Dilution of ¹⁰Be in detrital quartz by earthquake-induced landslides: Implications for determining denudation rates and potential to provide insights into landslide sediment dynamics

The concentration of ¹⁰Be in detrital quartz (¹⁰Be_qtz) from river sediments is now widely used to quantify catchment-wide denudation rates but may also be sensitive to inputs from bedrock landslides that deliver sediment with low ¹⁰Be_qtz. Major landslide-triggering events can provide large amounts of low-concentration material to rivers in mountain catchments, but changes in river sediment ¹⁰Be_qtz due to such events have not yet been measured directly. Here we examine the impact of widespread landslides triggered by the 2008 Wenchuan earthquake on ¹⁰Be_qtz in sediment samples from the Min Jiang river basin, in Sichuan, China. Landslide deposit material associated with the Wenchuan earthquake has consistently lower ¹⁰Be_qtz than in river sediment prior to the earthquake. River sediment ¹⁰Be_qtz decreased significantly following the earthquake downstream of areas of high coseismic landslide occurrence (i.e., with greater than ~0.3% of the upstream catchment area affected by landslides), because of input of the ¹⁰Be-depleted landslide material, but showed no systematic changes where landslide occurrence was low. Changes in river sediment ¹⁰Be_qtz concentration were largest in small first-order catchments but were still significant in large river basins with areas of ¹⁰⁴-^{105 km2}. Spatial and temporal variability in river sediment ¹⁰Be_qtz has important implications for inferring representative denudation rates in tectonically active, landslide-dominated environments, even in large basins. Although the dilution of ¹⁰Be_qtz in river sediment by landslide inputs may complicate interpretation of denudation rates, it also may provide a possible opportunity to track the transport of landslide sediment. The associated uncertainties are large, but in the Wenchuan case, calculations based on ¹⁰Be mixing proportions suggest that river sediment fluxes in the 2-3 years following the earthquake increased by a similar order of magnitude in the 0.25-1 mm and the <0.25 mm size fractions, as determined from ¹⁰Be_qtz mixing calculations and hydrological gauging, respectively. Such information could provide new insight into sediment transfer, with implications for secondary sediment-related hazards and for understanding the removal of mass from mountains.