Abstract. Concentrations of in-situ-produced
cosmogenic 10Be in river sediment are
widely used to estimate catchment-average denudation rates. Typically, the
10Be 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 10Be 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 10Be concentrations is incomplete or
has only been investigated in modelling studies. Here we present an
empirical study on the occurrence of grain-size-dependent 10Be
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 10Be 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 10Be concentrations in
different grain sizes (n=73 catchments) – comprising catchments with
contrasting hillslope angles, climate, lithology, and catchment size – revealed
a similar pattern. Lower 10Be 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∘) 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.
Cosmogenic <sup>10</sup>Be in river sediment: where grain size matters and why
