Carbon trapped in ocean floor rocks mostly sinks into the deep Earth

16-07-2021

Scientists from Cambridge University and NTU Singapore have found that when carbon, trapped in ocean floor rocks, gets drawn deep into the Earth at subduction zones, it tends to stay deep and less of it pops back out again through volcanoes than previously thought. They used ID27 to study this process. Their results are published this week in Nature Communications.

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One of the solutions to tackle climate change is to find ways to reduce the amount of CO2 in Earth’s atmosphere. Studying how carbon behaves in the deep Earth is an important part of the entire “lifecycle” of carbon of Earth, from atmosphere to oceans to as deep as Earth’s core.

With these aims, scientists study the carbon cycle on Earth, which starts in the atmosphere, then goes to the oceans, then the deep Earth and back in the atmosphere. Carbon (as CO2) in the atmosphere gets incorporated into carbonate minerals via weathering processes in rivers and in the oceans, when silicate minerals weather chemically (from rivers flowing through mountains) and react with CO2 in river water to form carbonates and bicarbonate.

When the water reaches the oceans, more of the dissolved carbon is used to make carbonate shells of organisms (seashells and micro-organisms with shells – this is what makes chalk rocks). Most of the sediments, along with the carbonated ocean floor crustal volcanic rocks that form in places like the mid-Atlantic ridge and mid-Pacific rise, are then transported (on geological time scales, i.e. hundreds of millions of years) to the edges of the oceanic plates. In places like the Pacific Ocean and the Caribbean, they are drawn down beneath other tectonic plates that they collide with, and “fall” into Earth’s mantle, called subduction.

As they are drawn down into Earth’s mantle, the carbonate minerals in the sediments and volcanic rocks that form the part of the subducted oceanic crust begin to dissolve. The dissolved carbonate-rich fluid then rises into the overlying rocks above the subducted slab and causes them to melt. This results in volcanoes that form above the subducted slab. Examples of these volcanoes can be found in Indonesia, or the Aleutian Islands, and the volcanoes surrounding the Pacific in the ring of fire. The volcanoes emit lava, but also steam (from the water that was released at depth) and CO2 (from the carbonate minerals).

How much carbon returns to the atmosphere in the form of CO2 depends on how much these carbonates dissolve in subduction zones, where they are present with fluids.

A team of scientists from Cambridge University, United Kingdom, and NTU Singapore have used the ESRF to study different carbonate minerals under the conditions of the subduction zones that sit beneath the volcanoes of the Pacific rim of fire. They found that about one third of the carbonate dissolves in the fluid they are soaking in, but the remaining two thirds sinks into the deep Earth, in contrast to earlier suggestions that what goes down mostly comes back up.

41467_2021_24533_Fig1_HTML.jpg

White headed black arrows indicate carbonate flux and blue arrows water flux. Blue shaded areas indicate water-rich regions. The melting of carbonated igneous oceanic crust is not shown as it starts at depths of 300 km22. The image is to scale, apart from the thickness of oceanic sediments that has been exaggerated. Credits: Farsang S, et al, Nature Communications. 

 

There is increasing evidence that the subducted carbonate, which is mainly the same sort of calcium carbonate that makes chalk, becomes less calcium-rich and more magnesium-rich as it is drawn deep into the Earth at the edges of subducting oceans. The results show that the solubility of magnesium-rich carbonate minerals is much less than the calcium carbonates, and so the reactions to form magnesium rich carbonates, going from minerals like calcite to ones like dolomite, means that the carbon gets locked into the rock.

The consequence of this decreasing solubility is that only one third of the subducting slab carbonates will dissolve in subduction zone fluids, while the rest will sink to the deep mantle and much of it will likely become diamond.

Reproducing the conditions of a subduction zone

The team went to ID27’s beamline, where they carried out an experiment using in-situ X-ray fluorescence. Simon Redfern, Dean of the College of Science at NTU Singapore and corresponding author of the paper, explains: “When a metal carbonate mineral dissolves in water in the conditions of the subduction zone, it releases metal atoms into the fluid. ESRF has the ability to measure very low concentrations of these metals in the water at the very high pressure and temperature conditions of interest to us. This is a key part of putting the whole picture together, and the team at ID27 of ESRF have world-leading facilities and expertise that we needed to get our results.”

 “This was a very challenging experiment”, recalls Angelika Rosa, scientist at the ESRF and co-author of the study. “Because we needed very low energy, we had to use an anvil cell with a very thin diamond, as below 5keV most of the X-rays are absorbed in a regular diamond anvil cell”, she adds. “EBS will make this kind of experiments much easier, as we’ll have more flux and we will be able to work at lower energies”.

“There is still a lot of research to be done in this field”, explains Stefan Farsang, who carried out this work as part of his PhD. “In the future, we aim to refine our estimates by studying carbonate solubility in a wider temperature, pressure range and in several fluid compositions”, he says.

Towards ‘negative emissions’?

The processes that the team has studied are important for understanding carbonate formation and stability 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’”, says Redfern. The team have been looking in to the use of these sorts of methods for future draw down of atmospheric CO2 into rocks and into 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” concludes Redfern.

On a more general level, these findings allow scientists to understand how the Earth system regulates its environment through processes like plate tectonics and volcanism. “Ironically, the processes that gave rise to the 2011 Tohoku earthquake and tsunami are, at the same time, those that are responsible also for the development of a liveable environment on Earth”, says Redfern.

Reference: 

Farsang, S. et al, Nat Commun 12, 4311 (2021). https://doi.org/10.1038/s41467-021-24533-7.

Text Montserrat Capellas Espuny

 

Top image: The Aleutian Islands, with their 57 volcanoes, form the northernmost part of the Pacific Ring of Fire. Credit: NASA