Abstract
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.
Highlights
We used a field study in Chile and a global compilation of previously published data to assess in what types of catchments grain-size-dependent 10Be concentrations may lead to biased estimates in catchment-average denudation rates
Our results suggest that mean basin slope and mean annual precipitation (MAP) control grain-size-dependent 10Be concentration through their effect on the presence and thickness of a mixed soil layer and the depth of erosion processes
Our global compilation results show that the influence of MAP is limited to humid catchments (> 2000 mm yr−1), whereas our case study in Chile suggests an additional control in arid catchments (< 100 mm yr−1)
Summary
Catchment-average denudation rates are commonly estimated with in-situ-produced cosmogenic 10Be concentrations in river sediment (Bierman and Steig, 1996; Brown et al, 1995; Granger et al, 1996). 10Be is a rare isotope that is produced within quartz minerals by high-energy cosmic rays in the upper few metres of the Earth’s surface (Gosse and Phillips, 2001). R. van Dongen et al.: Cosmogenic 10Be in river sediment only if it is spatially and temporally representative for all erosion sources within the catchment (Bierman and Steig, 1996; von Blanckenburg, 2005; Brown et al, 1995; Gonzalez et al, 2017; Granger et al, 1996; Neilson et al, 2017; Willenbring et al, 2013) Evaluating this condition is challenging and requires a detailed understanding of the catchment and its erosion processes. Which types of catchments are sensitive to grain-size-dependent 10Be concentrations and biased catchment-average denudation rates remain elusive (e.g. Carretier et al, 2015), or they have only been addressed in modelling studies (e.g. Lukens et al, 2016)
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