Deep root activity overprints weathering of petrogenic organic carbon in shale

The oxidation of organic carbon in sedimentary bedrock (petrogenic OC, OC_petro) is increasingly recognized as a potential source of CO₂ to the atmosphere. Recent studies provide evidence for the mobilization and oxidation of OC_petro in sedimentary bedrock during rock weathering. However, the mechanisms and rates remain uncertain, particularly where overlying soils and vegetation drive contemporaneous oxidation of recently fixed organic carbon. Here, we quantify OC_petro weathering across a 16 m shale depth profile in a steep, rapidly eroding forested hillslope in the Northern California Coast Ranges. We report solid and gas phase radiocarbon and stable isotope analyses of samples extracted from specialized in-situ samplers, and a supporting laboratory incubation experiment of the shale regolith. OC_petro is removed from the weathered bedrock at a rate of approximately 0.12 gC/m³yr, which is orders of magnitude lower than the rate of OC_petro oxidation we achieved in the laboratory with crushed samples (557.1 gC/m³/yr). This disparity occurs despite high O_2(g) content across the depth profile, indicating that physical accessibility of OC_petro can regulate oxidative weathering. There is no direct radiocarbon evidence of OC_petro oxidation in CO_2(g) across the upper 13 m of the weathering profile during both wet and dry seasons. Instead, vadose zone CO_2(g) production at the site is dominated by respiration of recently fixed carbon associated with deep rooting. OC_petro is clearly mobilized across the vadose zone during weathering in this rapidly eroding, oxygen-rich, biologically dynamic hillslope, but at rates far below what can be measured given the contribution of root-derived CO_2(g).