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 ₂ ) 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 ₂ (1.02 mol C/m ² /y) is exported to the subsurface during large infiltration events. Here, net atmospheric CO ₂ drawdown primarily occurs during spring snowmelt, governs the aqueous C exports (61%), and exceeds the CO ₂ flux generated by pyrite and petrogenic organic matter oxidation (~0.2 mol C/m ² /y). We show that shale CO ₂ 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 ₂ transport to the weathering front. Diffusion-limited transport of gases under transient hydrological conditions exerts an important control on CO _2(g) egress patterns and thus must be considered when inferring soil CO ₂ 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 ₂ sink.
Competing Interests: Competing interests statement:The authors declare no competing interest.