Scientists from Cambridge University and NTU Singapore have found that
slow-motion collisions of tectonic plates drag more carbon into Earth's
interior than previously thought.
They found that the carbon drawn into Earth's interior at subduction
zones—where tectonic plates collide and dive into Earth's interior—tends to
stay locked away at depth, rather than resurfacing in the form of volcanic
emissions.
Their findings, published in Nature Communications, suggest that only about
a third of the carbon recycled beneath volcanic chains returns to the
surface via recycling, in contrast to previous theories that what goes down
mostly comes back up.
One of the solutions to tackle climate change is to find ways to reduce the
amount of CO2 in Earth's atmosphere. By studying how carbon behaves in the
deep Earth, which houses the majority of our planet's carbon, 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-understood parts of the carbon cycle are at or near Earth's
surface, but deep carbon stores play a key role in maintaining the
habitability of our planet 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, who conducted the research while a Ph.D. student
at Cambridge's Department of Earth Sciences.
There are a number of ways for carbon to be released back to the atmosphere
(as CO2) but there is 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 and send
it deeper into Earth's interior—stopping some of it coming back to Earth's
surface.
The team conducted a series of experiments at the European Synchrotron
Radiation Facility, "The ESRF have world-leading facilities and the
expertise that we needed to get our results," said co-author Simon Redfern,
Dean of the College of Science at NTU Singapore, "The facility can measure
very low concentrations of these metals at the high pressure and temperature
conditions of interest to us." 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—meaning 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 have 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.
Reference:
Stefan Farsang et al, Deep carbon cycle constrained by carbonate solubility,
Nature Communications (2021). DOI:
10.1038/s41467-021-24533-7
Tags:
Planet and Environment