Modelling the systematics of cosmogenic nuclide signals in fluvial sediments following extreme events

Abstract

AbstractThe effect of punctuated mass‐wasting events on longer‐term erosion rates is not fully understood, and yet it is key to quantifying sediment generation, source‐to‐sink dynamics, and landscape evolution in active orogens. The measurement of terrestrial cosmogenic nuclides (TCNs) in river sediments is a common method for determining basin‐averaged erosion rates over centennial to millennial timescales and is often used to compare erosional processes between catchments. However, these comparisons often overlook the role of landsliding rates and their spatial distribution in the measurement and potential variability of TCN signals. While it is widely accepted that basin‐scale perturbations should temporarily dilute TCN concentrations as landsliding mobilizes new, low‐concentration material, the impact of the catastrophic release of hillslope sediment caused by a single event on TCN signatures has not yet been systematically investigated. In this modelling study, we use a catchment in central Nepal to build upon previous modelling efforts to consider how TCNs are recorded in landscapes with varying erosion rates, landsliding rates, and spatial distribution of landslides. We then use the 25 April 2015 Mw 7.8 Gorkha earthquake, Nepal, as a case study to investigate how perturbations like earthquakes are recorded in TCN time series and transferred to and ultimately archived in the sedimentary record. We find that the likelihood of a perturbation being measured by TCN dilution is based on a multitude of factors, including background erosion rates, long‐term landsliding rates, and the connectivity of newly released material to the fluvial system. Especially in landscapes like the central Himalaya with high background erosion and landsliding rates, changes in detrital TCN concentrations are not a reliable indicator of an upstream perturbation, nor should we expect a clear dilution signal following a major event. Our modelling results emphasize that TCN dilution is not a universal characteristic of high‐magnitude landslide‐triggering events.