Climate forcing controls on carbon terrestrial fluxes during shale weathering.

Climate influences near-surface biogeochemical processes and thereby determines the partitioning of carbon dioxide (CO) in shale, and yet the controls on carbon (C) weathering fluxes remain poorly constrained. Using a dataset that characterizes biogeochemical responses to climate forcing in shale regolith, we implement a numerical model that describes the effects of water infiltration events, gas exchange, and temperature fluctuations on soil respiration and mineral weathering at a seasonal timescale. Our modeling approach allows us to quantitatively disentangle the controls of transient climate forcing and biogeochemical mechanisms on C partitioning. We find that ~3% of soil CO (1.02 mol C/m/y) is exported to the subsurface during large infiltration events. Here, net atmospheric CO drawdown primarily occurs during spring snowmelt, governs the aqueous C exports (61%), and exceeds the CO flux generated by pyrite and petrogenic organic matter oxidation (~0.2 mol C/m/y). We show that shale CO consumption results from the temporal coupling between soil microbial respiration and carbonate weathering. This coupling is driven by the impacts of hydrologic fluctuations on fresh organic matter availability and CO transport to the weathering front. Diffusion-limited transport of gases under transient hydrological conditions exerts an important control on CO egress patterns and thus must be considered when inferring soil CO drawdown from the gas phase composition. Our findings emphasize the importance of seasonal climate forcing in shaping the net contribution of shale weathering to terrestrial C fluxes and suggest that warmer conditions could reduce the potential for shale weathering to act as a CO sink.