Observing decarbonation: field study reveals massive CO2 release during subduction of ocean crust

Abstract:

Planet Earth has supported life for billions of years. Despite profound changes in the surface environment and the deep Earth, our climate has remained relatively stable — and critically, habitable — over this time period. This stability is a result of the geologic Global Carbon Cycle, which acts to exchange carbon between the solid Earth, oceans, and atmosphere on timescales of ~1 million years or longer.

Subduction of oceanic crust is particularly important: it is the only known pathway for delivering surface carbon to the deep mantle, but devolatilization reactions in the subducting slab may also return mineral bound carbon back into Earth’s atmosphere.

The fate of subducted CO₂ is a subject of long-standing debate, with recent papers arguing either that the vast majority of this CO₂ is devolatilized (Kelemen & Manning 2015 PNAS) or that the majority is delivered to the deep mantle (Hirschmann 2018 EPSL). Critically, these incompatible interpretations both rely largely on numerical modeling or geochemical proxies, with direct observations from subducted rocks lacking.

We therefore present the first regional observation of widespread CO₂-release in subducted rocks. A field-based study of the Cycladic Blueschist Unit in Greece reveals that metamorphic reactions release ~40 to ~65% of subducted carbon from the slab into the overlying lithosphere at forearc depths (< ~70km). Other processes such as sediment melting or congruent dissolution of carbonate could release even more carbon and continue at greater depths.

We use detailed petrography in combination with chemical analysis and newly-developed numerical modeling techniques to track CO₂ mobility in hundreds of samples and resolve the long-standing debate. We find that deep subduction of carbon is inefficient, potentially driving carbon depletion of the mantle and a corresponding increase in the carbon content of the oceans + atmosphere + lithosphere over billions of years.