Can rock weathering halt global warming?

The Lockdown has hardly been a subject for celebration, but there have been two aspects that are, to some extent, a comfort: the trickle of road traffic and the absence of convection trails. As a result the air is less polluted and much clearer, and the quietness, even in cities, has been almost palpable. Wildlife seems to have benefitted and far less CO2 has been emitted. Apart from the universal tension of waiting for one of a host of potential Covid-19 symptoms to strike and the fact that the world economy is on the brink of the greatest collapse in a century, it is tempting to hope that somehow business-as-usual will remain this way. B*gger the gabardine rush to work and the Great Annual Exodus to ‘abroad’. The crisis in the fossil fuel industry can continue, as far as I am concerned, But then, of course, I am retired, lucky to have a decent pension and live rurally. Despite the health risks, however, global capital demands that business-as-it-was must return now. A planet left to that hegemonic force has little hope of staving off anthropogenic ecological decline. But is there a way for capital to ‘have its cake and eat it’? Some would argue that there are indeed technological fixes. Among them is sweeping excess of the main greenhouse gas ‘under the carpet’ by burying it. There are three main suggestions: physically extracting CO2 where it is emitted and pumping it underground into porous rocks; using engineered biological processes in the oceans to take carbon into planktonic carbohydrate or carbonate shells and disposing the dead remains in soil or ocean-floor sediments; enhancing and exploiting the natural weathering of rock. The last is the subject of a recent cost-benefit analysis (Beerling, D.J. and 20 others 2020. Potential for large-scale CO2 removal via enhanced rock weathering with croplands. Nature, v. 583, p. 242–248; DOI: 10.1038/s41586-020-2448-9).

Carbon dioxide in the rock cycle (Credit: Skeptical Science, in Wikipedia)

Research into the climatic effects of rock weathering has a long history, for it represents one of the major components of the global carbon cycle, as well as the rock cycle. Natural chemical weathering is estimated to remove about a billion metric tons of atmospheric carbon annually. That is because the main agent of weathering is the slightly acid nature of rainwater, which contains dissolved CO2 in the form of carbonic acid (H2CO3). This weak acid comprises hydrogen ions (H+), which confer acidity, that are released by the dissolution of CO2 in water, together with HCO3ions (bicarbonate, now termed hydrogen carbonate). During weathering the hydrogen ions break down minerals in rock. This liberates metals that are abundant in the silicate minerals that make up igneous rocks – predominantly Na, Ca, K, and Mg – as their dissolved ions, leaving hydrated aluminium silicates (clay minerals) and iron oxides as the main residues, which are the inorganic basis of soils. The dissolved metals and bicarbonate ions may ultimately reach the oceans. However, calcium and magnesium ions in soil moisture readily combine with bicarbonate ions to precipitate carbonate minerals in the soil itself, a process that locks-in atmospheric carbon. Another important consequence of such sequestration is that it may make the important plant nutrient magnesium – at the heart of chlorophyll – more easily available and it neutralises any soil acidity built-up by continuous agriculture.  But carbon sequestration naturally achieved by weathering amounts to only about a thirtieth of that emitted by the burning of fossil fuels, and we know that is incapable of coping with the build-up of anthropogenic CO2 in the atmosphere: it certainly has not since the start of the Industrial Revolution.

What could chemical weathering do if it was deliberately enhanced?  One of the most common rocks, basalt, is made up of calcium-rich feldspar and magnesium-rich pyroxene and olivine. In finely granulated form this mix is particularly prone to weathering, and the magnesium released would enrich existing soil as well as drawing down CO2. Hence the focus by David Beerling and his British, US and Belgian colleagues on systematic spreading of ground-up basalt on cropland soils, in much the same way as crushed limestone is currently applied to reverse soil acidification. It is almost as cheap as conventional liming, with the additional benefit of fertilising: it would boost to crop yields. The authors estimate that removal of a metric ton of CO2 from the atmosphere by this means would cost between US$ 55 to 190, depending on where it was done. One of their findings is that the three largest emitters of carbon dioxide – China, the US and India – happen to have the greatest potential for carbon sequestration by enhanced weathering. Incidentally, increased fertility also yields more organic waste that itself could be used to increase the actual carbon content of soils, if converted through pyrolysis to ‘biochar’ .

It all sounds promising, almost ‘too good to be true’. The logistics that would be needed and the carbon emissions that the sheer mass of rock to be finely ground and then distributed would entail, for as long as global capital continues to burn fossil fuels, are substantial, as the authors admit. The grinding would have to be far more extreme than the production of igneous-rock road aggregate. Basalt or related rock is commonly used for resurfacing motorways, not especially well known for degrading quickly to a clay-rich mush. It would probably have to be around the grain size achieved by milling to liberate ore minerals in metal mines, or to produce the feedstock for cement manufacture: small particles create a greater surface area for chemical reactions. But there remains the issue of how long this augmented weathering would take to do the job: its efficiency. Experimental weathering to test this great-escape hypothesis is being conducted by a former colleague of mine, using dust from an Irish basalt quarry to coat experimental plots of a variety of soil types. After two months Mg and Ca ions were indeed being released from the dust, and tiny fragments of olivine, feldspar and pyroxene do show signs of dissolution. Whether this stems from rainwater – the main objective – or from organic acids and bacteria in the soils is yet to be determined. No doubt NASA is doing much the same to see if dusts that coat much of Mars can be converted into soils  Beerling et al. acknowledge that the speed of weathering is a major uncertainty. Large-scale field trials seem some way off, and are likely to be plagued by cussedness! Will farmers willingly change their practices so dramatically?

See also: Lehmann, J & Possinger, A. 2020. Removal of atmospheric CO2 by rock weathering holds promise for mitigating climate change. Nature, v. 583, p. 204-205; DOI: 10.1038/d41586-020-01965-7

Note (added 15 July 2020): Follower Walter Pohl has alerted me to an interesting paper on using ultramafic rocks in the same way (Kelemen, P.B. et al. 2020. Engineered carbon mineralization in ultramafic rocks for COremoval from air: Review and new insights. Chemical Geology, v.  550, Article 119628; DOI:10.1016/j.chemgeo.2020.119628). Walter’s own blog contains comments on the climatic efficacy of MgCO3 (magnesite) formed when olivine is weathered.

Turmoil in Roman Republic followed Alaskan volcanic eruption

That activities in the global political-economic system are now dramatically forcing change in natural systems is clear to all but the most obdurate. In turn, those changes increase the likelihood of a negative rebound on humanity from the natural world. In the first case, data from ice cores suggests that an anthropogenic influence on climate may have started with the spread of farming in Neolithic times. Metal pollution of soils had an even earlier start, first locally in Neanderthal hearths whose remains meet the present-day standards for contaminated soil, and more extensively once Bronze Age smelting of copper began. Global spread of anomalously high metal concentrations in atmospheric dusts shows up as ‘spikes’ in lead within Greenland ice cores during the period from 1100 BCE to 800 CE. This would have resulted mainly from ‘booms and busts’ in silver extraction from lead ores and the smelting of lead itself. In turn, that may reflect vagaries in the world economy of those times

Precise dating by counting annual ice layers reveals connections of Pb peaks and troughs with major historic events, beginning with the spread of Phoenician mining and then by Carthaginians and Romans, especially in the Iberian Peninsula. Lead reaches a sustained peak during the acme of the Roman Republic from 400 to 125 BC to collapse during widespread internal conflict during the Crisis of the Republic. That was resolved by the accession of Octavian/Augustus as Emperor in 31 BCE and his establishment of Pax Romana across an expanded empire. Lead levels rose to the highest of Classical Antiquity during the 1st and early 2nd centuries CE. Collapse following the devastating Antonine smallpox pandemic (165 to 193 CE) saw the ice-core records’ reflecting stagnation of coinage activity at low levels for some 400 years, during which the Empire contracted and changed focus from Rome to Constantinople. Only during the Early Medieval period did levels rise slowly to the previous peak.

The Okmok caldera on the Aleutian island of Umnak (Credit: Desert Research Institute, Reno, Nevada USA)

Earth-logs has previously summarised how natural events, mainly volcanic eruptions, had a profound influence in prehistory. The gigantic eruption of Toba in Sumatra (~73 ka ago) may have had a major influence on modern-humans migrating from Africa to Eurasia. The beginning of the end for Roman hegemony in the Eastern Mediterranean was the Plague of Justinian (541–549 CE), during which between 25 to 50 million people died of bubonic plague across the Eastern Empire. This dreadful event followed the onset of famine from Ireland to China, which was preceded by signs of climatic cooling from tree-ring records, and also with a peak of volcanogenic sulfate ions in the Greenland and Antarctic ice caps around 534 CE. Regional weakening of the populace by cold winters and food shortages, also preceded the Black Death of the mid-14th century. In the case of the Plague of Justinian, it seems massive volcanism resulted in global cooling over a protracted period, although the actual volcanoes have yet to be tracked down. Cooling marked the start of a century of further economic turmoil reflected by lead levels in ice cores (see above). Its historical context is the Early Medieval equivalent of world war between the Eastern Roman Empire, the Sassanid Empire of Persia and, eventually, the dramatic appearance on the scene of Islam and the Arabian, Syrian and Iraqi forces that it inspired (see: Holland, T. 2013. In the Shadow of the Sword: The battle for Global Empire and the End of the Ancient World. Abacus, London)

An equally instructive case of massive volcanism underlying social, political and economic turmoil has emerged from the geochemical records in five Greenlandic ice cores and one from the Siberian island of Severnaya Zemlya (McConnell, J.R. and 19 others 2020. Extreme climate after massive eruption of Alaska’s Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom. Proceedings of the National Academy of Sciences, recent article (22 June 2020); DOI: 10.1073/pnas.2002722117). In this case the focus was on ice layers in all six cores that contain sulfate spikes and, more importantly, abundant volcanic dust, specifically shards of igneous glass. Using layer counting, all six show major volcanism in the years 45 to 43 BCE. The Ides (15th) of March 44 BCE famously marked the assassination of Julius Caesar, two years after the Roman Republic’s Senate appointed him Dictator, following four years of civil war. This was in the later stages of the period of economic decline signified by the fall in ice-core levels of Pb (see above). The Roman commentator Servius reported “…after Caesar had been killed in the Senate on the day before, the sun’s light failed from the sixth hour until nightfall.” Other sources report similar daytime dimming, and unusually cold weather and famine in 43 and 42 BCE.

As well as pinning down the date and duration of the volcanic dust layers precisely (to the nearest month using laser scanning of the ice cores’ opacity), Joseph McConnell and the team members from the US, UK, Switzerland, Germany and Denmark also chemically analysed the minute glass shards from one of the Greenlandic ice cores. This has enabled them to identify a single volcano from 6 possible candidates for the eruption responsible for the cold snap: Okmok, an active, 8 km wide caldera in the Aleutian Islands of Alaska. Previous data suggest that its last major eruption was 2050 years ago and blasted out between 10 to 100 km3 of debris, including ash. Okmok is an appropriate candidate for a natural contributor to profound historic change in the Roman hegemony. The authors also use their ice-core data to model Okmok’s potential for climate change: it had a global reach in terms of temperature and precipitation anomalies. Historians may yet find further correlations of Okmok with events in other polities that kept annual records, such as China.

See also: Eruption of Alaska’s Okmok volcano linked to period of extreme cold in ancient Rome (Science Daily, 22 June 2020); Kornei, K. 2020. Ancient Rome was teetering. Then a volcano erupted 6,000 miles away. (New York Times, 22 June 2020)

Did an impact affect hunter gatherers at the start of the Younger Dryas?

Whether or not the return to a glacial climate between 12.8 and 11.7 thousand years (ka) ago, known as the Younger Dryas (YD), was triggered by some kind of extraterrestrial impact has been a hot and sometimes fractious issue since 2007 (see: Whizz-bang view of Younger Dryas; Earth-logs, July 2007). Before then the most favoured causal mechanism was a shutdown of the Gulf Stream’s Arctic warming influence as a result of some kind of catastrophic flooding of fresh water into the North Atlantic. That would have lowered the density of surface waters, thereby preventing them from sinking to drive the deep circulation that draws surface water from the tropics into high northern latitudes (see: The Younger Dryas flood; May 2010). In 2008 the melt-water flood supporters were sufficiently piqued by the suggestion of a hitherto unsuspected impact event to mount a powerful rejoinder (see: Impact cause for Younger Dryas draws flak; May 2008), casting doubt on the validity of the data that had been presented. It seemed like a repeat of the initial furore over claims for a ‘mountain falling out of the sky’ wiping out the dinosaurs and much else. Yet, like the claims by Alvarez pere et fils for the K-T impact, accumulated weight of evidence published by its protagonists eventually has given the idea of an impact trigger for the YD a measure of respectability. This began with evidence of an impact crater beneath the Greenland icecap (see: Subglacial impact structure in Greenland: trigger for Younger Dryas?; November 2018), then signs of a 12.8 ka fire storm in Chile followed by geochemical evidence from South Carolina, USA for a coinciding impact (see: More on the Younger Dryas causal mechanism; November 2019).

Colour-coded subglacial topography from radar sounding over the Hiawatha Glacier of NW Greenland, showing a possible impact crater (Credit: Kjaer et al. 2018; Fig. 1D)

The YD played havoc with humans who had begun to repopulate northern Europe from their Ice Age refuges in the south and those who had first ventured into the Americas  across the Beringia land bridge between Siberia and Alaska. The climate decline was extremely rapid, spanning a mere decade or so, and many would have been trapped to perish in what again became frigid steppe land. There is now evidence that late-Palaeolithic to Mesolithic hunter gatherers living far south of the reglaciated zone also suffered devastation at the start of the YD (Moore, A.M.T. and 13 others 2020. Evidence of Cosmic Impact at Abu Hureyra, Syria at the Younger Dryas Onset (~12.8 ka): High-temperature melting at > 2,200 °C. Nature Science Reports, v. 10, p. 1-22; doi: 10.1038/s41598-020-60867-w). Abu Hureyra is a tell – a mound settlement – originally on the banks of the Euphrates in northern Syria. It now lies beneath Lake Assad, but was excavated in the early 1970s to reveal a charcoal-littered habitation surface with signs of a settlement and some cultivation. Charcoal from archived samples yielded a precise radiocarbon age of 12825 ± 55 ka, coinciding with the start of the YD. The sediment from the habitation floor also contained signs compatible with ejecta from a high-energy impact: tiny diamonds and glass spherules. Analyses of the glass by the authors suggests that it formed at a temperature up to 2200°C, far greater than that of magma associated with a volcanic eruption or in hearths used by the inhabitants. However, others have analysed the glass and suggest more mundane temperatures that could be explained more simply by accidental burning of thatched huts. That possibility might explain the lack of other impact indicators, such as shocked mineral grains and anomalous geochemistry, particularly the platinum-group metals that were the original ‘smoking gun’ for the K-T boundary event and other major impacts. Incidentally, these crucial indicators have been reported from other YD sites investigated by several members of the team behind this paper. My view is that what seems to be a remarkable coincidence will not settle the matter, but will probably draw the same kind of ‘flak’ as did others on this topic. It is hardly likely that new samples will be collected from the now submerged Abu Hureyra site.

See also: Cometary Debris may have destroyed Paleolithic settlement 12,800 years ago (Science News. 2 July 2020)

Fossil fuel, mercury and the end-Palaeozoic catastrophe

Siberian flood-basalt flows in the Putorana Plateau, Taymyr Peninsula, Russia. (Credit: Paul Wignall)

The end of the Permian Period (~252 Ma ago) saw the loss of 90% of marine fossil species and 70% of those known from terrestrial sediments: the greatest known extinction in Earth’s history. In their naming of newly discovered life forms, palaeontologists can become quite lyrical. Extinctions, however, really stretch their imagination. They call the Permo-Triassic boundary event ‘The Great Dying’. Why not ‘Permageddon’? Sadly, that was snaffled in the 1980s by an astonishingly short-haired heavy-metal tribute band. Enough bathos … The close of the Palaeozoic left a great many ecological niches to be filled by adaptive radiation during the Triassic and later Mesozoic times. Coinciding with the largest known flood-basalt outpouring – the three million cubic kilometres of Siberian Traps – the P-Tr event seemed to be ‘done and dusted’ after that possible connection was discovered in the mid 1990s. Notwithstanding, the quest for a gigantic, causative impact crater continues (see: Palaeobiology Earth-logs, May, September and October 2004), albeit among a dwindling circle of enthusiasts. The Siberian Traps are suitably vast to snuff the fossil record, for their eruption must have belched all manner of climate-changing gases and dusts into the atmosphere; CO2 to encourage global warming; SO2 and dusts as cooling agents. There is also evidence of a role for geochemical toxicity (see: Nickel, life and the end-Permian extinction, June 2014). The extinctions accompanied not only climate change but also a catastrophic fall in atmospheric oxygen content (see: Homing in on the great end-Permian extinction, April 2003; When rain kick-started evolution, December 2019). Recovery of the biosphere during the early Triassic was exceedingly slow.

Research focussed on the P-Tr boundary eventually uncovered an element of pure chance. Shales in Canada that span the boundary show major, negative δ13C excursions in the carbon-isotope record that coincide with fly ash in the analysed layers. This material is similar in all respects to that emitted from coal-fired power stations (see: Coal and the end-Permian mass extinction, March 2011). The part of Siberia onto which the flood basalts were erupted is rich in Permian coal measures and oil shales that lay close to the surface 252 Ma ago. The coal ash and massive emissions of CO2 may have resulted from their burning by the flood basalt event. Now evidence has emerged that this did indeed happen (Elkins-Tanton, L.T. et al. 2020. Field evidence for coal combustion links the 252 Ma Siberian Traps with global carbon disruption. Geology, v. 48, early publication; DOI: 10.1130/G47365.1).

The US, Canadian and Russian team found large quantities of burnt coal and woody material, and bituminous blobs in 600 m thick volcanic ashes at the base of the Siberian traps themselves. They concluded that the magma chamber from which the flood basalts emerged had incorporated sizeable volumes of the coal measures, leading to their combustion and distillation. This would have released CO2 enriched in light 12C due to isotopic fractionation by biological means, i.e. its δ13C would have been sufficiently negative to affect the carbon locked up in the Canadian P-Tr boundary-layer shales that show the sharp isotopic anomalies. The magnitude of the anomalies suggest that between six to ten thousand billion tons of carbon released as CO2 or methane by interaction of the Siberian Traps with sediments through which their magma passed could have created the global δ13C anomalies. That is about one tenth of the organic carbon originally locked in the Permian coal measures beneath the flood basalts

Another paper whose publication coincided with that by Elkins-Tanton et al. suggests that environmental mercury appears to have followed the same geochemical course as did carbon at the end of the Palaeozoic Era (Dal Corso, J. and 9 others 2020. Permo–Triassic boundary carbon and mercury cycling linked to terrestrial ecosystem collapse. Nature Communications, v. 11, paper 2962; DOI: 10.1038/s41467-020-16725-4). This group, based at Leeds and Oxford Universities, UK and the University of Geosciences in Wuhan, China, base their findings on biogeochemical modelling of the global carbon and mercury cycles at the end of the Permian. Their view is that the coincidence in marine sediments at the P-Tr boundary of a short-lived spike in mercury and an anomaly in its isotopic composition with the depletion in 13C, described earlier, shows an intimate link between mercury and the biological carbon cycle in the oceans at the time. They suggest that this synergy marks ecosystem collapse and derives ‘from a massive oxidation of terrestrial biomass’; i.e. burning of organic material on the land surface. Their modelling hints at huge wildfires in equatorial peatlands but also a role for the Siberian flood-basalt volcanism and the incorporation of coal measures into the Siberian Trap magma chamber.

A protein clue to H. antecessor’s role in human evolution

Homo_antecessor child
Forensic reconstruction of the remains of a Homo antecessor child from Gran Dolina Cave in northern Spain (credit Élisabeth Daynès, Museo de la Evolución, Burgos, Spain)

The older a fossil, no matter how well preserved it is, the less chance it has to contain enough undegraded DNA for it to be extracted and sequenced using the most advanced techniques. At present the oldest fossil DNA not to have passed its ‘sell-by’date is that of a 560 to 780 thousand year-old horse’s legbone found in Canadian permafrost. For human remains the oldest mtDNA is that of a ~430 ka individual from the Sima de los Huesos in northern Spain (see: Mitochondrial DNA from 400 thousand year old humans; Earth-logs December 2013). But there is another route to establishing genetic relatedness from the amino-acid sequences of proteins recovered from ancient individuals (see: Ancient proteins: keys to early human evolution?). Fossil teeth have proved to be good repositories of ancient protein and are the most commonly found hominin fossils.

A key species for unravelling the origins of the three most recent human groups (ourselves, Neanderthals and Denisovans) is thought to be Homo antecessor who inhabited the Gran Dolina Cave in the Atapuerca Mountains in northern Spain between about 1.2 Ma and 800 ka ago (see: Human evolution: bush or basketwork? Earth-logs, January 2014). Palaeoanthropologists excavated 170 skeletal fragments from six individuals in the most productive layer at Gran Dolina. Incomplete facial bones suggest a ‘modern-like’ face, although the remains as a whole are insufficient to reconstruct the oldest Europeans with sufficient detail to place them in anatomical relation to the younger groups. But there are several teeth. One of them, a permanent molar, has yielded informative proteins (Welker, F. and 26 others 2020. The dental proteome of Homo antecessor. Nature, v. 580, p. 235-238; DOI: 10.1038/s41586-020-2153-8) and has been dated to between 772 to 949 ka.

Amino acids in the dental proteins, sequenced using mass spectrometry, were compared with those of other hominins. Because protein sequences are coded by an animal’s genome they are a ‘proxy’ for DNA. The outcome is that the Gran Dolina proteins are roughly equally related to Denisovans, Neanderthals and ourselves, suggesting that, although the younger three groups are closely related, H. antecessor is an ‘outlier’. Being significantly older, it is likely to be the common ancestor of all three. Another species with close anatomical affinities is H. heidelbergensis (700 to 300 ka) found in Africa as well as in Europe. Its mtDNA (see: Mitochondrial DNA from 400 thousand year old humans; Earth-logs December 2013) matches that of Denisovans better than it does Neanderthals, yet without protein and full-genome analysis all that can be concluded is that it may be an intermediary between H. antecessor and the well known interbreeding triad of more recent times.

We are getting closer to a documented web of interrelationships between humans in general whose time span from 2 Ma ago is now well established. The remaining genetic link to be documented is that to H. erectus, the longest lived and most travelled of all ancient humans. Frido Welker and co-workers also had a shot at the proteomics of one of the first humans known to have migrated from Africa, using an isolated, presumably H. erectus, molar found at the 1.77 Ma site at Dmanisi in the Caucasus foothills of Georgia. Although inconclusive in placing that precociously intrepid group firmly in the human story, the fact that dental proteins were discovered is cause for optimism.

See also: Campbell, M. 2020. Protein analysis of 800,000-year-old human fossil clarifies dispute over ancestors (Technology Networks, 1 April 2020)

What controls the height of mountains?

‘Everybody knows’ that mountains grow: the question is, ‘How?’ There is a tale that farmers once believed that they grew from pebbles: ‘every year I try to rid my field of stones, but more are back the following year, so they must grow’… Geoscientists know better – or so they think[!] – and for 130 years have referred to ‘orogeny’, a classically-inspired term (from the Ancient Greek óros and geneia – high-ground creation’) adopted by the US geologist Grove Gilbert. It incorporates the concept of crustal thickening that results from lateral forces and horizontal compression. Another term, now rarely used, is ‘epeirogeny’ (coined too by G.K. Gilbert), wherein the continental surface rises or falls in response to underlying gravitational forces. That could include: changing mantle density over a hot, rising plume; detachment or delamination into the mantle of dense lower lithosphere; loading or unloading by ice during glacial cycles. Epeirogeny is bound up with isostasy, the maintenance of gravitational balance of mass in the outermost Earth.

A small part of the High Himalaya (credit: Access-Himalaya)

In 1990, Peter Molnar and Philip England pointed out that the incision of deep valleys into mountain ranges results in stupendous and rapid removal of mass from orogenic belts, which adds a major isostatic force to mountain building (Molnar, P. & England, P. 1990. Late Cenozoic uplift of mountain ranges and global climate change: chicken or egg? Nature, v. 346, p. 29–34; DOI: 10.1038/346029a0). In their model, the remaining peaks are driven higher by isostasy. They, and others, coupled climate change with compressional tectonics in a positive feedback that drives peaks to elevations that they would otherwise never achieve. Molnar and England’s review saw complex interplays contributing to mountain building, accompanying chemical weathering even changing global climate by sequestering atmospheric CO2 into the minerals that it produces. As well as the height of peaks in active zones of crustal shortening and thickening, such as the Himalaya, Molnar and England’s theory explained the aberrant high peaks at the edge of high plateaus that are passively subject to erosion. Examples of the latter are the isolated peaks beyond the eastern edge of the Ethiopian Plateau that locally have the greatest elevation than the flood basalts that form the plateau: unloading around these peaks has caused them to rise isostatically.

Thirty years on, this paradigm is being questioned, at least as regards active orogens (Dielforder, A. et al. 2020. Megathrust shear force controls mountain height at convergent plate margins. Nature, v. 582, p. 225–229; DOI: 10.1038/s41586-020-2340-7). Armin Dielforder and colleagues at the German Research Centre for Geosciences in Potsdam and The University of Münster consider that overall mountain height is sustained by interactions between three forces. 1. They are prevented from falling apart under their own weight or being pushed up further against gravity by lateral tectonic force. 2. Climate controlled erosion limits mountain height by removing material from the highest elevations. 3. Isostasy keeps the mountains ‘afloat’ above the asthenosphere. The authors have attempted to assess and balance all three major forces that determine the overall elevation of mountain belts.

At a convergent plate margin where one plate is shoved beneath another, the megathrust above the subduction zone behaves in a brittle fashion, with associated friction, towards the surface. At depth this transitions to a zone of ductile deformation dominated by viscosity. A major assumption in this work is that stress in the crust below a mountain belt is neutral; i.e. horizontal, tectonic compression is equal to the weight of the mountains themselves and thus to their height. So, the greater the tectonic compressive force the higher the mountain range that it can support. The test is to compare the actual elevation with that predicted from plate-tectonic considerations. For 10 active orogenic belts there is a remarkable correspondence between the model and actuality. the authors conclude that variation over time of mountain height reflects log-term variations in the force balance, in which they find little sign of a climatic/erosional control. But that doesn’t resolve the issue satisfactorily, at least for me.

The study focuses on the mean elevation, and this leaves out the largest mountains; for instance, their maximum mean elevation for the Himalaya is about 5.46 km (in fact for a narrow  NE-SW swath that may not be representative of the whole range). Yet the Himalaya contains 10 of the world’s highest mountains, all over 8 km high and 50 peaks that top 7 km, adjacent to the Tibetan Plateau. The mean elevation of the whole Himalayan range is 6.1 km. Consequently, it seems to me, the range’s maximum mean elevation must be somewhat higher than that reported by Dielforder et al.  The difference suggests that non-tectonic forces do contribute significantly to Himalayan terrain

See also:  Wang, K. 2020. Mountain height may be controlled by tectonic force, rather than erosion. Nature, v. 582, p. 189-190; DOI: 10.1038/d41586-020-01601-4

Geochemistry and the Ediacaran animals

Hopefully, readers will be fairly familiar with the sudden appearance of the Ediacaran fauna – the earliest abundant, large animals – at the start of the eponymous Period of the Neoproterozoic around 635 Ma. If not, use the Search Earth-logs box in the side bar to find extensive coverage since the start of the 21st century. A June 2019 Earth-logs review of the general geochemical background to the Ediacaran Period can be found here. Ten years ago I covered the possible role of the element phosphorus (P) – the main topic here – in the appearance of metazoans (see: Phosphorus, Snowball Earth and origin of metazoans – November 2010).

One of the major changes in marine sedimentation seen during the Ediacaran was a rapid increase in the deposition on the ocean floor of large bodies of P-rich rock (phosphorite), on which a recent paper focuses (Laakso, T.A. et al. 2020. Ediacaran reorganization of the marine phosphorus cycle. Proceedings of the National Academy of Sciences, v. 117, p. 11961-11967; DOI: 10.1073/pnas.1916738117). It has been estimated that on million-year time scales phosphorites remove only a tiny amount of the phosphorus carried into the oceans by rivers. So, conversely, an increase in deposition of marine P-rich sediment would have little effect on the overall availability of this essential nutrient from the oceans. The Ediacaran boost in phosphorites suggests a connection between them and the arrival of totally new ecosystems: the global P-cycle must somehow have changed. This isn’t the only change in Neoproterozoic biogeochemistry. Thomas Laakso and colleagues note signs of slightly increased ocean oxygenation from changes in sediment trace-element concentrations, a major increase in shallow-water evaporites dominated by calcium sulfate (gypsum) and changes in the relative proportions of different isotopes of sulfur.

Because all marine cycles, both geochemical and those involving life, are interwoven, the authors suggest that changes in the fate of dead organic matter may have created the phosphorus paradox. Phosphorus is the fifth most abundant element in all organisms after carbon, hydrogen, nitrogen and oxygen, followed by sulfur (CHNOPS), P being a major nutrient that limits the sheer bulk of marine life. Perhaps changes to dead organic matter beneath the ocean floor released its phosphorus content, roughly in the manner that composting garden waste releases nutrients back to the soil. Two chemical mechanisms can do this in the deep ocean: a greater supply of sinking organic matter – essentially electron donors – and of oxidants that are electron acceptors. In ocean-floor sediments organic matter can be altered to release phosphorus bonded in organic molecules into pore water and then to the body of the oceans to rise in upwellings to the near surface where photosynthesis operates to create the base of the ecological food chain.

Caption The Gondwana supercontinent that accumulated during the Neoproterozoic to dominate the Earth at the time of the Ediacaran (credit: Fama Clamosa, at Wikimedia Commons)

There is little sign of much increase in deep-ocean oxygen until hundreds of million years after the Ediacaran. It is likely, therefore, that increased availability of oxidant sulfate ions (SO42-) in ocean water and their reduction to sulfides in deep sediment chemically reconstituted the accumulating dead organic matter to release P far more rapidly than before. This is supported by the increase in CaSO4 evaporites in the Ediacaran shallows. So, where did the sulfate come from? Compressional tectonics during the Neoproterozoic Era were at a maximum, particularly in Africa, South America, Australia and Antarctica, as drifting continental fragments derived from the break-up of the earlier Rodinia supercontinent began to collide. This culminated during the Ediacaran around 550 Ma ago with assembly of the Gondwana supercontinent. Huge tracts of it were new mountain belts whose rapid erosion and chemical weathering would have released plenty of sulfate from the breakdown of common sulfide minerals.

So the biological revolution and a more productive biosphere that are reflected in the Ediacaran fauna ultimately may have stemmed from inorganic tectonic changes on a global scale

Is there water in the Earth’s core?

Understandably, the nature of what lies at the centre of the Earth is as much the subject of speculation as tangible evidence. That there must be something very dense within the planet emerged once the Earth’s bulk density was calculated. Because a high proportion of meteorites are dominated by an alloy of the metals iron and nickel, geoscientists adopted that combination as plausible core material. Study of the arrival times around the globe of seismic waves from earthquakes then revealed the actual size of the Earth’s core. Iron-nickel alloy fitted the bill quite nicely. It also fits geochemical evidence, such as the crust and mantle’s depletion in some trace elements that theoretically have an affinity for iron. The fact that seismology showed also that the outer core was molten and able to flow, together with metals’ high electrical conductivity, gave rise to the current concept of the geomagnetic field being generated by a dynamo effect in the core. However the density of Fe-Ni is not ‘quite right’ because the core is somewhat lighter than predicted for the pure alloy under stupendous pressure: it must contain a substantial amount – up to 13% – of lower density materials.  Silicon, sulfur and oxygen have been suggested as candidates, with evidence from a variety of minor minerals in metallic meteorites.

A recent model for core formation (credit Crystal Y. Shi et al 2013; DOI: 10.1038/NGEO1956 Fig. 5)

The world is currently awash with models that attempt to throw light on the course of the Covid-19 pandemic. Many are based on highly uncertain data, leading to suggestions by some people that they have become tools for political elites and a means of helping ambitious scientists into the limelight: a sort of fuel for hubris. In the midst of this unprecedented turmoil there has appeared a suggestion (from modelling) that the core also contains abundant hydrogen (Li, Y. et al. 2020. The Earth’s core as a reservoir of water. Nature Geoscience, v. 13, published online; DOI: 10.1038/s41561-020-0578-1). Yunguo Li and colleagues, from University College London, the Chinese Academy of Science and the University of Oslo, explore the idea that the dominant hydrogen of the pre-planetary Solar nebula, which accreted to form the Earth, may have joined iron during core formation. This had been predicted from the thermodynamics of chemical reactions between water and iron. The team takes this further through the geochemical theory that elements and compounds tend to enter other materials preferentially. For example, during partial melting of the crust alkali metals (Na, K etc) are more likely to enter the granitic melt than to remain in the solid residue. Li et al. have used thermodynamics to predict the partitioning of hydrogen between iron and silicate melts under the very high temperature and pressure conditions at the boundary between the core and mantle.

Their calculations suggest that hydrogen then behaves in much the same manner as, say gold and platinum: it becomes ‘iron-loving’ or siderophile and prefers the molten core, as would H2O. The amount that gets in depends on the water content of the molten silicate that eventually becomes the mantle. If the water now making up Earth’s ocean was ‘degassed’ from the mantle during core formation then the original silicate melt would have been ‘wetter’ than it is now. The implication of such early degassing is that the core may contain 5 ‘oceans worth’ of water! The alternative scenario for Earth’s becoming a watery world is the later accretion of, for instance, cometary material. In that case, the early core would have been drier. Yet, the continual subduction of hydrated oceanic lithosphere into the deep mantle during billions of years of plate tectonics would steadily have added water to the core, in the form of iron oxides and hydrogen. So, the core might, in either case, contain several ‘oceans’ of the components of water. One line of indirect evidence is the deficiency in Earth’s actual water of the heavier isotope of hydrogen (deuterium) relative to the D/H ratio of chondritic meteorites. Theory suggests that D has slightly more affinity for joining iron than does H. Substantial water in the core does help explain the core’s apparent low density, but that notion requires as much faith as politicians seem to have in ‘following the Science’ during the current pandemic …

Arsenic hazard on a global scale

I have been following the harrowing story of how arsenic gets into domestic water supplies for 20 years (see: Earth-logs Geohazards for 2002; 2003; 2004; 2005; 2006; 2008; 2009; 2011; 2013; 2017). In my opinion, it is the greatest natural hazard in terms of the numbers at risk of poisoning. In 2006 I wrote about the emergence in Bangladesh of arsenic poisoning on a huge scale during the mid 1990s for a now defunct Open University course. If people depend for drinking water on groundwater from tube wells driven into alluvium they would not know of the risk, unless the water is rigorously analysed for levels of As greater than 10 micrograms per litre (μg l-1), the WHO recommended maximum. The sad fact is that the affected population were advised to switch from surface water supplies, which carry a high risk of biological infection, to well water. That is because during downward percolation from the surface oxidation destroys bacteria and viruses as well as parasites. Opportunities provided by a massive UN-funded drilling programme and local well digging made the choice seemingly obvious. Most people came to prefer well water as gastro-intestinal infections and child mortality fell rapidly.

Arsenic adds no taste, which is why it was once the ‘poison of choice’. How it gets into groundwater is difficult to judge, unless wells are downflow of areas riddled with metal mines. Years of research uncovered an unsuspected mechanism. The most common colorant of mineral grains, and thus sedimentary rocks, is brownish iron hydroxide (goethite), and that is able to adsorb a range of dissolved elements, including arsenic. One would think, therefore, that groundwater should be made safe by such a natural ‘filtering’ process: indeed goethite can be used in decontamination. The problem is that iron hydroxide, which contains Fe-3, is only stable in water with a high capacity for oxidation. Under reducing conditions it breaks down to soluble Fe-2 and water, thereby releasing to solution any other element that it has adsorbed. In alluvium, beds containing organic matter are prone to this ‘reductive dissolution’ of goethite. If weathering upstream has released even seemingly insignificant amounts of arsenic during the build up of alluvium, there is a potential life-threatening problem as arsenic builds up in the goethite coating of sedimentary grains to become ‘locked in’, with the potential to be released in high concentrations if subsurface chemical conditions change. The colour of the alluvial sediments penetrated by wells is a clue. If they are reddish brown, groundwater is safe, if they are greyish and goethite-free then, ‘beware’. But it is rare to examine ‘cuttings’ from a drill site aimed at groundwater, unlike those aimed at ores or oil

Since the tragedy of Bangladesh, which resulted after 5 years or so in obvious signs of arsenicosis – dark wart-line keratoses on hands and feet or black blotches on facial and torso skin – several alluvial basins in large river systems have had their well water tested. But by no means all such basins have been screened in this way, and there are many less-obvious signs of arsenic poisoning. After long exposure to the lower range of dangerous arsenic levels a variety of cancers develop in known areas of arsenic risk. There are also high levels of endemic respiratory problems, cardiovascular disease, reduced intellectual development in children and even diabetes. Geochemical monitoring of all populated and farmed river systems is a huge task that is far beyond the resources of many countries through which they run. One approach to ‘screening’ for hazard or safety is to use geological, hydrological, soil, climate and topographic data. Those from known arsenic-prone basins and those where its levels are shown to be consistently below the 10 μg l-1 danger threshold help to develop a predictive model (Podgorski, J. & Berg, M. 2020. Global threat of arsenic in groundwater. Science, v. 368, p. 845-850; DOI: 10.1126/science.aba1510).

Modelled global probability of arsenic concentration in groundwater exceeding 10 μg l-1. Click to display a larger map in a separate browser tab. (credit: Podgorski & Berg; Fig 2A, with enhanced colour)

Rather than trying to model the full range of arsenic concentrations, Joel Podgorski and Michael Berg of the Swiss Federal Institute of Aquatic Science and Technology focussed on assessing probabilities that arsenic in well water exceeds the WHO recommended maximum safe level of 10 μg l-1. Their global map highlights areas of concern for environmental health. Thankfully, huge (blue) areas are suggested to present low risk, the pale, yellow, orange and red patches signifying areas of increasing concern. No populated continent is hazard-free. What is very clear is that Asia presents the greatest worries. Most of the Asian ‘hot zones’ are spatially close to large mountain ranges and plateaus. In the case of the Indus and Ganges-Brahmaputra plains the sources for excessive arsenic in groundwater implicated by previous geochemical investigations lie in the Himalaya. The factor common to all major hot spots seems to be rapid transport of huge amounts of sediment released by weathering from areas of high topographic relief, rather than local large-scale mining operations. There are hazardous areas related to historic and active mining, such as the Andes of Bolivia, Peru and Chile and the western USA, but they are tiny by comparison with the dominance of natural arsenic mobilisation.

Despite the WHO recommended maximum of 10 μg l-1 of arsenic, many countries base their policy on levels that are five times higher, largely because of the difficulty of analysing for the lower concentration without expensive analytical facilities. Field analyses are often done using simple semi-quantitative tests based on paper impregnated with reagents that show a colour range for different concentrations, which are unreliable for those lower than 100 μg l-1. Thankfully, despite the many risky areas, most of them have population densities less than 1 per km2.

If you are interested in the geological details of the arsenic problems of Bangladesh, the course text that I produced for the Open University (Drury, S. 2006. Water and well-being: arsenic in Bangladesh. The Open University: Milton Keynes, UK. ISBN 0-7492-1435-X), the course itself (S250 Science in Context) was withdrawn some years ago.  It may be possible to arrange a PDF for private use.

See also: Zheng, Y. 2020. Global solutions to a silent poison. Science, v. 368, p. 818-819; DOI: 10.1126/science.abb9746

Update on climate and sea-level change during the Cenozoic

The Cenozoic Era was a period of fundamental change in the outer part of the Earth system. It culminated in the greatest climatic cooling since the Permian Period, during which upright apes emerged between 6 to 10 Ma ago.  The most decisive part of hominin evolution – the appearance of our own genus Homo – took place in the last 2.5 Ma that saw icecaps plastered over both polar regions and repeated pulses of major climate upheaval that dramatically affected all parts of the continents. Whereas the Mesozoic was dominated by reptiles, most famously the dinosaurs, the Cenozoic is rightly known as the age of mammals and of birds. The flowering plants, especially grasses, also transformed terrestrial ecosystems. The background to what has become ‘our time’ is not only climate change, but massive shifts in sea level and the outlines of the continents. For more than two decades many palaeoclimatologists have focused on the Cenozoic, gathering data using a variety of rapidly advancing technologies from a growing number of sites, in sediments from the continents and the ocean floor. One problem has been correlating all this global data precisely, coming as it does from many incomplete sedimentary sequences dotted around the planet. A great deal of basic information has come from the petroleum industry, which, of course, has continually eyed sedimentary rocks as the source of hydrocarbons through the 20th century. It was seismic reflection surveying that first gave clues to global ups and downs of sea level from onlaps and offlaps of strata that are visible on seismic sections, amplified by sequence stratigraphy. Six geoscientists from Rutgers University in New Jersey, USA have blended oil-industry archives with academic research to produce the first fully calibrated, comprehensive record of the Cenozoic (Miller, K.G. et al. 2020. Cenozoic sea-level and cryospheric evolution from deep-sea geochemical and continental margin records. Science Advances, v. 6, article eaaz1346; DOI: 10.1126/sciadv.aaz1346).

Latest palaeoclimate data for the Cenozoic Era. A oxygen-isotope data from benthic foraminifera (pale blue = polar icecaps, green = ice-free, pink – hothouse); B estimated mean sea-surface temperature from the calcium/magnesium ratio in Pacific Ocean cores; C variation in global mean sea level estimated from A and corrected for changes in the density of seawater due to water temperature (B); D atmospheric CO2 variations estimated using various proxies – see top right box. Click on the image to show a full-size version in a new browser tab. (Credit: Miller et al. 2020; Fig. 1)

Fluctuations in the proportion of 18O (δ18O) in the tests of foraminifera that lived in deep water are the key to global changes in sea level  and point to the influence of glacial ice accumulating on land (see Cooling sets in: Stepping Stones, Chapter 17). This is because glaciers are made from water that has evaporated from the oceans. When this happens, water that incorporates the lighter 16O isotope evaporates more easily and becomes enriched in atmospheric water vapour. When this water falls as snow that accumulates on land to form ice, the oceans are slightly enriched in the heavier 18O: δ18O incorporated into the shelly material of organism dwelling in the deep ocean increases at levels of a few parts per thousand. Conversely, their δ18O decreases when huge ice caps melt (A on the figure). The oxygen isotope records from fossils in ocean floor sediments give a far more precise impression of fluctuating sea level than do seismic sections and sequence stratigraphy of sedimentary rocks that interest the oil industry. But it is an ‘impression’, because other factors affect sea level.

Not only the global volume of ocean water is involved: the volume of the ocean basins changes too. This can occur because of changes in the rate of sea-floor spreading: when that is fast the hot new oceanic lithosphere is less dense and so buoys-up part of the ocean floor to drive sea level upwards. Slow spreading does the converse, as more lithosphere cools and sinks slightly. Another factor is the changing rate of marine sedimentation of material eroded from the continents. That fills ocean basins to some extent, again displacing the water upwards. When sediments are compacted as they become more deeply buried that has an effect too, to increase basin volume and result in sea-level fall. Oil industry geoscientists have attempted to allow for these long-term, slow mechanisms, to give a more accurate sea-level record.

Yet there is another important factor: the density and thus volume of ocean water changes with temperature. The warmer it is the greater the volume of ocean water and the higher is sea level. This is where academic work comes in handy. Two common elements that are dissolved in ocean water are magnesium and calcium. They also occur in the carbonate tests of the same deep-water forams that are used for oxygen-isotope measurements. It turns out that the warmer the water is the more magnesium enters the foram tests, and vice versa: their Mg/Ca ratio is a reliable proxy for mean ocean temperature and can be measured easily, centimetre-by-centimetre through cores. Kenneth Miller and colleagues have used this with the oxygen isotope proxy for land-ice volume to correct the sea-level record.

The Cenozoic ocean temperature record (B on the figure) is, in itself, interesting. It reveals far more large fluctuations than previously thought, especially in the Palaeocene and early Eocene. Yet, overall, the trend is one of steady cooling compared with the sudden shifts in δ18O that mark the onset of the Antarctic ice cap at the Eocene-Oligocene boundary around 34 Ma ago, and the apparent, temporary emergence from ‘ice-house conditions in the Middle Miocene. Also, sea level corrected for ocean temperature effects (C on the figure) suggests that for much of the Cenozoic sea level was lower than expected; i.e. it rarely exceeded 60 m above the current level, which is that expected when no substantial mass of  land ice exists.

The other important compilation made by Miller et al. is that of the CO2 content of the atmosphere estimated using six different proxies. It is a lot more fuzzy than the oceanic records because the proxies are not precise. Nevertheless, it is interesting. The current, partly anthropogenic level of around 400 parts per million (ppm) is not unique. In fact from 55 to 23 Ma it was consistently above this ‘Anthropocene’ level, peaking at twice that level at the end of the Eocene. That’s odd, because it doesn’t tally with the oxygen isotopes that indicate the onset of large scale Antarctic glaciation shortly afterwards. In fact most of the climatic highlights shown by A on the figure are not reflected in the Cenozoic history of the most influential greenhouse gas. In the short term of glacial-interglacial cycles during the late Pleistocene, atmospheric CO2 levels are very closely related to fluctuations of land-ice volume. In the 65 Ma of the Cenozoic such a link is hard to argue for. There are more puzzles than revelations in this otherwise major addition to palaeoclimatology.