Forearc Carbon Sink Reduces Long-Term Volatile Recycling Into the Mantle

Carbon and other volatiles are transported from Earth’s surface into the mantle at subduction margins. The efficiency of this transfer has profound implications for the nature and scale of geochemical heterogeneities in Earth’s deep (mantle) and shallow (crustal) reservoirs, as well as Earth’s oxidation state. However, the proportion of volatiles released in the forearc and backarc are not well constrained compared to fluxes from the volcanic front. Here, we use helium and carbon isotope data from deeply sourced springs along two cross-arc transects to show that ~91% of carbon released from the slab/mantle beneath the Costa Rica forearc is sequestered within the crust by calcite deposition, and an additional ~3% is incorporated into biomass through microbial chemolithoautotrophy. We estimate that ~1.2 × 10(exp 8) to 1.3 × 10(exp 10) mol CO2/yr are released from the slab beneath the forearc, resulting in up to ~19% less carbon being transferred to Earth’s deep mantle than previously estimated.