Validating the rhenium proxy for rock organic carbon oxidation using weathering profiles

Abstract

Chemical weathering over geological timescales acts as a source or sink of atmospheric carbon dioxide (CO₂), while influencing long-term redox cycling and atmospheric oxygen (O₂) at Earth's surface. There is a growing recognition that the oxidative weathering of rock organic carbon (OC_petro) can release more CO₂ than is locally drawn down by silicate weathering, and may vary due to changes in erosion and climate. The element rhenium (Re) has emerged as a proxy to track the oxidative weathering of OC_petro, yet uncertainties in its application remain namely that we lack a systematic assessment of the comparative mobility of Re and OC_petro during sedimentary rock weathering. Here we measure Re and OC_petro loss across gradients in rock weathering at 9 global sites, spanning a range of initial OC_petro values from ∼0.2 % to >10 %. We use titanium to account for volume changes during weathering and assess Re and OC_petro loss alongside major elements that reflect silicate (Na, Mg), carbonate (Ca, Mg) and sulfide (S) weathering. Across the dataset, Re loss is correlated with OC_petro loss but not with loss of any other major element. Across the weathering profiles, the average molar ratio of OC_petro to Re loss was 0.84 ± 0.15, with 8 out of 9 sites having a ratio >0.74. At one site (Marcellus Shale), the average ratio was lower at 0.58 ± 0.11. The excess loss of Re matches expectations that, typically, between ∼0 and 20 % of the Re liberated by sedimentary rock weathering derives from silicate or sulfide phases, while some OC_petro may be physically or chemically protected from weathering. Overall, our measurements provide validation for the Re proxy of OC_petro oxidation and allow future work to further improve our knowledge of regional and global-scale rates of this important source of CO₂ in the geochemical carbon cycle.