Fluxes of mantle and subducted carbon along convergent plate boundaries

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Abstract
The potential impact of increases in atmospheric CO2 is a topic of considerable controversy. Even though volcanic emission of CO2 may be very small as compared to anthropogenic emissions, evaluation of natural degassing of CO2 is important for any model of the geochemical C cycle and evolution of the Earth's atmosphere. We report here the mantle C flux in subduction zones based on He and C isotopes and CO2/³ He ratios of high‐temperature volcanic gases and medium‐ and low‐temperature fumaroles in circum‐Pacific volcanic regions. The calculated volcanic C flux of 3.1 × 1012 mol/a from subduction zones is larger than the flux of 1.5 × 1012 mol/a from mid‐ocean ridges, while contributions from the mantle in subduction zone is only 0.30 × 1012 mol/a, equivalent to about 20% of the C flux in mid‐ocean ridges. Since the estimated mantle C flux in hot spot regions is insignificant, 0.029 × 1012 mol/a, we propose that the global mantle C flux is 1.8 × 1012 mol/a in total. The flux, if accumulated over 4.5 billion year of geological time, amounts to 8.3 × 1021 mol which agrees well with 9 × 1021 mol of the present inventory of C at the Earth's surface. This may support a continuous degassing model of C or the idea that subducted C is recycled into the lower mantle.