Cosmogenic <sup>10</sup>Be in river sediment: where grain size matters and why

Abstract

<p><strong>Abstract.</strong> Concentrations of <i>in situ</i>-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 cover a much wider range. Where ¹⁰Be concentrations depend on grain size, denudation rate estimates based on the sand fraction alone could potentially be 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=62 catchments), comprising catchments with contrasting hillslope angles, climate, lithology and catchment size revealed a similar pattern. Lower ¹⁰Be concentrations in coarse grains (defined as <q>negative grain size dependency</q>) 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°), arid (&amp;lt;100<span class="thinspace"></span>mm<span class="thinspace"></span>yr^&amp;minus;1) and humid catchments (>2000<span class="thinspace"></span>mm<span class="thinspace"></span>yr^&amp;minus;1). Furthermore, we found that an additional cause of negative grain size dependencies may emerge in large catchments with long sediment travel distances (>2300&amp;ndash;7000<span class="thinspace"></span>m, depending on lithology) where abrasion and sediment provenance 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.</p>