Erosion sensitivity of rock organic carbon oxidation reinforces CO2 release at high erosion rates

Victoria Alcock , Aaron Bufe, Ella Victoria Walsh, Niels Hovius, Li-Hung Lin, Robert G Hilton

Oxidation of rock organic carbon (OCpetro) can exert a substantial control on atmospheric CO2 over geological timescales, yet the erosion sensitivity of this reaction has proved challenging to isolate and quantify due to the covariation of erosion with other environmental variables in many existing datasets. Here we apply the rhenium (Re) proxy to quantify OCpetro oxidation across a three order-of-magnitude erosion-rate gradient in Taiwan with minimal variation in runoff, temperature, and lithology, and use a weathering model to describe the erosion sensitivity of OCpetro oxidation. OCpetro oxidation fluxes exhibit a positive non-linear scaling with erosion, similar to sulfide mineral weathering, although with a weaker erosion rate dependence likely reflecting slower oxidation kinetics and the presence of recalcitrant graphitic carbon. Integrating OCpetro oxidation with existing silicate, carbonate, and sulfide weathering carbon budgets reveals erosion-dependent transitions in the net weathering carbon balance. Moderate erosion rates around 0.1 mm yr-1 maximise CO2 drawdown, whereas higher erosion rates produce net CO2 release due to oxidative weathering of OCpetro and sulfide minerals. Accounting for OCpetro oxidation increases CO2 emissions at high erosion rates and dampens CO2 drawdown at moderate erosion rates relative to previous characterisations that considered emissions only from sulfide oxidation. Unlike sulfide oxidation, CO2 emissions from OCpetro oxidation remain a relevant carbon source on timescales of 10 Myr or more, extending the CO2 drawdown optimum over longer timescales. Erosion and burial of biospheric organic carbon (OCbio) may provide an erosion-sensitive CO2 sink that could partially offset oxidative CO2 release at high erosion rates.