Influence of topography and human activity on apparent in situ <sup>10</sup>Be-derived erosion rates in Yunnan, SW China

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.

Spatial and temporal replicability of meteoric and in situ10Be concentrations in fluvial sediment

“…Over centuries to millennia, this would lead to a decrease in 10 Be i concentration on slopes and an apparent increase in erosion rates for agricultural areas compared to non-agricultural areas (Von Blanckenburg et al, 2004). Prior research suggests that this is the case in the more heavily agricultural parts of our study region (Schmidt et al, 2016).…”

“…Similarly, increased precipitation, particularly concentrated in the monsoon season (Yang et al, 2010), could also increase the likelihood of debris flows and landslides, episodically altering the isotopic concentration of channel sediment (Niemi et al, 2005;Binnie et al, 2006;Savi et al, 2014). Upstream agricultural land use increases modern sediment yield relative to long-term rates of sediment generation (Hewawasam et al, 2003;Schmidt et al, 2016) through increased linear erosional features or widespread sheetwash (Von Blanckenburg et al, 2004). Such rapid sediment transfer mechanisms could increase sediment yield from one part of the basin only during certain times of the year, thus seasonally affecting the isotopic concentration of in channel sediment.…”

“…Because many landscapes in China have been disturbed by natural tectonic and climatic driven surface processes as well as human impacts on the landscape, it is a challenging place to apply the 10 Be method and so provides, in some ways, a 'worst case' test of replicability; however, China is not the only highly disturbed landscape in which cosmogenic nuclides have been measured. Many regions where 10 Be has been used for geomorphic analysis are tectonically active (Lupker et al, 2012;West et al, 2014), steep (Portenga et al, 2015), subject to extreme rainfall events (Foster and Anderson, 2016) and/or have experienced widespread land use change (Hewawasam et al, 2003;Reusser and Bierman, 2010;Schmidt et al, 2016). By performing spatial and temporal replicate analyses in a setting where many factors are likely to increase the temporal and spatial heterogeneity of analyte concentration in sediment, our results illustrate the limitations of detrital sediment studies and inform sampling strategies not only for cosmogenic nuclides but also for all geochemical detrital sediment analyses.…”

Characterizing landscape‐scale erosion using 10Be in detrital fluvial sediment: Slope‐based sampling strategy detects the effect of widespread dams

“…Res., 53, 4476-4486, doi:10.1002 and biological stirring [e.g., Jungers et al, 2009]. These data have been used to understand the relationship between erosion rate and climate, tectonic setting, and lithology [e.g., Matmon et al, 2003b;Portenga and Bierman, 2011;Schmidt et al, 2016;Kirchner et al, 2001;Schaller et al, 2001]. Because the method is both expensive and time consuming, some regional studies rely on a limited number of 10 Be measurements made in subbasins and averaged, sometimes using area weighting, to generate a landscape-scale, average erosion rate [e.g., Lupker et al, 2012].…”

“…Large dams with high trap efficiencies [Brune, 1953] by design impede the flow of water and sediment; thus, the interpretation of 10 Be concentrations as basin-scale erosion rates, when measured in sediment collected from channels below such dams, is widely known to be uncertain [Schmidt et al, 2016]. The uncertainty arises because the sampled material may not represent that carried into the reservoir from upstream.…”