Provenance of sediments derived from karst and non-karst areas and deposited behind sequential check dams in a small watershed

Abstract

This study uses a composite fingerprinting technique, supported by 137Cs and 210Pbex chronologies, to determine the proportion of sediment derived from karst (limestone) and non-karst (purple sandstone) areas and deposited behind sequential check dams within a small watershed in the Three Gorges Reservoir Region and to calculate the sediment deposition rates (DRs) derived from the two lithological areas. Nine sediment cores were extracted from three terraces behind the sequential check dams, and a total of 115 samples were taken from the cores. In addition, 67 and 99 soil samples were collected from the karst and non-karst areas, respectively. Geochemical elements were used as the fingerprints to identify the sediment provenance. The results indicate that the sediment contribution from the karst area generally decreases in the upcore direction, whereas that from the non-karst area shows the opposite trend. The karst and non-karst areas occupy 60% and 37% of the watershed, respectively, while contributing 26% and 74% of the sediment in the nine cores overall, respectively. The DRs from both lithological areas first increase and then decrease through three stages (1920–1954, 1955–1963, and 1964–2020) over the last 100 years. However, the increased proportion of the DR from the karst area during the second stage was less than that from the non-karst area, while the decreased proportion of the DR from the karst area during the final stage was greater than that from the non-karst area. Overall, the sediment provenance, DRs, and grain-size distributions determined for the nine cores show little change in the downstream direction across the three check dams. The sediment provenance and DRs conjointly indicate the sediment contributions from the karst and non-karst areas, and deposition behind the sequential check dams within a small watershed influenced by human activity over the last 100+ years. They reflect the characteristic patterns of sediment yields from the two areas with contrasting lithology, which originate from the fact that the soil inventory and human activity are generally both limited in the karst area but not in the non-karst area. The grain-size sorting across the check dams, and its influence on the sediment provenance, are minimal, which is related to the small size of the watershed and the hydrological interception and retention of the check dams. This study provides new insights into sediment contributions from karst and non-karst areas, as well as sediment deposition behind sequential check dams, at the scale of a small watershed. It also improves our understanding of surface processes on karst hillslopes experiencing rocky desertification and on non-karst hillslopes subjected to human disturbance, as well as differences between the two.