Tracking landslide sediment: ¹⁰Be dilution of detrital quartz in multiple grain sizes following the 2016 M_w 7.8 Kaikōura earthquake, New Zealand

Abstract Cosmogenic radionuclides (CRNs) have been proposed as a tool for tracking landslide sediment, yet interpreting the in‐situ 10 Be signal through time with respect to the post‐seismic sediment cascade has proved challenging in the few examples where it has been applied. To determine how useful the 10 Be concentration is in providing insights into the fluvial response of co‐seismic sediment delivery, we measured the 10 Be concentration in multiple grain sizes in two catchments in New Zealand affected by the 2016 Kaikōura earthquake. A 6‐year post earthquake time series of 10 Be measurements was compared to a sediment budget produced by differencing lidar surveys over the same study period. Results showed that the 10 Be concentrations in the sand fraction are controlled by the post‐seismic hillslope delivery, whereas the 10 Be concentrations in the coarser grains respond to the total volume of landslide material in the active channel. Faster 10 Be dilution and recovery of the sand fraction indicates that the transport rate of the sediment pulse is highly dependent on grain size. The residence of the fine‐grained sediment is estimated to be within a decade of the earthquake, although it may take several centuries to evacuate the entirety of the landslide pulse. We observe different magnitudes of 10 Be dilution between two study catchments, which is attributable to a difference in connected landslide volume from hillslopes. We also demonstrate that the dilution factor has little relationship to conventional landslide metrics on a global scale. Our analysis suggests that the magnitude of dilution is controlled through a complex relationship between the connected landslide volume, the degree of sediment mixing and the delivery and evacuation time of landslide sediment. Additionally, if coarse sediment is primarily sourced through mass‐wasting events, measuring just the sand 10 Be concentration will bias the catchment‐averaged erosion rate.