Low rates of rock organic carbon oxidation and anthropogenic cycling of rhenium in a slowly denuding landscape

The oxidation of petrogenic organic carbon (OC_petro) is a source of carbon dioxide to the atmosphere over geological timescales. The rates of OC_petro oxidation in locations that experience low rates of denudation remain poorly constrained, despite these landscapes dominating Earth's continental surface area. Here, we track OC_petro oxidation using radiocarbon and the trace element rhenium (Re) in the deep weathering profiles, soils and stream waters of the Susquehanna Shale Hills Critical Zone Observatory (PA, USA). In a ridge-top borehole, radiocarbon measurements reveal the presence of a broad OC_petro weathering front, with a first-order assessment of ~40% loss occurring over ~6 m. However, the low OC_petro concentration (< 0.05 wt%) and inputs of radiocarbon throughout the deepest parts of the profile complicate the assessment of OC_petro loss. The OC_petro weathering front coincides with a zone of Re depletion (~90% loss), and we estimate that > 80% of Re in the rock is associated with OC_petro, based on Re/Na and Re/S ratios. Using estimates of long-term denudation rates, the observed OC_petro loss and the Re proxy are equivalent to a low OC_petro oxidation yield of < 1.7 × 10⁻² tC km⁻² yr⁻¹. This is consistent with the low OC_petro concentrations and low denudation rates at this location. In addition, we find the surface cycle of Re is decoupled from that of deep weathering, with an enrichment of Re in surface soils and elevated Re concentrations in stream water, precipitation, and shallow groundwater. A mass balance model shows that this can be explained by a historical anthropogenic contribution of Re through atmospheric deposition. We estimate that the topsoil Re pool could take decades to centuries to deplete and call for a renewed focus on anthropogenic perturbation of the surface Re cycle in low denudation rate settings.