As tectonic plates slide beneath each other, carbon is carried into the Earth's mantle – a factor in shaping the element's abundance at the surface. New research explains why less of this carbon gets returned to the atmosphere than some models predicted, resolving a discrepancy in how permanent this removal is.
Before atmospheric carbon can reach the mantle it must be deposited on the seafloor, usually in the form of shells or microorganisms that sink to the bottom of the deep ocean. A process increasing the deposition rate can lock carbon away there for thousands or even millions of years – but until the oceanic plate it is sitting on subducts beneath another plate, there are always processes that can bring it back into circulation.
Some of the carbon subducted into the mantle eventually returns to the atmosphere through volcanoes – in rare cases dramatically enough to cause a mass extinction. Geologists have thought it is likely that most eventually returns this way. However, a paper in Nature Communications makes the case that about two-thirds of the carbon never leaves the mantle once it gets there.
Around 78 million tonnes of carbon are pushed into the mantle each year for more permanent burial – tiny compared to the billions of tonnes humans release to the atmosphere, but a world-shaping quantity over time.
"We currently have a relatively good understanding of the surface reservoirs of carbon and the fluxes between them, but know much less about Earth's interior carbon stores, which cycle carbon over millions of years," said lead author and Cambridge University PhD student Stefan Farsang in a statement.
Farsang and co-authors tried to fill this knowledge gap by exploring the chemical reactions that occur once carbon finds itself in the heat and pressure of the mantle. They used a heated “diamond anvil” to expose small amounts of carbonate to conditions similar to those 35 kilometers (20 miles) below the Earth's surface.
Most of the sinking carbon is locked in carbonate rocks (which have the same chemical makeup as chalk), but the researchers found that as pressure rises, calcium is replaced by magnesium. Under the conditions they tested, MgCaO3 is at least a hundred times less soluble than CaCO3, impeding its absorption into liquids that eventually erupt from volcanoes. Instead, magnesium-carbonate molecules sink ever deeper into the mantle, potentially transforming into diamonds. Nevertheless, the authors acknowledge that conditions in the mantle can vary. Future studies will test carbon's behavior over a wider range of temperatures and pressures.
"These results will also help us understand better ways to lock carbon into the solid Earth, out of the atmosphere. If we can accelerate this process faster than nature handles it, it could prove a route to help solve the climate crisis," said senior author Professor Simon Redfern.