Science. Earth’s interior absorbs more carbon than previously thought

2021-07-26 Aerial view of the University of Cambridge’s Volcano for Policy Research and Technology

Madrid, 26 (European Press)

Scientists from the University of Cambridge and NTU in Singapore have discovered that slow-moving plate tectonic collisions are pulling more carbon into Earth than previously thought.

They found that carbon drawn into the Earth’s interior at subduction zones, where tectonic plates collide and sink into Earth’s interior, tends to remain trapped at depth, rather than appearing as volcanic emissions.

Their findings, published in Nature Communications, suggest that about a third of the carbon recycled under volcanic chains returns to the surface through recycling, contrary to previous theories that what falls mostly is due to uplift.

One solution to tackle climate change is to find ways to reduce the amount of carbon dioxide in the Earth’s atmosphere. By studying how carbon behaves deep in the Earth, which is home to most of the carbon on our planet, scientists can understand the full life cycle of carbon on Earth and how it flows between the atmosphere, oceans and life at the surface.

The best understood parts of the carbon cycle are at or near the Earth’s surface, but deep carbon stocks play a key role in maintaining our planet’s habitability by regulating levels of carbon dioxide in the atmosphere.

“We currently have a relatively good understanding of surface carbon pools and the flow between them, but we know little about intra-Earth carbon pools, which cycle carbon over millions of years,” said lead author Stefan Farsang, who conducted the research. While he was a PhD student at the Department of Earth Sciences at Cambridge.

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There are several ways to release carbon into the atmosphere (such as carbon dioxide), but there is only one way for it to return to Earth’s interior: through plate subduction. Here, carbon from the surface, for example in the form of seashells and microorganisms that trap atmospheric carbon dioxide in their shells, is directed to the Earth’s interior.

Scientists had thought that much of this carbon was returned to the atmosphere in the form of carbon dioxide through emissions from volcanoes. But the new study reveals that chemical reactions that occur in rocks that are swallowed up in subduction zones trap carbon and send it deeper into the Earth’s interior, preventing some of it from returning to the Earth’s surface.

The team conducted a series of experiments at the European Synchrotron Radiation Facility. “The ESRF has world-leading facilities and the expertise we need to obtain our results,” co-author Simon Redfern, dean of the NTU Singapore College of Science, said in a statement. “The facility can measure very low concentrations of these metals under conditions of high pressure and temperature of interest to us,” he added. To reproduce the high pressures and temperatures of the subduction zones, they used a hot “diamond anvil”, in which extreme pressures are achieved by pressing two small diamond anvils onto the sample.

The work supports mounting evidence that carbonate rocks, which have the same chemical composition as chalk, become less calcium-rich and more magnesium-rich when channeled deeper into the mantle. This chemical shift makes the carbonate less soluble, which means it is not absorbed into the fluids that supply volcanoes. Instead, most of the carbonates sink deep into the mantle where they can eventually turn into diamond.

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“There is still a lot of research to be done in this area,” Varsang said. “In the future, our goal is to improve our estimates by studying the solubility of carbonates over a wider range of temperature and pressure and in different fluid compositions.”

The findings are also important for understanding the role of carbonate formation in our climate system in general. “Our results show that these metals are very stable and can certainly retain carbon dioxide from the atmosphere in solid metallic forms that can lead to negative emissions,” Redfern said. The team has been studying the use of similar carbon capture methods, which move carbon dioxide in the atmosphere to storage in rocks and oceans.

“These findings will also help us understand better ways to sequester carbon on solid Earth, outside the atmosphere,” Redfern said. “If we can accelerate this process faster than nature manages to do, it could be a pathway to help solve the climate crisis.”

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