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The potential impact of increases in atmospheric CO 2 is a topic of considerable controversy. Even though volcanic emission of CO 2 may be very small as compared to anthropogenic emissions, evaluation of natural degassing of CO 2 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 CO 2 /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 × 10 12 mol/a from subduction zones is larger than the flux of 1.5 × 10 12 mol/a from mid‐ocean ridges, while contributions from the mantle in subduction zone is only 0.30 × 10 12 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 × 10 12 mol/a, we propose that the global mantle C flux is 1.8 × 10 12 mol/a in total. The flux, if accumulated over 4.5 billion year of geological time, amounts to 8.3 × 10 21 mol which agrees well with 9 × 10 21 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.
Sano et al. (Tue,) studied this question.