Cosmogenic 10Be in river sediment: where grain size matters and why

Concentrations of in-situ-produced cosmogenic Be-10 in river sediment are widely used to estimate catchment-average denudation rates. Typically, the Be-10 concentrations are measured in the sand fraction of river sediment. However, the grain size of bedload sediment in most bedrock rivers covers a much wider range. Where Be-10 concentrations depend on grain size, denudation rate estimates based on the sand fraction alone are potentially biased. To date, knowledge about catchment attributes that may induce grain-size-dependent Be-10 concentrations is incomplete or has only been investigated in modelling studies. Here we present an empirical study on the occurrence of grain-size-dependent Be-10 concentrations and the potential controls of hillslope angle, precipitation, lithology, and abrasion. We first conducted a study focusing on the sole effect of precipitation in four granitic catchments located on a climate gradient in the Chilean Coastal Cordillera. We found that observed grain size dependencies of Be-10 concentrations in the most-arid and most-humid catchments could be explained by the effect of precipitation on both the scouring depth of erosion processes and the depth of the mixed soil layer. Analysis of a global dataset of published Be-10 concentrations in different grain sizes (n = 73 catchments) - comprising catchments with contrasting hillslope angles, climate, lithology, and catchment size - revealed a similar pattern. Lower Be-10 concentrations in coarse grains (defined as negative grain size dependency) emerge frequently in catchments which likely have thin soil and where deep-seated erosion processes (e.g. landslides) excavate grains over a larger depth interval. These catchments include steep (> 25 degrees) and humid catchments (> 2000 mm yr(-1)). Furthermore, we found that an additional cause of negative grain size dependencies may emerge in large catchments with weak lithologies and long sediment travel distances (> 2300-7000 m, depending on lithology) where abrasion may lead to a grain size distribution that is not representative for the entire catchment. The results of this study can be used to evaluate whether catchment-average denudation rates are likely to be biased in particular catchments.