Abstract
Abstract. In order to understand better if and where erosion rates calculated using in situ 10Be are affected by contemporary changes in land use and attendant deep regolith erosion, we calculated erosion rates using measurements of in situ 10Be in quartz from 52 samples of river sediment collected from three tributaries of the Mekong River (median basin area = 46.5 km2). Erosion rates range from 12 to 209 mm kyr−1 with an area-weighted mean of 117 ± 49 mm kyr−1 (1 standard deviation) and median of 74 mm kyr−1. We observed a decrease in the relative influence of human activity from our steepest and least altered watershed in the north to the most heavily altered landscapes in the south. In the areas of the landscape least disturbed by humans, erosion rates correlate best with measures of topographic steepness. In the most heavily altered landscapes, measures of modern land use correlate with 10Be-estimated erosion rates but topographic steepness parameters cease to correlate with erosion rates. We conclude that, in some small watersheds with high rates and intensity of agricultural land use that we sampled, tillage and resultant erosion has excavated deeply enough into the regolith to deliver subsurface sediment to streams and thus raise apparent in situ 10Be-derived erosion rates by as much as 2.5 times over background rates had the watersheds not been disturbed.
Highlights
Understanding the source and volume of sediment moving across the landscape, and the role of humans in sediment generation and transport, are fundamental issues in earth science (NRC, 2012)
Considering the study area as a whole, erosion rates are significantly and positively correlated with mean basin slope, mean local relief, median ksn, the fraction of the watershed covered by forest, and the fraction of the watershed covered by grassland or shrubland (R2 ≥ 0.18, p < 0.05, Fig. 6)
In the Weiyuan watershed, erosion is significantly correlated with median ksn, mean annual precipitation (MAP), the fraction of the upstream watershed that is agriculture, the fraction of the upstream watershed that is forested, mean basin slope, and mean local relief (R2 ≥ 0.20, p < 0.05); the relationship is positive for all terms except MAP and the fraction of the upstream watershed that is forested
Summary
Understanding the source and volume of sediment moving across the landscape, and the role of humans in sediment generation and transport, are fundamental issues in earth science (NRC, 2012). In situ 10Be (10Bei; t1/2 = 1.39 My) (Chmeleff et al, 2009), measured in detrital quartz sand, is widely used (Portenga and Bierman, 2011) to estimate basin-wide erosion rates integrated over 103–105 years (Brown et al, 1995; Bierman and Steig, 1996; Granger et al, 1996). The technique is useful even in areas disturbed by people (Brown et al, 1995, 1998; Granger et al, 1996; Bierman and Steig, 1996; Hewawasam et al, 2003; Reusser et al, 2015; Vanacker et al, 2007; West et al, 2015), because human activity does not typically erode sediment below the mixed layer over the scale of sampled watersheds.
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