Carbon sources in arc volcanism, with implications for the carbon cycle

New CO₂/³ He data from the East Sunda Arc (Indonesia) confirm earlier observations that arc volcanic gases have higher CO₂/³ He ratios than MOR environments.On average, > 80% of arc volcanic CO₂ is recycled, exogene carbon. Addition of a few percent of carbonate-bearing sediments to the mantle wedge explains much of the carbon abundance andcarbon isotopic data of arc gases, but can not explain the He isotope observations. The CO₂/³He in arc volcanoes is not strongly dependent on the composition of modem trough sediments (e.g. deep sea clays vs carbonate-rich sequences), and calcite veins in the hydrothermally altered subducted slab may provide a contribution to the recycled carbon flux of, arcs. The sum of globally deep-subducted sediment and slab carbon exceeds the estimated arc CO₂ flux, and approximately 3.5 teramole of carbon may return annually to the mantle in convergent zones. The modem combined processes of MOR volcanism, slab alteration, and subduction volcanism do not produce a substantial carbon flux into the exosphere, and rate-changes in ocean floor spreading are unlikely to cause major changes in atmospheric CO₂ as a result of changes in the volcanic CO₂ fluxes. Intense pulses of flood basalt volcanism, however, may alter the CO₂ contents of the atmosphere over the course of a millenium or so, and influence global climate.