each ton of the 15 m deep zone of active weathering has naturally absorbed CO2 at a rate of about 1 g m-3 year-1 equivalent to 1000 tons per cubic km per year. But parts of the ophiolite have been fully altered to serpentinite plus carbonates since the Cretaceous, probably at depth. Dating some of the near-surface carbonate veins revealed that they had formed in only a few thousand years rather than the tens of million years expected. Natural sequestration could therefore be happening at depth about 10,000 times faster than theory predicts. Also natural springs emerging from the peridotite are highly alkaline and by combining with atmospheric carbon dioxide precipitate carbonate to form travertine deposits at the surface. This is so rapid that if the carbonate is scraped off the exposed rock, within a year it has recoated the surface.

This year, deep drilling into the Oman ophiolite has begun. To the surprise of members of the team, carbonate minerals are not present in the bedrock below 100 m depth: CO2 is not penetrating naturally beyond that depth. If it becomes possible to inject CO2 deep beneath the surface the exothermic reactions could be kick-started. This would involve sinking pairs of boreholes to set up a flow of carbon-charged water from the ‘injection’ hole to the other that would return decarbonised water to the surface for re-use. The carbon-capture experiment in Iceland (Carbfix) has been running since 2012. Carbon dioxideseparated from hot water passing through a geothermal power plant is re-injected into basalt at a depth of half a kilometre. This small pilot runs at a cost of US$25 per ton of sequestered gas. But it uses already concentrated CO2, whereas global-scale sequestration would require capturing, compressing and dissolving it directly from the atmosphere, probably costing about $120 to $220 per ton injected into mantle rock. The engineering required – about 5,000 boreholes – to capture a billion tons of CO2 deep in the Oman ophiolites is achievable with current technology. Since 2005 almost 140 thousand fracking wells have been sunk in the US alone; they are analogous to the paired holes needed for sequestration. Worldwide, the petroleum industry has driven tens of million wells for conventional oil and gas extraction.

The energy needed to run carbon capture in mantle rocks in an arid country like Oman could be solar derived. Moreover, there are possible by-products such as hydrogen released by the chemical reactions. The alternative, more conventional approach of pumping CO2 into deep, permeable sedimentary reservoirs also carries substantial costs but has the disadvantage of possible leakage. Ophiolites are not rare, occurring as they do in areas of ancient destructive plate margins. So permanently locking away excess atmospheric greenhouse gases currently driving global warming would require only a tiny proportion of the volume of peridotite that is easily accessible by drilling. It would clearly cost an eye-watering sum, but bear in mind that the four biggest petroleum companies – BP, Shell, Chevron and Exxon – have harvested profits of around US$ 2 trillion since 1990. Along with the global coal industries, they are the source of the present climate emergency.

Massive hominin skull from China: is it a Denisovan?

In 1933 labourers unearthed a very large skull during the construction of a bridge near Harbin, Northern China. At that time, the area was under occupation by Imperial Japanese forces. To keep it out of the invaders’ hands the skull was quickly wrapped in a cloth and hidden in an old well. It was only in 2018 that the original finder’s grandson recovered it to pass on to archaeologists at  Hebei Geo University. It lacks a lower jawbone, so technically it is a cranium, but is very well preserved. The face has very large brow ridges – generally taken as a primitive feature – but also some more modern features. With a 1,420 ml brain case, it is significantly larger than most modern human crania. Apparently, it is of an adult male. As well as a big head, he had a large nose, as do Neanderthals: a possible adaptation to very cold conditions. Without waiting to see if the bones might yield DNA, five of the team who examined the cranium claimed it as a new species, Homo longi or ‘Dragon Man’; i.e. distinct from modern humans and Neanderthals and all known older hominins (Ni, X. et al. 2021. Massive cranium from Harbin establishes a new Middle Pleistocene human lineage in China. The Innovation, v. 2, article 100130; DOI: 10.1016/j.xinn.2021.100130. Ji, Q. et al. 2021. Late Middle Pleistocene Harbin cranium represents a new Homo species. The Innovation, v. 2, article 100132; DOI: 10.1016/j.xinn.2021.100132). They based this phylogenetic interpretation on morphology alone. At least one of the team, Chris Stringer a leading hominin palaeoanthropologist at The Natural History Museum in London, demurred. The cranium is not unique and bears close similarity to another from the central Chinese province of Shaanxi, which was found in the late 1970s. In fact there are three other Chinese crania that resemble that from Harbin, although they are less well preserved.

All-sided views of the Harbin cranium. (Credit: Ni et al., Fig 2)

Dating the fossil was not easy, as the site where labourers discovered him was destroyed during construction of the bridge. Researchers used a variety of geochemical analyses, including from sediment stuck in his nasal cavity, to derive a likely stratigraphic profile from which the cranium may have been excavated. The best fit is with Middle Pleistocene sediments in the Harbin area. Uranium-series dating of the bone suggests that it is older than 146 ka (Shao, Q.  et al. 2021. Geochemical locating and direct dating of the Harbin archaic human craniumThe Innovation, v. 2, article 100131; DOI: 10.1016/j.xinn.2021.100131). So it is likely that this man and his companions did not cohabit China with anatomically modern humans, who arrived no more than about 50 ka ago. The highly robust nature of all the similar crania suggests that the individuals must have been large and physically active. Like the Neanderthals, they had adapted to harsh conditions over several hundred thousand years of repeated climate change. Even today, winters in northern China average around -16°C, and far inland conditions are semi-arid to arid. For them to migrate would have involved traversing some of highest, bleakest passes in the world. These people evolved to survive extreme climatic and environmental change, much as did the Neanderthals in West Asia and Europe. By comparison anatomically modern humans evolved in the more stable environments of Africa and the Middle East, surviving only the last ice age once they had migrated northwards. Those who made it to northern Siberia and crossed the Bering Strait via Beringia around the last glacial maximum did evolve physical traits that helped them survive, but minor ones compared with the earlier humans.

So what do these Chinese fossils represent? Using cranial features alone to propose distinct species smacks of the techniques of 19th and early 20th century anatomical anthropologists, albeit with powerful statistical analysis. We know that anatomically modern humans carry genetic signatures of interbreeding with at least two known ‘species’ with whom they cohabited Eurasia – Neanderthals and Denisovans. Indeed, traces in  the DNA of living African and Eurasian humans hint at other unknown and probably very ancient ‘ghost’ populations. Genetic, physical and probably cultural differences did not deter repeated interbreeding with these ‘others’. To be frank, erecting new human ‘species’ these days seems to serve little purpose. ‘Dragon Man’ is just as likely to represent the Denisovans as the fully sequenced DNA from a couple of bones from caves in Siberia and Tibet. The latter matched stretches of the DNA from living people of East Asia and parts of the Pacific. There are no other such live genetic tracers awaiting a different candidate to fill the role that we know Neanderthals and Denisovans to have filled. That may yet change, but the first job for the mainly Chinese consortium of scientists is to get genetic material from these crania and sequence it, or invite other highly successful palaeogeneticists who would leap at the opportunity.

See also: Jones, N. 2021. Mysterious skull fossils expand human family tree — but questions remain. Nature, v. 595, p. 50; DOI: 10.1038/d41586-021-01738-w

Sample I. 2021. Massive human head in Chinese well forces scientists to rethink evolution. The Guardian, 25 June 2021.

Tectonics on Venus

The surface of Venus is not easily observed because of the almost opaque nature of its atmosphere. The planet is veiled by a mixture of CO2 (96.5%) and nitrogen (3.5%), with a little sulfur dioxide and noble gases. The atmosphere’s mass is almost 100 times that of the Earth’s, and has a density about 6.5% that of liquid water at the surface. The opacity stems from a turbulent upper layer of mainly sulfuric acid. Venus is the victim of runaway greenhouse conditions. Despite that, radar can penetrate the atmosphere to reveal details of its surface morphology – roughness and elevation – at a spatial resolution of 150 m. Although coarser than that available from radar remote sensing of the Earth from orbit, the Magellan data are still geologically revealing.

Earlier interpretation of Venus radar images revealed the surface to be far simpler than that of the Earth, Mars and all other rocky bodies in the Solar System. Yet it has more volcanoes than does the Earth or Mars. However, despite being subject to very little erosion – Venus is a dry world – only around 1000 impact craters have been found: far short of the number seen on Mars or the Moon. This deficiency of evidence for bombardment suggests that Venus was ‘repaved’ by vast volcanic outpourings in the geologically recent past, estimated to have occurred 300 to 600 Ma ago. This early work concluded that plate tectonics was absent; indeed that for half a billion years the lithosphere on Venus had been barely deformed. It has been suggested that Venus has been involved in megacycles of sudden, planet-wide magmatic activity separated by long periods of quiescence. This could be attributed to the lack of plate tectonics, which is the principal means that Earth continuously rids itself of heat produced at depth by decay of radioactive isotopes in the mantle. Venus has been suggested to build up internal temperatures until they reach a threshold that launches widespread partial melting of its mantle. Planet-wide eruption of magma then reduces internal temperatures.

Polygonal blocks or ‘campuses’ on the lowland surface of Venus. Note the zones of ridges that roughly parallel ‘campus’ margins. Credit: Paul K. Byrne, North Carolina State University and Sean C. Solomon, Lamont-Doherty Earth Observatory

It comes as a surprise that 26 years after Magellan plunged into the Venusian atmosphere new interpretation of its radar images suggests a completely different scenario (it may be that academic attention generally switched to research on Mars because of all the missions to the ‘Red Planet’ since Magellan disappeared). It is based on features of the surface of Venus so large that their having been missed until now may be a planetary-scale example of ‘not seeing the woods for the trees’! Geoscientists from the US, Turkey, the UK and Greece have mapped out features ranging from 100 to 1000 km across that cover the lowland parts of Venus (Byrne, P.K. et al. 2021. A globally fragmented and mobile lithosphere on Venus. Proceedings of the National Academy of Science, v. 118, article e2025919118; DOI: 10.1073/pnas.2025919118). They resemble 1950s ‘crazy paving’ or floes in Arctic pack ice, but on a much larger scale. Extending the ice floe analogy, the polygonal blocks are separated by what resemble pressure ridges that roughly parallel the block margins. Paul Byrne of North Carolina State University, USA, and co-workers also found evidence that the large blocks of lithosphere had rotated and moved laterally relative to one another: they had ‘jostled’. Moreover, some of the movement has disturbed the youngest materials on the surface.

To distinguish what seem to be characteristic of Venus’s tectonics from Earthly tectonic plates, the team hit on the name ‘campus’, meaning ‘field’ in Latin. Rather than having remained a single spherical skin of lithosphere, the surface of at least part of Venus has broken into a series of ‘campuses’. It does display tectonics, but not as we know it on planet Earth. This could be ascribed to an outcome of stress transfer from deep convective motion in the Venerean mantle. Being in the virtually non-magmatic phase of Venus’s thermal cycling, there is neither formation of new lithosphere nor subduction of old, cold plates that characterise terrestrial plate tectonics. ‘Campus’ tectonics seems likely to be another form of planetary energy and matter redistribution, and Byrne et al. have likened it to how the Earth may have functioned during the ‘missing’ 600 Ma of the Hadean Eon on Earth. But perhaps not …

The runaway greenhouse has resulted in surface temperatures on Venus being 450°C higher than on Earth: enough to melt lead. It is not just solar heat that is trapped by the atmosphere, but that from the Venerean interior. This must result in a very different geotherm (the way temperature varies with depth in a planet) from that characterising the Earth. The temperature of the beginning of mantle melting – about 1200°C – must be much shallower on Venus. On Earth that is at depths between 50 and 100 km below active plate margins and within-plate hotspots, and is not reached at all for most of the Earth that lies beneath the tectonic plates. If the mantle of Venus contained a similar complement of heat-producing isotopes to that of Earth wouldn’t we expect continual volcanism on Venus rather than the odd dribble that has been observed by Magellan? Or does the jostling of ‘campuses’ absorb the thermal energy and help direct it slowly to space by radiation through the dense, greenhouse atmosphere. Here’s another poser: If the Earth and Venus are geochemically similar and Hadean Earth went through such a phase of ‘campus tectonics’ – perhaps our world had a CO2-rich atmosphere too – what changed to allow plate tectonics here to replace that system of thermal balance? And, why hasn’t that happened on Venus? Perhaps some light will be thrown on these enigmas once a series of new missions to Venus are launched between now and the 2030s, by NASA and the European Space Agency.