The Deep Carbon Cycle: Implications for Volcanism, Diamonds, and Life

In a new paper in Nature Geoscience, researchers provide insight into the behavior of carbon in supercritical aqueous fluids in Earth’s upper mantle and lower crust.

A thorough understanding of Earth’s global carbon cycle is critical for many scientific disciplines. While much research has focused on carbon cycling in the lithosphere, processes occurring in Earth’s deep crust and mantle have proved challenging to study. In a new paper in Nature Geoscience, DCO’s Dimitri Sverjensky (Johns Hopkins University, USA), Vincenzo Stagno (Carnegie Institution of Washington, USA), and Fang Huang (Johns Hopkins University, USA) provide new insight into the behavior of carbon in supercritical aqueous fluids in Earth’s upper mantle and lower crust [1].

Sverjensky and colleagues used a recently-developed theoretical model, called the Deep Earth Water model, to calculate the equilibrium constants of aqueous ions, metal complexes, neutral species, and minerals at up to 6 GPa and 1,200°C. They then used these data to infer the identity and quantity of carbon species expected in silicate fluids in Earth’s upper mantle and lower crust.

“Up until now, the amount of carbon that could be carried by fluids liberated from subducting plates seemed too low to account for the CO2 released by volcanoes,” said Sverjensky. “Now that we can predict the solubility of C in fluids in subducting plates, it will be possible to quantify this part of the deep carbon cycle.”

The team’s results point to the importance of the deep carbon cycle in several Earth processes. First, their data suggest that the concentration of carbon in subduction zone fluids could drive mantle oxidation during subduction. Affecting the oxidation state of the mantle wedge in turn drives mantle melting, surface volcanism, and volatile degassing into the atmosphere.

They also highlight a previously unknown mechanism for the formation of diamond in subduction zones. Traditional models of diamond formation, unable to account for the role of ions in mantle fluids, invoke redox reactions such as the oxidation of methane. However, the new model results suggest diamond may actually precipitate from mantle fluids that contain dissolved ionic organic carbon species.

Lastly, the authors note that organic carbon species in mantle fluids could act as energy sources for the deep microbial biosphere. Early life on Earth may have also relied on these products of the deep carbon cycle.

“This study represents a major breakthrough in predicting and quantifying the amount of carbon mobilized from deep reservoirs, where natural rocks and complex aqueous fluids interact with each other at high pressure and temperature,” said Stagno. “It opens the door to future experiments on the stability of C-bearing aqueous fluids at extreme conditions that are needed to understand the origin, speciation, and influence of deep carbon on the history of life on Earth.”

Jay Ague (Yale University, USA; Reservoirs and Fluxes), contributed a News and Views article regarding this paper to the same edition of Nature Geoscience [2], which highlights the impact of this work on our understanding of the deep carbon cycle.

This research was supported by the Deep Carbon Observatory, the WDC Research Fund, Johns Hopkins University, and the US Department of Energy.

Image: Póas volcano, Costa Rica. Credit: Katie Pratt. 

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