Hydrologic and geomorphic controls on bedrock organic carbon export

The top meter of the Earth’s crust holds nearly equal amounts of rock-derived organic carbon (OC petro ) and soil organic carbon. Due to this large reservoir size, the oxidation of OC petro may be an important flux of CO 2 to the atmosphere over geologic time. However, determining the magnitude and drivers of OC petro oxidation remains difficult at the watershed scale. To understand the balance of OC sources eroded from a landscape and prone to breakdown, we measured the OC concentrations, stable isotope ratios, and the radiocarbon content of suspended sediment samples from the East River: a shale-dominated alpine watershed near Crested Butte, Colorado. Suspended sediment samples were studied because they integrate multiple sources of sediment across a watershed. With these OC measurements, we employ a Monte Carlo mass balance inversion to constrain inputs of carbon from plants, soil, and bedrock. We find that along the main stem of the river, δ13C values range from -27 to -26 ‰ VPDB and fraction modern OC (FmC) ranges from 0.73 to 0.95, suggesting millennial-scale floodplain residence times. In contrast, lighter δ13C values (-29 to -27 ‰) and older FmC values are observed within tributaries. Notably, samples collected at different river stages at the downstream-most locality exhibit the widest variability in carbon isotope data. A decrease in sediment OC abundances and FmC values with increasing discharge is likely the result of an increased proportion in bedrock carbon in transport. These data illuminate hydrologic and geomorphic controls on the balance of OC export, which has consequences for watershed-scale OC fluxes. Whereas the river main stem primarily transports young, labile, plant- and soil-derived organic carbon, small steeper tributaries appear to transport older, rock-derived organic carbon, likely due to erosion of bedrock. However, the relative contributions of bedrock and soil-derived carbon in floodplains can fluctuate in response to changing hydrologic states, which are influenced by climate. Ongoing measurements of rhenium and carbon isotopes on suspended sediment, bedrock, and soil will help refine endmembers and shed light on the role of bedrock OC in riverine carbon fluxes.