Understanding the phase associations and weathering behavior of rhenium to assess the use of Re as a tracer of georespiration

Abstract

Rhenium (Re) has been proposed as a tracer of petrogenic organic carbon (OC_petro) oxidation, also known as georespiration, but information regarding the solid phases hosting Re in rocks and Re weathering behavior is limited. Examining grey shale rocks from two small mountainous river basins in the Pacific Northwest, USA, we found that the majority of parent material (bedrock) Re in both basins is associated with OC_petro (average 71 %, range 50 to 93 %), with minimal Re hosted in sulfide minerals. In soil and weathered rock profiles, mass transfer calculations (τ) reveal a clear distinction between the weathering behavior of Re and that of other elements, indicating that Re weathering in these siliclastic rocks and soils does not primarily trace sulfide oxidation or dissolution of primary minerals. We find evidence that Re likely traces OC_petro oxidation, although patterns of Re and OC weathering are influenced by heterogeneity in bedrock composition and inputs of modern OC near the surface. In some cases Re loss exceeds that of OC during weathering, suggesting that the kinetics of Re oxidation are sometimes faster than those of OC oxidation. We observe greater Re loss in systems with slower erosion rate, which implies that systems subject to faster erosion export more unoxidized particulate Re. Our observations of Re phase associations and weathering behavior in soil and rock weathering profiles support the use of the Re proxy for georespiration, but also provide qualifications on its application in future work quantifying georespiration fluxes at regional and global scales.