Closing the geologic carbon cycle

Estimates of sedimentary organic carbon burial fluxes based on inventory and isotope mass balance methods have been divergent. A new calculation of the isotope mass balance using a revised assessment of the inputs to the ocean-atmosphere system resolves the apparent discrepancy. Inputs include weathering of carbonate and old kerogen, geogenic methane oxidation, and volcanic and metamorphic degassing. Volcanic and metamorphic degassing comprise ≈23% of the total C input. Inputs from isotopically light <i>OCpetro</i> and <i>CH_4-geo</i> drive the mean δ¹³C of the input to =-8.0 ± 1.9‰, notably lower than the commonly assumed volcanic degassing value. The isotope mass balance model yields a modern burial flux =15.9 ± 6.6 Tmol y^-1. The impact of the mid-Miocene Climatic Optimum isotope anomaly is an integrated excess deposition ≈ 4.3 × 10⁶ Tmol between 18 and 11 Ma, which is both longer and larger than estimates for the total degassing by the Columbia River Basalt eruptions, implying a complex carbon system response to large eruptive events. Monte Carlo evaluation finds that late Cenozoic net growth of the carbonate reservoir is very likely while net growth of the <i>C_org</i> reservoir is less certain but more likely than not. At present, subduction does not appear to keep up with net sedimentation and the overall masses of sedimentary carbonate and organic carbon are likely increasing. Growth in the sedimentary <i>C_org</i> reservoir implies oxidation of the surface environment and likely increases in atmospheric pO₂.