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Volcanic arcs recycle crustal carbon

last modified Jul 20, 2017 08:18 PM
New research by Cambridge scientists is helping answer a key question about the origin of carbon emitted from Earth’s volcanoes.
Volcanic arcs recycle crustal carbon

Image credit: Richard Herd

The flow of carbon in and out of Earth’s surface environment has a considerable impact upon Earth’s climate and habitability because the total carbon at Earth’s surface affects the amount of CO₂ in the atmosphere and hence surface temperatures. It is not widely appreciated that until the Industrial Revolution most (around 90%) of Earth’s surface carbon was sourced from volcanic outgassing. And volcanoes derive the carbon from Earth’s interior core, mantle and crust. 

In contrast, the biosphere, atmosphere and oceans only contain around 10% of Earth’s carbon reservoir. This huge disparity is a result of carbon being removed from the surface reservoir and ‘locked up’ in carbonate minerals and organic carbon, which are deposited on the seafloor in the form of carbonate and associated sediments. From there the carbon is recycled over geological time by tectonic, metamorphic and volcanic processes. 

Emily Mason and colleagues from the Department of Earth Science in the University of Cambridge have compiled a new global data set of carbon and helium isotopes which are emitted by volcanic arcs. From this data they demonstrate for the first time that the largest and most important volcanic sources of outgassed carbon are volcanic arcs rather than other volcanic settings such as rift and intraplate volcanoes. And of the volcanic arc settings, the most important are mature continental arcs. Furthermore, they suggest that volcanic arc carbon is continental derived through the remobilization of carbon from deeply buried ancient shelf limestone deposits.

Volcanic arcs are formed by the geological processes of plate subduction. In mature continental arcs, large volumes of continental-shelf shallow-water marine carbonates are accreted to the continental margin. Organic carbon and inorganic carbon from these deposits is dragged down into Earth’s interior by the down-going ocean-floor plate during subduction. With increasing depth carbon is released by metamorphic decarbonation and carbonate dissolution. The ascending magmas which generate arc volcanoes incorporate the CO₂-rich fluids which then outgas during ascent and eruption at the surface.

Douglas Palmer, Sedgwick Museum

The research is published in the journal Science

Link identified between continental breakup, volcanic carbon emissions and evolution