December 2021 ESRFnews16

DEEP CARBON CYCLE

There are several ways in which carbonates could help sequester carbon from the atmosphere. One is known as enhanced weathering, and involves grinding up huge quantities of rocks and spreading the resultant dust over fields or beaches. Then, as the dust is wetted by rainwater or seawater, it reacts with any dissolved carbon dioxide to form carbonates, which do not release the carbon for thousands of years or more.

Another idea involves directly capturing carbon dioxide from industry, dissolving it in water, and pumping that water deep underground, where it reacts to form the long-lived carbonates. Since 2014, Carbfix, a project led by Reykjavik Energy in Iceland, the University of Iceland and CNRS in Toulouse, France, claims to have buried 70,000 tons of carbon dioxide at an injection site in Hellisheiði, Iceland, using this method.

BURYING THE PROBLEM

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F C A N D É /E S R F

The ID27 beamline, run by Mohamed Mezouar, delivered sufficient X-ray flux to detect low concentrations of metal ions, and establish carbonate solubility with high precision.

crustaceans, molluscs and other sea-borne organisms, which turn it into carbonate shells. Over hundreds of millions of years, by tectonic activity, these rocks and shells are transported to the edges of oceanic plates, where sub- duction draws them down into the Earth s mantle.

A fork in the road There are two possibilities for what happens next. One is that the carbonates sink deeper, and are subjected to higher temperatures and pressures until they form diamond; indeed, at depths of around 100 km, these pre- cious stones may be commonplace. The other possibility is that the carbonates dissolve. If so, then they are carried with fluids to the rocks above the subducting plate and, like salt on ice, melt the rocks by lowering their melting temperature. Should any of this melt find a path to the surface, the sudden pressure drop like unscrewing the cap on a fizzy drink bot- tle releases the dissolved carbonates and any water present, as carbon dioxide and steam. The result: volcanoes, such as those in Indonesia, or the Aleutian Islands, or surrounding the Pacific in the Ring of Fire . In this second sce- nario, therefore, carbon that was once dragged into the Earth ends up back in the atmosphere, ready for the cycle to start again. In 2015, Earth scientists Peter Kelemen at Colum-

bia University in New York state, US, and Craig Man- ning at the University of California, Los Angeles, US, analysed the relative take-up of the two possible paths. They estimated that the second path is dominant that relatively little carbon is withheld by the mantle (PNAS

112 E3997). Their paper s title summed up their findings with the phrase, what goes down, mostly comes up . Kelemen and Manning s analysis was based on the

solubility of calcium carbonate (CaCO3), which takes forms such as calcite and aragonite. But carbonates can also contain substantial amounts of magnesium, form- ing minerals such as dolomite (CaMg(CO3)2, the main constituent of the Dolomite Mountains in Italy) and magnesite (MgCO3). Studies of subducted rocks that have made their way back to the surface have shown that dolomite and magnesite are probably the most sta- ble carbonate phases at increasing depth, where pres- sures top 2 GPa. The question is whether the solubility of these carbonates at depth affects the carbon s overall journey. Scientists have postulated that it would make a difference, but no-one had measured it before, says Simon Redfern, an environmental materials scientist at NTU Singapore and the corresponding author of the latest ESRF research. Measuring the solubilities of carbonates in extreme

conditions is not straightforward. Redfern, Rosa and colleagues (who are based in the UK, China and Swit- zerland) sealed samples of metal carbonates in hydrous saline fluid similar to that found in subducted slabs in the gasket of a heated diamond-anvil cell. With the help of Mohamed Mezouar (pictured, above), ESRF scientist-in-charge at the ID27 beamline, they used X-ray fluorescence at very high resolution to detect the concen- trations of dissociated metal ions in the fluid at tempera- tures of up to 400 °C and pressures of more than 8 GPa. The