The sources and selectivity characteristics of organic carbon transported by debris flow events in a mountainous catchment, Southwest China.

Purpose: This study aims to obtain a preliminary understanding on the sources of debris flow sediments and the characteristics of organic carbon (OC) transported by debris flow events. Methods: Samples from debris flow and debris flow deposits as well as potential sources in the catchment were collected. For the collected samples, we measured grain size compositions, OC concentrations and properties such as C:N ratio, stable C isotopic composition and OC components derived from nuclear magnetic resonance (NMR) spectroscopy. Results: As inferred from OC concentrations, C:N ratios and δ¹³C values, we found that OC in debris flow sediments and bare land soils are mainly petrogenic OC, while OC in forest, cropland and grassland soils contained a large fraction of biospheric OC. Fine particles were found to be preferentially transported in debris flow, and the particle selectivity decreased with increasing debris flow erosion intensity. However, no selectivity was observed for OC contained in debris flow, which is different from the widely observed preferential mobilization of OC in sediments by surface erosion. Conclusions: Our results show that sediments in debris flow are mainly sourced from bare lands. The fact that there are no significant differences in the petrogenic OC concentrations in various size fractions of debris flow sediment sources leads to no selectivity of OC in debris flow despite of selectivity of sediment particles. [ABSTRACT FROM AUTHOR]