Global warming: Can mantle rocks reduce the greenhouse effect?

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

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

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

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

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

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

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

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

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.

Ants and carbon sequestration

Aside from a swift but highly unlikely abandonment of fossil fuels, reduction of greenhouse warming depends to a large extent, possibly entirely, on somehow removing CO2 from the atmosphere. Currently the most researched approach is simply pumping emissions into underground storage in gas permeable rock, but an important target is incorporating anthropogenic carbon in carbonate minerals through chemical interaction with potentially reactive rocks. In a sense this is a quest to exploit equilibria involving carbon compounds that dominate natural chemical weathering and to sequester CO2 in solid, stable minerals.

The two most likely minerals to participate readily in weathering that involves CO2 dissolved in water are plagioclase feldspar, a calcium-rich aluminosilicate and olivine, a magnesium silicate. Both are abundant in mafic and ultramafic rocks, such as basalt and peridotite, which themselves are among the most common rocks exposed at the Earth’s surface. The two minerals, being anhydrous, are especially prone to weathering reactions involving acid waters that contain hydrogen ions, and in the presence of CO2 they yield stable carbonates of calcium and magnesium respectively. Despite lots of exposed basalts and ultramafic rocks, clearly such natural sequestration is incapable of absorbing emissions as fast as they are produced.

One means of speeding up weathering is to grind up plagioclase- and olivine-bearing rocks and spread the resulting gravel over large areas; as particles become smaller their surface area exposed to weathering increases. Yet it doesn’t take much pondering to realise that a great deal of energy would be needed to produce sufficient Ca- and Mg-rich gravel to take up the approximately 10 billion tonnes of CO2 being released each year by burning fossil fuels: though quick by geological standards the reaction rates involved are painfully slow in the sense of what the climatic future threatens to do. So is there any way in which these reactions might be speeded up?

Two biological agencies are known to accelerate chemical weathering, or are suspected to do so: plant roots and animals that live in soil. Ronald Dorn of Arizona State University set out to investigate the extent to which such agencies do sequester carbon dioxide, under the semi-arid conditions that prevail in Arizona and Texas (Dorn, R.I. 2014. Ants as a powerful biotic agent of olivine and plagioclase dissolution. Geology, v. 42, p. 771-774). His was such a simple experiment that it is a wonder it had not been conducted long ago; but it actually took more than half his working life. Spaced over a range of topographic elevations, Dorn used an augur at each site to drill five half-metre holes into the root mats of native trees, established ant and termite colonies and bare soil surfaces free of vegetation or animal colonies, filling each with sand-sized crushed basalt.

Empire of the Ants (film)
Film poster for Empire of the Ants (starring Joan Collins) (credit: Wikipedia)

Every five years thereafter he extracted the basalt sand from one of the holes at each site and each soil environment. To assess how much dissolution had occurred he checked for changes in porosity, and heated the samples to temperatures where carbonates break down to discover how much carbonate had been deposited. That way he was able to assess the cumulative changes over a 25 year period relative to the bare-ground control sites. The results are startling: root mats achieved 11 to 49 times more dissolution than the control; termites somewhat less, at 10 to 19 times; while ants achieved 53 to 177 times more dissolution. While it was certain that the samples had been continuously exposed to root mats throughout, the degree of exposure to termites and ants is unknown, so the animal enhancements of dissolution are probably minima.

Microscopic examination of mineral grains exposed to ant activity shows clear signs of surface pitting and other kinds of decay. Chemically, the samples showed that exposure to ants consistently increased levels of carbonate in the crushed basalt sand compared with controls, with levels rising by 2 to 4% by mass, with some variation according to ant species. Clearly, there is some scope for a role for ants in carbon sequestration and storage; after all, there are estimated to be around 1013 to 1016 individual ants living in the world’s soils. In the humid tropics the total mass of ants may be up to 4 times greater than all mammals, reptiles and amphibians combined. There is more to learn, but probably a mix of acid secretions and bioturbation by ants and termites is involved in their dramatic effect on weathering. One interesting speculation is that ants may even have played a role in global cooling through the Cenozoic, having evolved around 100 Ma ago.