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.

How flowering plants may have regulated atmospheric oxygen

Ultimately, the source of free oxygen in the Earth System is photosynthesis, but that is the result of a chemical balance in the biosphere and hydrosphere that operates at the surface and just beneath it in sediments. Burial of dead organic carbon in sedimentary rocks allows free oxygen to accumulate whereas weathering and oxidation of that carbon, largely to CO2, tends to counteract oxygen build-up. The balance is reflected in the current proportion of 21% oxygen in the atmosphere. Yet in the past oxygen levels have been much higher. During the Carboniferous and Permian periods it rose dramatically to an all-time high of 35% in the late Permian (about 250 Ma ago). This is famously reflected in fossils of giant dragonflies and other insects from the later part of the Palaeozoic Era.  Insects breathe passively by tiny tubes (trachea) through whose walls oxygen diffuses, unlike active-breathing quadrupeds that drive air into lung alveoli to dissolve O2 directly in blood. Insect size is thus limited by the oxygen content of air; to grow wing spans of up to 2 metres a modern dragon fly’s body would consist only of trachea with no room for gut; it would starve.

Woman holding a reconstructed Late Carboniferous dragonfly (Namurotypus sippeli)

During the early Mesozoic oxygen fell rapidly to around 15% during the Triassic then rose through the Jurassic and Cretaceous Periods to about 30%, only to fall again to present levels during the Cenozoic Era. Incidentally, the mass extinction at the end of the Cretaceous (the K-Pg boundary event) was marked in the marine sedimentary record by unusually high amounts of charcoal. That is evidence for the Chixculub impact being accompanied by global wild fires that a high-oxygen atmosphere would have encouraged. The high oxygen levels of the Cretaceous marked the emergence of modern flowering plants – the angiosperms. Six British geoscientists have analysed the possible influence on the Earth System of this new and eventually dominant component of the terrestrial biosphere. (Belcher, C.M. et al. The rise of angiosperms strengthened fire feedbacks and improved the regulation of atmospheric oxygenNature Communications, v. 12, article 503; DOI 10.1038/s41467-020-20772-2)

The episodic occurrence of charcoal in sedimentary rocks bears witness to wildfires having affected terrestrial ecosystems since the decisive colonisation of the land by plants at the start of the Devonian 420 Ma ago. Fire and vegetation have since gone hand in hand, and the evolution of land plants has partly been through adaptations to burning. For instance the cones of some conifer species open only during wildfires to shed seeds following burning. Some angiosperm seeds, such as those of eucalyptus, germinate only after being subject to fire . The nature of wildfires varies according to particular ecosystems: needle-like foliage burns differently from angiosperm leaves; grassland fires differ from those in forests and so on. Massive fires on the Earth’s surface are not inevitable, however. Evidence for wildfires is absent during those times when the atmosphere’s oxygen content has dipped below an estimated 16%. The current oxygen level encourages fires in dry forest during drought, as those of Victoria in Australia and California in the US during 2020 amply demonstrated. It is possible that with oxygen above 25% dry forest would not regenerate without burning in the next dry season. Wet forest, as in Brazil and Indonesia, can burn under present conditions but only if set alight deliberately. Evidence of a global firestorm after the K-Pg extinction implies that tropical rain forest burns easily when oxygen is above 30%. So, how come the dominant flora of Earth’s huge tropical forests – the flowering angiosperms – evolved and hung on when conditions were ripe for them to burn on a massive scale?

Early angiosperms had small leaves suggesting small stature and growth in stands of open woodland [perhaps shrubberies] that favoured the fire protection of wetlands. ‘Weedy’ plants regenerate and reach maturity more quickly than do those species that are destined to produce tall trees. With endemic wildfires, tree-sized plants – e.g. the gymnosperms of the Mesozoic – cannot attain maturity by growing above the height of flames. Diminutive early angiosperms in a forest understory would probably outcompete their more ancient companions.  Yet to become the mighty trees of later rain forests angiosperms must somehow have regulated atmospheric oxygen so that it declined well below the level where wet forest is ravaged by natural wild fires. The oldest evidence for angiosperm rain forest dates to 59 Ma, when perhaps more primitive tropical trees had been almost wiped-out by wildfires. Did angiosperms also encourage wildfires, that consumed oxygen on a massive scale, as well as evolving to resist their affects on plant growth? Claire Belcher et al. suggest that they did, through series of evolutionary steps. Key to their stabilising oxygen levels at around 21%, the authors allege, was angiosperms’ suppression of weathering of phosphorus from rocks and/or transfer of that major nutrient from the land to the oceans. On land nitrogen is the most important nutrient for biomass, whereas phosphorus is the limiting factor in the ocean. Its reduction by angiosperm dominance on land thereby reduces carbon burial in ocean sediments. In a very roundabout way, therefore, angiosperms control the key factor in allowing atmospheric build-up of oxygen; by encouraging mass burning and suppressing carbon burial.  Today, about 84 percent of wildfires are started by anthropogenic activities. As yet we have little, if any, idea of how such disruption of the natural flora-fire system is going to affect future ecosystems. The ‘Pyrocene’ may be an outcome of the ‘Anthropocene’ …

Tsunami risk in East Africa

The 26 December 2004 Indian Ocean tsunami was one of the deadliest natural disasters since the start of the 20th century, with an estimated death toll of around 230 thousand. Millions more were deeply traumatised, bereft of homes and possessions, rendered short of food and clean water, and threatened by disease. Together with that launched onto the seaboard of eastern Japan by the Sendai earthquake of 11 March 2011, it has spurred research into detecting the signs of older tsunamis left in coastal sedimentary deposits (see for instance: Doggerland and the Storegga tsunami, December 2020). In normally quiet coastal areas these tsunamites commonly take the form of sand sheets interbedded with terrestrial sediments, such as peaty soils. On shores fully exposed to the ocean the evidence may take the form of jumbles of large boulders that could not have been moved by even the worst storm waves.

Sand sheets attributed to a succession of tsunamis, interbedded with peaty soils deposited in a swamp on Phra Thong Island, Thailand. Note that a sand sheet deposited by the 2004 Indian Ocean tsunami is directly beneath the current swamp surface (Credit: US Geological Survey)

Most of the deaths and damage wrought by the 2004 tsunami were along coasts bordering the Bay of Bengal in Indonesia, Thailand, Myanmar, India and Sri Lanka, and the Nicobar Islands. Tsunami waves were recorded on the coastlines of Somalia, Kenya and Tanzania, but had far lower amplitudes and energy so that fatalities – several hundred – were restricted to coastal Somalia. East Africa was protected to a large extent by the Indian subcontinent taking much of the wave energy released by the magnitude 9.1 to 9.3 earthquake (the third largest recorded) beneath Aceh at the northernmost tip of the Indonesian island of Sumatra. Yet the subduction zone that failed there extends far to the southeast along the Sunda Arc. Earthquakes further along that active island arc might potentially expose parts of East Africa to far higher wave energy, because of less protection by intervening land masses.

This possibility, together with the lack of any estimate of tsunami risk for East Africa, drew a multinational team of geoscientists to the estuary of the Pangani River  in Tanzania (Maselli, V. and 12 others 2020. A 1000-yr-old tsunami in the Indian Ocean points to greater risk for East Africa. Geology, v. 48, p. 808-813; DOI: 10.1130/G47257.1). Archaeologists had previously examined excavations for fish farming ponds and discovered the relics of an ancient coastal village. Digging further pits revealed a tell-tale sheet of sand in a sequence of alluvial sediments and peaty silts and fine sands derived from mangrove swamps. The peats contained archaeological remains – sherds of pottery and even beads. The tsunamite sand sheet occurs within the mangrove facies. It contains pebbles of bedrock that also litter the open shoreline of this part of Tanzania. There are also fossils; mainly a mix of marine molluscs and foraminifera with terrestrial rodents fish, birds and amphibians. But throughout the sheet, scattered at random, are human skeletons and disarticulated bones of male and female adults, and children. Many have broken limb bones, but show no signs of blunt-force trauma or disease pathology. Moreover, there is no sign of ritual burial or weaponry; the corpses had not resulted from massacre or epidemic. The most likely conclusion is that they are victims of an earlier Indian Ocean tsunami. Radiocarbon dating shows that it occurred at some time between the 11th and 13th centuries CE. This tallies with evidence from Thailand, Sumatra, the Andaman and Maldive Islands, India and Sri Lanka for a major tsunami in 950 CE.

Computer modelling of tsunami propagation reveals that the Pangani River lies on a stretch of the Tanzanian coast that is likely to have been sheltered from most Indian Ocean tsunamis by Madagascar and the shallows around the Seychelles Archipelago. Seismic events on the Sunda Arc or the lesser, Makran subduction zone of eastern Iran may not have been capable of generating sufficient energy to raise tsunami waves at the latitudes of the Tanzanian coast much higher than those witnessed there in 2004, unless their arrival coincided with high tide – damage was prevented in 2004 because of low tide levels. However, the topography of the Pangani estuary may well amplify water level by constricting a surge. Such a mechanism can account for variations of destruction during the 2011 Tohoku-Sendai tsunami in NE Japan.

If coastal Tanzania is at high risk of tsunamis, that can only be confirmed by deeper excavation into coastal sediments to check for multiple sand sheets that characterise areas closer to the Sunda Arc. So far, that in the Pangani estuary is the only one recorded in East Africa

Thawing permafrost, release of carbon and the role of iron

Projected shrinkage of permanently frozen ground i around the Arctic Ocean over the next 60 years

Global warming is clearly happening. The crucial question is ‘How bad can it get?’ Most pundits focus on the capacity of the globalised economy to cut carbon emissions – mainly CO2 from fossil fuel burning and methane emissions by commercial livestock herds. Can they be reduced in time to reverse the increase in global mean surface temperature that has already taken place and those that lie ahead? Every now and then there is mention of the importance of natural means of drawing down greenhouse gases: plant more trees; preserve and encourage wetlands and their accumulation of peat and so on. For several months of the Northern Hemisphere summer the planet’s largest bogs actively sequester carbon in the form of dead vegetation. For the rest of the year they are frozen stiff. Muskeg and tundra form a band across the alluvial plains of great rivers that drain North America and Eurasia towards the Arctic Ocean. The seasonal bogs lie above sediments deposited in earlier river basins and swamps that have remained permanently frozen since the last glacial period. Such permafrost begins at just a few metres below the surface at high latitudes down to as much as a kilometre, becoming deeper, thinner and more patchy until it disappears south of about 60°N except in mountainous areas. Permafrost is melting relentlessly, sometimes with spectacular results broadly known as thermokarst that involves surface collapse, mudslides and erosion by summer meltwater.

Thawing permafrost in Siberia and associated collapse structures

Permafrost is a good preserver of organic material, as shown by the almost perfect remains of mammoths and other animals that have been found where rivers have eroded their frozen banks. The latest spectacular find is a mummified wolf pup unearthed by a gold prospector from 57 ka-old permafrost in the Yukon, Canada. She was probably buried when a wolf den collapsed. Thawing exposes buried carbonaceous material to processes that release CO, as does the drying-out of peat in more temperate climes. It has long been known that the vast reserves of carbon preserved in frozen ground and in gas hydrate in sea-floor sediments present an immense danger of accelerated greenhouse conditions should permafrost thaw quickly and deep seawater heats up; the first is certainly starting to happen in boreal North America and Eurasia. Research into Arctic soils had suggested that there is a potential mitigating factor. Iron-3 oxides and hydroxides, the colorants of soils that overlie permafrost, have chemical properties that allow them to trap carbon, in much the same way that they trap arsenic by adsorption on the surface of their molecular structure (see: Screening for arsenic contamination, September 2008).

But, as in the case of arsenic, mineralogical trapping of carbon and its protection from oxidation to CO2 can be thwarted by bacterial action (Patzner, M.S. and 10 others 2020. Iron mineral dissolution releases iron and associated organic carbon during permafrost thaw. Nature Communications, v. 11, article 6329; DOI: 10.1038/s41467-020-20102-6). Monique Patzner of the University of Tuebingen, Germany, and her colleagues from Germany, Denmark, the UK and the US have studied peaty soils overlying permafrost in Sweden that occurs north of the Arctic Circle. Their mineralogical and biological findings came from cores driven through the different layers above deep permafrost. In the layer immediately above permanently frozen ground the binding of carbon to iron-3 minerals certainly does occur. However, at higher levels that show evidence of longer periods of thawing there is an increase of reduced iron-2 dissolved in the soil water along with more dissolved organic carbon – i.e. carbon prone to oxidation to carbon dioxide. Also, biogenic methane – a more powerful greenhouse gas – increases in the more waterlogged upper sediments. Among the active bacteria are varieties whose metabolism involves the reduction of insoluble iron in ferric oxyhdroxide minerals to the soluble ferrous form (iron-2). As in the case of arsenic contamination of groundwater, the adsorbed contents of iron oxyhydroxides are being released as a result of powerful reducing conditions.

Applying their results to the entire permafrost inventory at high northern latitudes, the team predicts a worrying scenario. Initial thawing can indeed lock-in up to tens of billion tonnes of carbon once preserved in permafrost, yet this amounts to only a fifth of the carbon present in the surface-to-permafrost layer of thawing, at best. In itself, the trapped carbon is equivalent to between 2 to 5 times the annual anthropogenic release of carbon by burning fossil fuels. Nevertheless, it is destined by reductive dissolution of its host minerals to be emitted eventually, if thawing continues. This adds to the even vaster potential releases of greenhouse gases in the form of biogenic methane from waterlogged ground. However, there is some evidence to the contrary. During the deglaciation between 15 to 8 thousand years ago – except for the thousand years of the Younger Dryas cold episode – land-surface temperatures rose far more rapidly than happening at present. A study of carbon isotopes in air trapped as bubbles in Antarctic ice suggests that methane emissions from organic carbon exposed to bacterial action by thawing permafrost were much lower than claimed by Patzner et al. for present-day, slower thawing (see: Old carbon reservoirs unlikely to cause massive greenhouse gas release, study finds. Science Daily, 20 February 2020) – as were those released by breakdown of submarine gas hydrates.

Doggerland and the Storegga tsunami

Britain is only an island when sea level stands high; i.e. during interglacial conditions. Since the last ice age global sea level have risen by about 130 m as the great northern ice sheets slowly melted. That Britain could oscillate between being part of Europe and a large archipelago as a result of major climatic cycles dates back only to between 450 and 240 ka ago. Previously it was a permanent part of what is now Europe, as befits its geological identity, joined to it by a low ridge buttressed by Chalk across the Dover Strait/Pas de Calais. All that remains of that are the white cliffs on either side. The drainage of what became the Thames, Seine and Rhine passed to the Atlantic in a much larger rive system that flowed down the axis of the Channel. Each time an ice age ended the ridge acted as a dam for glacial meltwater to form a large lake in what is now the southern North Sea. While continuous glaciers across the northern North Sea persisted the lake remained, but erosion during interglacials steadily wore down the ridge. About 450 ka ago it was low enough for this pro-glacial lake to spill across it in a catastrophic flood that began the separation. Several repeats occurred until the ridge was finally breached (See: When Britain first left Europe; September 2007). Yet sufficient remained that the link reappeared when sea level fell. What remains at present is a system of shallows and sandbanks, the largest of which is the Dogger Bank roughly halfway between Newcastle and Denmark. Consequently the swamps and river systems that immediately followed the last ice age have become known collectively as Doggerland.

The shrinkage of Doggerland since 16,000 BCE (Credit: Europe’s Lost Frontiers Project, University of Bradford)

Dredging of the southern North Sea for sand and gravel frequently brings both the bones of land mammals and the tools of Stone Age hunters to light – one fossil was a skull fragment of a Neanderthal. At the end of the Younger Dryas (~11.7 ka) Doggerland was populated and became a route for Mesolithic hunter-gatherers to cross from Europe to Britain and become transient and then permanent inhabitants. Melting of the northern ice sheets was slow and so was the pace of sea-level rise. A continuous passage across Dogger Land  remained even as it shrank. Only when the sea surface reached about 20 m below its current level was the land corridor breached bay what is now the Dover Strait, although low islands, including the Dogger Bank, littered the growing seaway. A new study examines the fate of Doggerland and its people during its final stage (Walker, J. et al. 2020. A great wave: the Storegga tsunami and the end of Doggerland? Antiquity, v. 94, p. 1409-1425; DOI: 10.15184/aqy.2020.49).

James Walker and colleagues at the University of Bradford, UK, and co-workers from the universities of Tartu, Estonia, Wales Trinity Saint David and St Andrews, UK, focus on one devastating event during Doggerland’s slow shrinkage and inundation. This took place around 8.2 ka ago, during the collapse of a section of the Norwegian continental edge. Known as the Storegga Slides (storegga means great edge in Norse), three submarine debris flows shifted 3500 km3 of sediment to blanket 80 thousand km2 of the Norwegian Sea floor, reaching more than half way to Iceland.  Tsunami deposits related to these events occur along the coast western Norway, on the Shetlands and the shoreline of eastern Scotland. They lie between 3 and 20 m above modern sea level, but allowing for the lower sea level at the time the ‘run-up’ probably reached as high as 35 m: more than the maximum of both the 26 December 2004 Indian Ocean tsunami and that in NW Japan on 11 March 2011. Two Mesolithic archaeological sites definitely lie beneath the tsunami deposit, one close to the source of the slid, another near Inverness, Scotland. At the time part of the Dogger Bank still lay above the sea, as did a wide coastal plain and offshore islands along England’s east coast. This catastrophic event was a little later than a sudden cooling event in the Northern Hemisphere. Any Mesolithic people living on what was left of Doggerland would not have survived. But quite possibly they may already have left as the climate cooled substantially

A seabed drilling programme financed by the EU targeted what lies beneath more recent sediments on the Dogger Bank and off the embayment known as The Wash of Eastern England. Some of the cores contain tsunamis deposits, one having been analysed in detail in a separate paper (Gaffney, V. and 24 others 2020. Multi-Proxy Characterisation of the Storegga Tsunami and Its Impact on the Early Holocene Landscapes of the Southern North Sea. Geosciences, v. 10, online; DOI: 10.3390/geosciences10070270). The tsunami washed across an estuarine mudflat into an area of meadowland with oak and hazel woodland, which may have absorbed much of its energy. Environmental DNA analysis suggests that this relic of Doggerland was roamed by bear, wild boar and ruminants. The authors also found evidence that the tsunamis had been guided by pre-existing topography, such as the river channel of what is now the River Great Ouse. Yet they found no evidence of human occupation. Together with other researchers, the University of Bradford’s Lost Frontiers Project have produced sufficient detail about Doggerland to contemplate looking for Mesolithic sites in the excavations for offshore wind farms.

See also: Addley, E. 2020.  Study finds indications of life on Doggerland after devastating tsunamis. (The Guardian, 1 December 2020); Europe’s Lost Frontiers website

Human impact on surface geological processes

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

‘Badlands’ formed by accelerated soil erosion.

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

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

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

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

Balanced boulders and seismic hazard

The seismometer invented by early Chinese engineer Zhang Heng

China has been plagued by natural disasters since the earliest historical writings. Devastating earthquakes have been a particular menace, the first recorded having occurred in 780 BC . During the Han dynasty in 132 CE, polymath Zhang Heng invented an ‘instrument for measuring the seasonal winds and the movements of the Earth’ (Houfeng Didong Yi, for short): the first seismometer. A pendulum mechanism in a large bronze jar activated one of eight dragons corresponding to the eight cardinal and intermediate compass directions (N, NE, E etc.) so that a bronze ball dropped from its mouth to be caught by a corresponding bronze toad. The device took advantage of unstable equilibrium in which a small disturbance will produce a large change: akin to a pencil balanced on its unsharpened end. Modern seismometers exploit the same basic principle of amplification of small motions. The natural world is also full of examples of unstable equilibrium, often the outcome of chemical and physical weathering. Examples are slope instability, materials that are on the brink of changing properties from those of a solid to a liquid state (thixotropic materials – see: Mud, mud, glorious mud August 2020) and rocks in which stress has built almost to the point of brittle failure: earthquakes themselves. But there are natural curiosities that not only express unstable equilibrium but have maintained it long enough to become … curious! Perched boulders, such as glacial erratics and the relics of slow erosion and weathering, are good examples. Seismicity could easily topple them, so that their continued presence signifies that large enough tremors haven’t yet happened.

A precarious boulder in coastal central California (credit: Anna Rood & Dylan Rood, Imperial College London)

Now it has become possible to judge how long their delicate existence has persisted, giving a clue to the long-term seismicity and thus the likely hazard in their vicinity (Rood, A.H. and 10 others 2020. Earthquake Hazard Uncertainties Improved Using Precariously Balanced Rocks. American Geological Union Advances, v. 1, ePDF e2020AV000182; DOI: 10.1029/2020AV000182). Anna Rood and her partner Dylan of Imperial College London, with colleagues from New Zealand, the US and Australia, found seven delicately balanced large boulders of silica-rich sedimentary rock in seismically active, coastal California. They had clearly withstood earthquake ground motions for some time. Using multiple photographs to produce accurate digital 3D renditions and modelling of resistance to shaking and rocking motions, the authors determined each precarious rock’s probable susceptibility to toppling as a result of earthquakes. How long each had withstood tectonic activity shows up from the mass-spectrometric determination of beryllium-10 isotopes produced by cosmic-ray bombardment of the outer layer. Comparing its surface abundance relative to that in the rock’s interior indicates the time since the boulders’ first exposure to cosmic rays. With allowance for former support from surrounding blocks, this gives a useful measure of the survival time of each boulder – its ‘fragility age’.

The boulder data provide a useful means of reducing the uncertainties inherent in conventional seismic hazard assessment, which are based on estimates of the frequency of seismic activity, the magnitude of historic ‘quakes, in most cases over the last few hundred years, and the underlying geology and tectonics. In the study area (near a coastal nuclear power station) the data have narrowed uncertainty down to almost a half that in existing risk models. Moreover, they establish that the highest-magnitude earthquakes to be expected every 10 thousand years (the ‘worst case scenario’) were 27% less than otherwise estimated. This is especially useful for coastal California, where the most threatening faults lie off shore and are less amenable to geological investigation.

See also:  Strange precariously balanced rocks provide earthquake forecasting clues. (SciTech Daily; 1 October 2020) 

Monitoring ground motions with satellite radar

By using artificially generated microwaves to illuminate the Earth’s surface it is possible to create images. The technology and the theory behind this radar imaging are formidable. After about 30 years of development using aircraft-mounted transmission and reception antennas, the first high resolution images from space were produced in the late 1970s. Successive experiments improved and expanded the techniques, and for the last decade radar surveillance has been routine from a number of orbiting platforms. Radar has two advantages over optical remote sensing: being an active system it can be done equally effectively day or night; it also penetrates cloud cover, which is almost completely transparent to microwaves with wavelengths between a centimetre and a metre. The images are very different from those produced by visible or infrared radiation, the energy returns from the surface being controlled by topography and the roughness of the surface. One of many complicating factors is that images can only be produced by oblique illumination.  That, together with deployment of widely separated transmission and reception antennas, opens up the possibility of extracting very-high precision (millimetre) measurements of topographic elevation.

In 1992 radar data from two overpasses of the European ERS-1 satellite over California were processed to capture interference due to changes in the ground elevation during the time between the two orbits: the first interferometric radar or InSAR. It revealed the regional ground motions that resulted from the magnitude 7.3 Landers earthquake at 4:57 am local time on June 28, 1992. For the last decade InSAR has become a routine tool to monitor globally both lateral and vertical ground movements, whether rapid, as in earthquakes, or slow in the case of continental plate motions, subsidence or the inflation of volcanoes prior to eruptions. Juliet Biggs and Tim Wright, respectively of the Universities of Bristol and Leeds, UK, have summarised InSAR’s potential (Biggs, J. & Wright, T.J. 2020. How satellite InSAR has grown from opportunistic science to routine monitoring over the last decade. Nature Communications, v. 11, p. 1-4; DOI: 10.1038/s41467-020-17587-6).

Ground motions associated with the 2016 Kaiköuea earthquake on the South Island of New Zealand. Each colour fringe represents 11.4 cm of displacement in the radar line-of-sight (LOS) direction. Known faults are shown as thick black lines (Credit: Hamling et al. 2017. Complex multifault rupture during the 2016 Mw 7.8 Kaikōura earthquake, New Zealand. Science, v. 356, article eaam7194; DOI: 10.1126/science.aam7194)

Since the ERS-1 satellite discovered the ground motions associated with the Landers earthquake, InSAR has covered more than 130 large seismic events. Although the data post-dated the damage, they have demonstrated the particular mechanics of each earthquake, allowing theoretical models to be tested and refined. In the image above it is clear that the motions were not associated with a single fault in New Zealand: the Kaikoura earthquake involved a whole network of them, at least at the surface. Probably, displacement jumped from one to another; a complexity that must be taken into account for future events on such notorious fault systems as those in densely populated parts of California and Turkey.

East to west speed of the Anatolian micro-plate south of the North Anatolian Fault derived from the first five years of the EU’s Sentinel-1 InSAR constellation. Major known faults shown by black lines (Credit: Emre, O. et al. 2018. Active fault database of Turkey. Bulletin of Earthquake Engineering, v. 16, p. 3229-3275; DOI: 10.1007/s10518-016-0041-2)

Since its inception, GPS has proved capable of monitoring tectonic motions over a number of years, but only for widely spaced, individual ground instruments. Using InSAR alongside years’ worth of GPS measurements helps to extend detected motions to much finer resolution, as the image above shows for Asiatic Turkey. An important parameter needed for prediction of earthquakes is the way in which crustal strain builds up in regions with dangerously active fault systems.

InSAR image of the Sierra Negra volcano on Isabela Island in the Galapagos Archipelago, at the time of a magma body intruding its flanks. Each colour fringe represents 2.8 cm of subsidence in the LOS direction (Credit: Anantrasirichai, N. et al. 2019. A deep learning approach to detecting volcano deformation from satellite imagery using synthetic datasets. Remote Sensing of Environment, v. 230, article 111179; DOI: 10.1016/j.rse.2019.04.032)

Volcanism obviously involves the movement of large masses of magma beneath the surface before eruptions. GPS and micro-gravity measurements show that charging of a magma chamber causes volcanoes to inflate so InSAR provides a welcome means of detecting the associated uplift, even if it only a few centimetres, as show by the example above from the Galapagos Islands. A volcano’s flanks may bulge, which could presage a lateral eruption or a pyroclastic flow such as that at Mount St Helens in 1980. Truly vast eruptions are associated with calderas whose ring faults may cause collapse in advance.

The presence of cavities beneath the surface, formed by natural solution of limestones, deliberately as in extraction of brines from salt deposits or after subsurface mining, present subsidence hazards. There have been several series of alarming TV programmes about sinkhole formation that demonstrate sudden collapse. Yet every case will have been preceded by years of gradual sagging. InSAR allows risky areas to be identified well in advance of major problems. Indeed estate agents (realtors) as well as planners, civil engineers and insurers form a ready market for such survey.

Natural sparkling water and seismicity

For all manner of reasons, natural springs have fascinated people since at least as long ago as the Neolithic. Just the fact that clear water emerges from the ground to source streams and great rivers seems miraculous. There are many occurrences of offerings having been made to supernatural spirits thought to guard springs. Even today many cannot resist tossing in a coin, hanging up a ring, necklace or strip of cloth beside a spring, for luck if nothing else. Hot springs obviously attract attention and bathers. Water from cool ones has been supposed to have health-giving properties for at least a couple of centuries, even if they stink of rotten eggs or precipitate yellow-brown iron hydroxide slime in the bottom of your cup. Spas now attribute their efficacy to their waters’ chemistry, and that depends on the rocks through which they have passed. Those in areas of volcanic rock are generally the most geochemically diverse: remember the cringe-making adverts for Volvic from the volcanic Chain des Puys in the French Auvergne. Far more ‘posh’ are naturally carbonated waters that well-out full of fizz from pressurised, dissolved CO2. Internationally the best known of these is Perrier from the limestone-dominated Gard region of southern France. Sales of bottled spring waters are booming and the obligatory water-chemistry data printed on their labels form  a do-it-yourself means of regional geochemical mapping (Dinelli, E. et al. 2010. Hydrogeochemical analysis on Italian bottled mineral waters: Effects of geology. Journal of Geochemical Exploration, v. 107, p. 317–335; DOI: 10.1016/j.gexplo.2010.06.004) But it appears from a study of variations in CO2 output from commercial springs in Italy that they may also help in earthquake prediction (Chiodini, G. et al. 2020. Correlation between tectonic CO2 Earth degassing and seismicity is revealed by a 10-year record in the Apennines, Italy. Science Advances, v. 6, article eabc2938; DOI: 10.1126/sciadv.abc2938).

Italy produces over 12 billion litres of spring water and the average Italian drinks 200 litres of it every year. There are more than 600 separate brands of acqua minerale produced in Italy, including acqua gassata (sparkling water). Even non-carbonated springs emit CO2, so it is possible to monitor its emission from the deep Earth across wide tracts of the country. High CO2 emissions are correlated worldwide with areas of seismicity, either associated with shallow magma chambers or to degassing from subduction zones. There are two possibilities: that earthquakes help release built-up fluid pressure or because fluids, such as CO2 somehow affect rock strength. Giovanni Chiodini and colleagues have been monitoring variations in CO2 release from carbonated spring water in the Italian Apennines since 2009. Over a ten-year period there have been repeated earthquakes in the area, including three of magnitude 6.0 or greater. The worst was that affecting L’Aquila in April 2009, the aftermath of which saw six geoscientists charged with – and eventually acquitted of – multiple manslaughter (see: Una parodia della giustizia?, October 2012). It was this tragedy that prompted Chiodini et al.’s unique programme of 21 repeated sampling of gas discharge rates at 36 springs, matched to continuous seismograph records. The year after the L’Aquila earthquake coincided with high emissions, which then fell to about half the maximum level by 2013. In 2015 emissions began to rise to reach a peak before earthquakes with almost the same magnitude, but less devastation, on 24 August and 30 October 2016. Thereafter emissions fell once again. This suggests a linked cycle, which the authors suggest is modulated by ascent of CO2 that originates from the melting of carbonates along the subduction zone that dips beneath central Italy. They suggest that gas accumulates in the lower crust and builds up pressure that is able to trigger earthquakes in the crust.

The variation in average emissions across central Italy (see figure above) suggests that there are two major routes for degassing from the subduction zone, perhaps focussed by fractures generated by previous crustal tectonic movements. In my opinion, this study does not prove a causal link, although that is a distinct possibility, which may be verified by extending this survey of degassing and starting similar programmes in other seismically active areas. Whether or not it might become a predictive tool depends on further work. However, other studies, particularly in China, show that other phenomena associated with groundwater in earthquake-prone areas, such as rise in well-water levels and an increase in their emissions of radon and methane, correlate in a similar manner.

‘Mud, mud, glorious mud’

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

Mud carnival in Brazil (Credit: africanews.com)

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

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

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

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