Articles | Volume 4, issue 4
Research article
04 Nov 2016
Research article |  | 04 Nov 2016

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

Amanda H. Schmidt, Thomas B. Neilson, Paul R. Bierman, Dylan H. Rood, William B. Ouimet, and Veronica Sosa Gonzalez

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.

Short summary
In order to test the assumption that erosion rates derived from Be-10 are not affected by increases in erosion due to contemporary agricultural land use, we measured erosion rates in three tributaries of the Mekong River. We find that in the most heavily agricultural landscapes, the apparent long-term erosion rate correlates best with measures of modern land use, suggesting that agriculture has eroded below the mixed layer and is affecting apparent erosion rates derived from Be-10.