Geothermal heat from coalfields: a ‘green’ revolution?

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

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

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

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

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

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

Is there a future for coal?

Burning coal is far and away the main culprit for elevated atmospheric CO2 and global climatic warming. That is a consequence of the Industrial Revolution and a mode of production that centres on the market value of commodities rather than their intrinsic usefulness and thus on the continual generation of profit. As the main original energy source for this capitalist mode, whose viability depends on incessant economic growth, world coal production grew at an exponential rate through the 19th and 20th centuries, as did its influence on global climate. Beginning in the 1920s, coal was joined by other carbon-based fossil fuels (oil and natural gas) in massively increasing energy and greenhouse-gas production. In the data there is little sign of an appetite to reduce this dependence on carbon burning, renewable energy output accounting today for only a few percent of the energy demands of capital. Although coal’s energy contribution is flattening off to be replaced by those of oil and gas, in terms of CO2 emissions it still dominates. That is because oil and gas are carbon-hydrogen compounds rather than almost pure carbon in the case of coal. Unless burning fossil fuels is outlawed and economic growth is drastically curtailed, we are set to live on a far warmer planet.

Growth in energy supply from different sources since 1800 CE (Credit: ourworldindata.org)

For various reasons coal is unique in a social sense. Every producing area has large communities who depend on mining for income. Britain now produces vastly less coal than it did up to the 1990s, with wholesale closure of underground mines. Since 1960 these communities have lost over half a million jobs in mining, let alone those in related industries and those that served mining communities. Three decades on from the last round of closures, those communities remain socially devastated in many respects. A huge amount of coal still lies beneath them. Can it make a come-back? A recent study suggests that perhaps it can, with diametrically opposite environmental consequences.

It seems that surfaces of coal particles are able to take-up and store gases, increasingly so as pressure increases. Miners faced the consequences of that in the form of adsorbed carbon dioxide and methane (‘choke damp’ and ‘fire damp’) that were released when coal was mined underground. Miners’ safety lamps (invented in 1815 by Humphry Davy) enable them to monitor the risks of suffocation by CO2 or explosion of CH4 when the lamps dim or flare, respectively. Engineers at Edith Cowan University in Perth, Western Australia, experimentally measured the amounts of hydrogen adsorbed by crushed coal at different pressures (Iglauer, S et al. 2021. Hydrogen adsorption on sub-bituminous coal: Implications for hydrogen geostorage. Geophysical Research Letters, v. 48, article 2021GL092976; DOI: 10.1029/2021GL092976). From surface atmospheric pressure to 40 times that, adsorption increases rapidly, especially for hydrogen (from 0.05 to 0.25 grams per kilogram). At higher pressures it rises less rapidly to about 0.6 g kg-1 at 100 times atmospheric pressure, which is equivalent to a depth of about 500 m in a sedimentary rock formation. At deeper levels hydrogen adsorption remains about the same.

The experimental results suggest that large amounts of hydrogen can be stored in coal at quite shallow depths. The potential storage in a ton of fractured coal is about 600 kg, equivalent to about 12 cubic metres of liquid hydrogen, but without the need for containment and refrigeration. In the absence of oxygen, such storage would be safe and long-term. If feasible from an engineering standpoint, underground storage of hydrogen in coal seams to overcome one of the current barriers to a hydrogen-based industrial economy through the storage of energy generated by carbon-free technologies, such as wind, wave, tidal and solar generation that operate at highly variable rates, not suited to energy use patterns. Effectively, coalfields could become giant ‘batteries’ without the need to mine vast amounts of elements, such as lithium, needed for conventional batteries; provided that a sustainable means of repeated hydrogen recovery can be devised. A central technology of a future ‘hydrogen economy’ is that of the fuel cell in which hydrogen and oxygen combine using a catalyst to generate electricity, without any combustion and emitting only water vapour.

‘Green’ metal mining?

A glance at statistics for the global consumption of any particular metal reveals much about the current unfairness of the world we live in. On a per capita basis, people in the developed, rich world use vastly more than do those in the less developed countries, on average. It is commonly said that in order for everyone to live in a fair world, the poor need more metals and other physical resources in order to match the living standards of the rich, or the wealthy will need to consume much less. A new factor in the equitability equation is the necessity to stave off CO2-induced global warming, largely through replacing energy from fossil fuels with that produced by a variety of ‘green’ sources. That carries with it another issue; the technologies for carbon-free energy generation, transmission, storage and use will consume a broad range of metals and other physical resources. These include cobalt and lithium, graphite, rare-earth elements and especially copper, whose annual production is set to soar.

Copper is particularly critical. If China alone fulfils its planned production of all-electric transportation, the demand for copper will over 2 billion tonnes requiring 119 years production at the current extraction rate of 20 Mt per year. The rush to electric cars has already forced copper demand above production, resulting in soaring prices on the world market. In the last year they have doubled, reaching US$10,000 per ton at the time ofwriting. Most metals are won by digging up their ores, often from considerable depths in the crust. Ores then have to be concentrated, smelted and the elemental metals refined. About 6 % of global energy is consumed by this process, adding CO2 and a variety of noxious gases to the atmosphere, let alone the stupendous amounts of uneconomic waste rock and polluted water. Copper is high up the list for environmental impact, being extracted from some of the world biggest mines. Like all physical resources, its extraction cannot be continued without further environmental deterioration. But is there a more sustainable way of extracting metals from the Earth?

Bingham Canyon copper mine in Utah, USA; at 4.5 km diameter and 1.2 km depth it is the world’s largest excavation. (Credit: Mining Magazine)

Under the right chemical conditions many metals can be dissolved, so longs as fluids can pass through the ore. One example is the use of sodium cyanide solution (known as a lixiviant) to dissolve gold from low-grade ore: so-called ‘heap leaching’. But this is done at the surface, either using newly crushed ore from an excavation or waste from earlier mining that could not extract fine-grained gold. A similar approach uses bacteria whose metabolism involves oxidation of sulfide ore minerals, resulting in chemical reactions that liberate their desirable metal content to solution in water. If buried orebodies are fractured in situ this kind of leaching will supposedly transform metal production, in an analogous fashion to fracking for gas and oil. Like fracking, current operations that involve both forms of hydrometallurgy generate highly toxic fluids, and in many cases extract only a fraction of the target metal. But a novel alternative has just emerged, which involves leaching based on electrical means (Martens, E. and 9 others 2021. Toward a more sustainable mining future with electrokinetic in situ leaching. Science Advances, v. 7, article eabf9971; DOI: 10.1126/sciadv.abf9971). It isn’t totally new, for it uses the same chemistry as in heap leaching. However, it does not involve shattering the orebody at depth. Instead, low-voltage currents are passed through the orebody which induce a lixiviant to migrate through the rock, along mineral-grain boundaries rather than through fractures. Fluid movement becomes more efficient over time as the host rock is artificially ‘weathered’ thereby making it permeable. In effect, electrokinetic leaching creates a kind of hydrothermal system in reverse, by replacing the chemically reducing conditions of ore deposition with oxidising dissolution and transportation.

So far, the method has only been demonstrated through a small-scale test of concept using drill core samples of ore from a copper mine. Tests over a few days consumed more than half the grains of a copper ore mineral (chalcopyrite) present in the ore sample. So, it seems to work and astonishingly rapidly too. No doubt metal-mining companies, who are currently coining it hand-over-fist during a boom in metal prices, will beat a path to the doors of the team of researchers. But is it an economic proposition? They authors will soon find out … More important, if it is deployed widely will it increase the sustainability of metal mining? At first glance, yes: by removing the need for excavation of ore, liberation of ore-mineral grains by milling and their separation from valueless waste and many other aspects of beneficiation at the surface. Yet, the bottom line is that mining companies deploy their capital not so much to make ingots of useful metal but primarily to yield profits.  Speeding up metal extraction and thereby its supply to the world market could drive down the price that they can get for each tonne. Perversely, it is perceived shortages on metals and the resulting inflation of price that really yield bonanzas. My guess is that the industry will continue mining in the present manner, with all its lack of sustainability and environmental impact, for that very reason. The real way to reduce damage is to reduce demand for metals: do people in general really need more of them and the goods in which they are bound up in such vast amounts?

Indian groundwater shortage threatens food production

Farmers in India have been engaged in mass protests since September 2020. Their anger is directed at a series of laws introduced by the central government of Narendra Modi’s  Bharatiya Janata Party (BJP) that change farmers’ terms of trade. Agriculture in India also faces a future of reduced availability of groundwater on which farmers have become increasingly dependent, especially in the vast alluvial plains of the Ganges river system. The twin satellites of the Gravity Recovery  and Climate Experiment (GRACE), which chart changes in mass beneath the Earth’s surface, detected a major change in gravity over 3 million km2 of India’s largest area of agriculture in the northwestern Gangetic plains (Rodell, M. et al. 2009. Satellite-based estimates of groundwater depletion in India. Nature, v. 460, p.999-1002; DOI: 10.1038/nature08238). The data suggested a loss between 2002 and 2008 of around 109 cubic kilometres of water from the aquifers that support regional irrigation and the livelihoods of about 114 million people (see NASA summary). The loss of water and decline in well-water levels have continued since then.

Colour-coded GRACE data  from 2002 to 2008 showing the estimated drawdown in water levels in wells in NW India and NE Pakistan during this period. Green to dark-red colours indicate from 0 to 12 metres of decline (credit: Trent Schindler and Matt Rodell, NASA)

A recent comprehensive survey (Jain, M. and 8 others 2021. Groundwater depletion will reduce cropping intensity in India. Science Advances, v. 9, article eabd2849; DOI: 10.1126/sciadv.abd2849) uses satellite image and census data to document the actual changes in winter crops (those most dependent on irrigation) over the period 2001 to 2012. It roughly measures the realities of the unsustainable extraction of groundwater indicated by GRACE from 2002 to 2008. The study projects an average reduction of 20% in winter cropping across the whole of India, with some of the worst-hit areas being likely to experience a 68% loss. The dominant supplies of irrigation water are from countless tube wells and systems of canals supplied by dams or rivers. India has witnessed impressive gains in food production in the last half century, thanks to rapid and continuing growth in the number of tube wells driven by individual farmers. The livelihoods of about 600 million people depend on agriculture. There is no prospect of substituting either form of irrigation to maintain current levels of production. If increased canal supply was used to replace well water and reduce groundwater depletion, cropping intensity would still decline, albeit at about half the projected rate; however, that doesn’t take into account unpredictable droughts in surface water accumulation and movement.

Faced with this situation, it is hardly surprising that farmers fear for their families future and react massively to state intervention in their marketing and crop storage strategies.

For a wider context to the Indian agricultural crisis see also: The ecological roots of India’s farming crisis (Deutche Welle, 1 February, 2021)

Diamonds and the deep carbon cycle

When considering the fate of the element carbon and CO2, together with all their climatic connotations, it is easy to forget that they may end up back in the Earth’s mantle from which they once escaped to the surface. In fact all geochemical cycles involve rock, so that elements may find their way into the deep Earth through subduction, and they could eventually come out again: the ‘logic’ of plate tectonics. Teasing out the various routes by which carbon might get to mantle is not so easily achieved. Yet one of the ways it escapes is through the strange magma that once produced kimberlite intrusions, in the form of pure-carbon crystals of diamond that kimberlites contain. A variety of petrological and geochemical techniques, some hinging on other minerals that occur as inclusions, has allowed mineralogists to figure out that diamonds may form at depths greater than about 150 km. Most diamonds of gem quality formed in unusually thick lithosphere beneath the stable, and relatively cool blocks of ancient continental crust known as cratons, which extends to about 250 km. But there are a few that reflect formation depths as great as 800 km that span two major discontinuities in the mantle (at 410 and 660 km depth). These transition zones are marked by sudden changes in seismic speed due to pressure-induced transformations in the structure and density of the main mantle mineral, olivine.

Diamond crystal containing a garnet and other inclusions (Credit: Stephen Richardson, University of Cape Town, South Africa)

Carbon-rich rocks that may be subducted are not restricted to limestones and carbon-rich mudstones. Far greater in mass are the basalts of oceanic crust. Not especially rich in carbon when they crystallised as igneous rocks, their progress away from oceanic spreading centres exposes them to infiltration by ocean water. Once heated, aqueous fluids cause basalts to be hydrothermally altered. Anhydrous feldspars, pyroxenes and olivines react with the fluids to break down to hydrated-silicate clays and dissolved metals. Dissolved carbon dioxide combines with released calcium and magnesium to form pervasive carbonate minerals, often occupying networks of veins. So there has been considerable dispute as to whether subducted sediments or igneous rocks of the oceanic crust are the main source of diamonds. Diamonds with gem potential form only a small proportion of recovered diamonds. Most are only saleable for industrial uses as the ultimate natural abrasive and so are cheaply available for research. This now centres on the isotopic chemistry of carbon and nitrogen in the diamonds themselves and the various depth-indicating silicate minerals that occur in them as minute inclusions, most useful being various types of garnet.

The depletion of diamonds in ‘heavy’ 13C once seemed to match that of carbonaceous shales and the carbonates in fossil shells, but recent data from carbonates in oceanic basalts reveals similar carbon, giving three possibilities. Yet, when their nitrogen-isotope characteristics are taken into account, even diamonds that formed at lithospheric depths do not support a sedimentary source (Regier, M.E. et al. 2020. The lithospheric-to-lower-mantle carbon cycle recorded in superdeep diamonds. Nature, v. 585, p. 234–238; DOI: 10.1038/s41586-020-2676-z). That leaves secondary carbonates in subducted oceanic basalts as the most likely option, the nitrogen isotopes more reminiscent of clays formed from igneous minerals by hydrothermal processes than those created by weathering and sedimentary deposition. However, diamonds with the deepest origins – below the 660 km mantle transition zone – suggest yet another possibility, from the oxygen isotopes of their inclusions combined with those of C and N in the diamonds. All three have tightly constrained values that most resemble those from pristine mantle that has had no interaction with crustal rocks. At such depths, unaltered mantle probably contains carbon in the form of metal alloys and carbides. Regier and colleagues suggest that subducted slabs reaching this environment – the lower mantle – may release watery fluids that mobilise carbon from such alloys to form diamonds. So, I suppose, such ultra-deep diamonds may be formed from the original stellar stuff that accreted to form the Earth and never since saw the ‘light of day’.

‘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.

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.

Sedimentary deposits of the ‘Anthropocene’

Economic activity since the Industrial Revolution has dug up rock – ores, aggregate, building materials and coal. Holes in the ground are a signature of late-Modern humanity, even the 18th century borrow pits along the rural, single-track road that passes the hamlet where I live. Construction of every canal, railway, road, housing development, industrial estate and land reclaimed from swamps and sea during the last two and a half centuries involved earth and rock being pushed around to level their routes and sites. The world’s biggest machine, aside from CERN’s Large Hadron Collider near Geneva, is Hitachi’s Bertha the tunnel borer (33,000 t) currently driving tunnels for Seattle’s underground rapid transit system. But the record muck shifter is the 14,200 t MAN TAKRAF RB293 capable of moving about 220,000 t of sediment per day, currently in a German lignite mine. The scale of humans as geological agents has grown exponentially. We produce sedimentary sequences, but ones with structures that are very different from those in natural strata. In Britain alone the accumulation of excavated and shifted material has an estimated volume six times that of our largest natural feature, Ben Nevis in NW Scotland. On a global scale 57 billion t of rock and soil is moved annually, compared with the 22 billion t transported by all the world’s rivers. Humans have certainly left their mark in the geological record, even if we manage to reverse  terrestrial rapacity and stave off the social and natural collapse that now pose a major threat to our home planet.

A self propelled MAN TAKRAF bucketwheel excavator (Bagger 293) crossing a road in Germany to get from one lignite mine to another. (Credit: u/loerez, Reddit)

The holes in the ground have become a major physical resource, generating substantial profit for their owners from their infilling with waste of all kinds, dominated by domestic refuse. Unsurprisingly, large holes have become a dwindling resource in the same manner as metal ores. Yet these stupendous dumps contain a great deal of metals and other potentially useful material awaiting recovery in the eventuality that doing so would yield a profit, which presently seems a remote prospect. Such infill also poses environmental threats simply from its composition which is totally alien compared with common rock and sediment. Three types of infill common in the Netherlands, of which everyone is aware, have recently been assessed (Dijkstra, J.J. et al. 2019. The geological significance of novel anthropogenic materials: Deposits of industrial waste and by-products. Anthropocene, v. 28, Article 100229; DOI: 10.1016/j.ancene.2019.100229). These are: ash from the incineration of household waste; slags from metal smelting; builders’ waste. What unites them, aside from their sheer mass, is the fact that are each products of high-temperature conditions: anthropogenic metamorphic rocks, if you like. That makes them thermodynamically unstable under surface conditions, so they are likely to weather quickly if they are exposed at the surface or in contact with groundwater. And that poses threats of pollution of soil-, surface- and groundwater

All are highly alkaline, so they change environmental pH. Ash from waste incineration is akin to volcanic ash in that it contains a high proportion of complex glasses, which easily break down to clays and soluble products. Curiously, old dumps of ash often contain horizons of iron oxides and hydroxides, similar to the ‘iron pans’ in peaty soils. They form at contacts between oxidising and reducing conditions, such as the water table or at the interface with natural soils and rocks. Soluble salts of a variety of trace elements may accumulate, such copper, antimony and molybdenum. Slags not only contain anhydrous silicates rich in the metals of interest and other trace metals, which on weathering may yield soluble chromium and vanadium, but they also have high levels of calcium-rich compounds from the limestone flux used in smelting, i.e. agents able to create high alkalinity. Portland cement, perhaps the most common material in builders’ waste, is dominated by hydrated calcium-aluminium silicates that break-down if the concrete is crushed, again with highly alkaline products. Another component in demolition debris is gypsum from plaster, which can be a source of highly toxic hydrogen sulfide gas generated in anaerobic conditions by sulfate-sulfide reducing bacteria.

Ancient oil migration

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

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

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

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

UK shale gas: fracking potential dramatically revised downwards

In 2013, much to the joy of the British government and the fracking industry, the British Geological Survey (BGS) declared that there was likely to be between 24 and 68 trillion m3 (TCM) of gas available to fracking ventures in the Carboniferous Bowland Shale, the most promising target in Britain. That is equivalent to up to about 90 years’ supply at the current UK demand for natural gas.  The BGS estimate was based on its huge archives of subsurface geology, including that of the Bowland Shale; they know where the rock is present and how much there is. But their calculations of potential gas reserves used data on the gas content of shales in the US where fracking has been booming for quite a while. Fracking depends on creating myriad cracks in a shale so that gas can escape what is an otherwise impermeable material.

Bowland Shale 1
Areas in Britain underlain by the Bowland Shale formation (credit: British Geological Survey)

How much gas might be available from a shale depends on its content of solid hydrocarbons (kerogen) and whether it has thermally matured and produced gas that remains locked within the rock. So a shale may be very rich in kerogen, but if it has not been heated to ‘maturity’ during burial it may contain no gas at all, and is therefore worthless for fracking. Likewise, a shale from which the gas has leaked away over millions of years. A reliable means of checking has only recently emerged. High-pressure water pyrolysis (HPWP) mimics the way in which oil and gas are generated during deep burial and then expelled as once deep rock is slowly uplifted (Whitelaw, P. et al. 2019. Shale gas reserve evaluation by laboratory pyrolysis and gas holding capacity consistent with field data. Nature Communications, v. 10, article 3659; DOI: 10.1038/s41467-019-11653-4). The authors from the University of Nottingham, BGS and a geochemical consulting company show that two samples of the Bowland Shale are much less promising than originally thought. Based on the HPWP results, it seems that the Bowland Shale as a whole may have gas reserves of only around 0.6 TCM of gas that may be recoverable from the estimated 4 TCM of gas that may reside in the shale formation as a whole. This is ‘considerably below 10 years supply at the current [UK] consumption’.

Unsurprisingly, the most prominent of the fracking companies, Cuadrilla, have dismissed the findings brusquely, despite having published analyses of other samples that consistent with results in this paper. Opinion in broader petroleum circles is that the only way of truly putting a number to potential reserves is to drill and frack many wells … The British government may well have a collective red face only a week after indicating that they were prepared to review regulation of fracking, which currently forces operations to stop if it causes seismic events above magnitude 0.5 on the Richter scale. A spokesperson for Greenpeace UK said that, ‘Fracking is our first post-truth industry, where there is no product, no profit and no prospect of either.’

See also: McGrath, M. 2019. Fracking: UK shale reserves may be smaller than previously estimated. (BBC News 20 August); Ambrose, J. 2019. Government’s shift to relax shale gas fracking safeguards condemned (Guardian 15 August); Fracking in the UK; will it happen? (Earth-logs June 2014)