Provenance, transport, and the fate of organic matter and sediments drained through Himalayan Rivers in Nepal

Sammanfattning: Despite its small area (ca. 595,000 km2), the Himalayan region transfers disproportionally high amounts of sediments and organic matter (OM) through a network of rivers into the oceanic sink. Such a high contribution is due to the synergistic effects of active tectonics, variable precipitation, and steep slopes aided by human perturbations on exhumed and young mountain terrains. Seasonality in the mobilization and transport of fresh biogenic OM and petrogenic carbon, including paleosols, is dominant in this unique landscape with implications for climate change and the global carbon budget. However, a comprehensive assessment of sources and the fate of organic carbon (OC) in Himalayan rivers remains elusive, driving considerable uncertainty in estimates of the fluvial transport of carbon, its budget, and its impacts on the global carbon cycle. Four rivers from diverse physiographic zones with specific rock types were selected from the Nepal Himalayas to characterize the OM sources and their fate using C:N ratios, lipid biomarkers, and lignin phenols. The seasonal OC fluxes were estimated, and strontium (Sr) and neodymium (Nd) isotope data were utilized to elucidate the provenance. Finally, monitoring of runoff plots and RUSLE modeling was conducted to estimate soil erosion from different land-use practices. The suspended sediment load in these rivers was proportional to the erosional intensity. Unlike suspended sediments, OM in bedload samples was derived from multiple sources with weak terrigenous dominance. The influence of seasonality on OM and elemental concentrations was evident in the new data. The abundance of sedimentary lipids in these small rivers represents high OM sequestration and corroborates the inference derived from diagnostic lignin ratios. These rivers transport > 90% POC and ca. 75% DOC during the short monsoon season, highlighting rapid transport/mobilization of OC from the Nepal Himalayas. The high strontium isotope (87Sr/86Sr) ratio in silicates drives the high radiogenic Sr input, which exceeds the global average. The clusters in Sr and Nd isotope data represent specific physiographic zones and rock types that can help infer OM provenance and trace the fate of carbon from source to sink. Data from the runoff plots suggest that irrigated croplands drive topsoil erosion. The soil erosion rate in the watersheds is high (> 24 tons ha-1 yr-1). This is attributed to anthropogenic disturbance associated with cropping patterns, soil disturbance, and waterlogging. The contrasting features in the Himalayan region and new data on the role of mountain rivers invite global attention to infer ongoing and future changes in OM flux.