Climate change

Earth’s interior taking up more carbon

Earth’s interior is taking up more carbon emissions than previously thought. To tackle climate change, scientists need to find ways to reduce the amount of CO2 in Earth’s atmosphere. 

Deep earth’s interior houses the majority of the planet’s carbon. And therefore, by studying how carbon behaves in the deep Earth scientists can better understand the entire lifecycle of carbon on Earth, and how it flows between the atmosphere, oceans, and life at the surface.

The best-known carbon cycle is at or near Earth’s surface, but deep carbon stores play a key role in maintaining the habitability of planet earth by regulating atmospheric CO2 levels. 

“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 Stefan Farsang.

Through their research, scientists from Cambridge University and NTU Singapore found that slow-motion collisions of tectonic plates drag more carbon into Earth’s interior than previously estimated.

The scientists found that the carbon drawn into Earth’s interior at subduction zones tends to stay locked away at depth, rather than resurfacing in the form of volcanic emissions. Subduction zones are where tectonic plates collide and dive into Earth’s interior.

They found that only about a third of the carbon recycled beneath volcanic chains returns to the surface via recycling This is in contrast to previous theories that what goes down mostly comes back up. The findings were published in Nature Communications

Carbon can be released back to the atmosphere (as CO2) through several ways but only one path in which it can return to the Earth’s interior: via plate subduction. Here, surface carbon, for instance in the form of seashells and micro-organisms which have locked atmospheric CO2 into their shells, is channeled into Earth’s interior. 

Scientists had thought that much of this carbon was then returned to the atmosphere as CO2 via emissions from volcanoes. But the new study reveals that chemical reactions taking place in rocks swallowed up at subduction zones trap carbon. It is then sent deeper into Earth’s interior; stopping some of it from coming back to Earth’s surface.

The team conducted a series of experiments at the European Synchrotron Radiation Facility. To replicate the high pressures and temperatures of subductions zones, they used a heated ‘diamond anvil,” in which extreme pressures are achieved by pressing two tiny diamond anvils against the sample.

The work supports growing evidence that carbonate rocks, which have the same chemical makeup as chalk, become less calcium-rich and more magnesium-rich when channeled deeper into the mantle. This chemical transformation makes carbonate less soluble. This means it doesn’t get drawn into the fluids that supply volcanoes. Instead, the majority of the carbonate sinks deeper into the mantle where it may eventually become diamond.

“There is still a lot of research to be done in this field,” said Farsang. “In the future, we aim to refine our estimates by studying carbonate solubility in a wider temperature, pressure range, and in several fluid compositions.”

The findings are also important for understanding the role of carbonate formation in our climate system more generally. “Our results show that these minerals are very stable and can certainly lock up CO2 from the atmosphere into solid mineral forms that could result in negative emissions,” said Redfern. The team has been looking into the use of similar methods for carbon capture, which moves atmospheric CO2 into storage in rocks and the oceans.

“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 Redfern.

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