Flux and Isotopic Composition of Sedimentary Carbon Subducting Along the World's Trenches

Abstract Here we provide comprehensive estimates for the subducted sedimentary flux of inorganic and organic carbon and δ 13 C at 16 trench segments worldwide. Our calculations provide minimum (incoming sediment), maximum (trench fill) and best estimates (underthrust) of carbon fluxes. Notably, our estimates are granular, at a scale of ∼10 km along the trench, relevant to individual volcanoes. Results highlight extreme regional heterogeneity, where the sedimentary carbon input to some trench segments is zero, while for others it exceeds that in the altered oceanic crust by more than a factor of four. The fraction of carbon that is organic is <10% in some regions and >70% in others, with a resulting large range in subducting sedimentary δ 13 C from +1 to −17‰. We find that the δ 13 C of high‐temperature volcanic gases at different arc volcanoes correlates with that in the local subducted sediment. We also provide a new estimate of the carbon flux in global subducting sediment (C‐GLOSS) of 26.5 ± 1 MtC/yr, more than a factor of two lower than previous estimates. Given the uncertainties in the global volcanic output rate of carbon, mantle inputs and outputs of carbon are still roughly in balance. The fraction of organic carbon relative to total carbon in C‐GLOSS is 21%, yielding an isotopic composition of −4.2 ± 0.4‰ δ 13 C, which is by coincidence within the mantle range.