Grain-size dependencies in fallout radionuclide activity have been attributed to either increase in specific surface area in finer grain sizes or differing mineralogical abundances in different grain sizes. Here, we consider a third possibility, that the concentration and composition of grain coatings, where fallout radionuclides reside, controls their activity in fluvial sediment. We evaluated these three possible explanations in two experiments: 1) we examined the effect of sediment grain size, lithology, mineralogy, and composition of the acidextractable materials on the distribution of 7Be, 10Be, 137Cs, and unsupported 210Pb in detrital sediment samples collected from rivers in China and the United States, and 2) we periodically monitored 7Be, 137Cs, and 210Pb retention in samples of known composition exposed to natural fallout in Ohio, USA for 294 days.Acid-extractable materials (made up predominately of Fe, Mn, Al, and Ca from secondary minerals and grain coatings produced during pedogenesis) are positively related to the abundance of fallout radionuclides in our sediment samples. Grain-size dependency of fallout radionuclide concentrations was significant in detrital sediment samples, but not in samples exposed to fallout under controlled conditions. Mineralogy had a large effect on 7Be and 210Pb retention in samples exposed to fallout, suggesting that sieving sediments to a single grain size or using specific surface area-based correction terms may not completely control for preferential distribution of these nuclides.We conclude that time-dependent geochemical, pedogenic, and sedimentary processes together result in the observed differences in nuclide distribution between different grain sizes and substrate compositions. These findings likely explain variability of measured nuclide activities in river networks that exceeds the variability introduced by analytical techniques as well as spatial and temporal differences in erosion rates and processes. In short, we suggest that presence and amount of pedogenic grain coatings is more important than either specific surface area or surface charge in setting the distribution of FRNs. Cs, and 210 Pb in river sediment (GCA manuscript GCA-D-16-0143R2).We are again appreciative of the through and thoughtful review from one reviewer who has previously reviewed the manuscript, particularly with respect to measurements of 7 Be in the "natural delivery" experiment. The reviewer has helped us to clarify the main points of that experiment and to better explain the sources of error in those data. We have made minor changes to the manuscript to reflect these concerns.Below we have enclosed a detailed response point by point to each reviewer concern. Reviewer comments are in black and our responses are in blue.The manuscript is now 7,239 words from the introduction through the acknowledgements and includes a 308 word abstract, 8 figures, 1 table, 66 references, and supporting information including text, 13 figures, and 11 tables. This manuscript is not under con...

Agricultural land use doubled sediment loads in western China's rivers Author information removed for double-blind review.

Using measurements of in situ and meteoric 10 Be in fluvial sand to measure erosion rates, quantify soil loss, and trace sediment sources and sinks relies on the assumption that such sediment is well-mixed and representative of the upstream area. We test this assumption at 13 river junctions in three tributary watersheds (200-2500 km 2) to the Mekong River, Yunnan, China, where human alteration of the landscape is significant and widespread. We find that two of the three watersheds mix well for in situ 10 Be and only one mixes well for meteoric 10 Be when considering the concentration of 10 Be at the outlet compared to the areaweighted mean of headwater samples. We also assessed mixing at 13 river junctions by comparing the erosion rate-weighted isotopic concentration of sediment taken from tributaries upstream of a junction to the concentration in a sample taken downstream of the junction. With this metric, mixing is generally poor for both in situ and meteoric 10 Be but is better for in situ 10 Be than for meteoric 10 Be (p < 0.05). This is likely because in situ 10 Be is measured in quartz,

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.

Schmidt et al. present an interesting study in which they aim to evaluate the effects of human activity on long-term apparent erosion rates calculated from measurements of in-situ 10Be collected from river sediments in a nested sampling scheme across three watersheds in Yunnan, China. The authors choose three watersheds differing in size across covarying gradients of elevation, precipitation, channel steepness and land use. The authors present the results of correlation and regression analyses of in-situ 10Be-derived long-term apparent erosion rates calculated using the CRONUS-Earth online calculator with topographic (basin average slope, normalized channel steepness and relief/elevation), climatic (mean annual precipitation), and land use/land cover variables.Through this analysis, the authors present results which suggest that in-situ 10Be derived apparent erosion rates are related predominantly to topographic variables in two C1 of the three watersheds (Yongchun and Weiyuan) which are not dominated by human land-use), while apparent erosion rates are not correlated with topographic variables (and instead correlated with area in agricultural land use) for a third watershed (Nankai) which was heavily dominated by human land-use. Further regression analysis suggests that human land-use may play a secondary but not controlling role in the Weiyuan watershed (which lies in the middle of the land use gradient from most intensive human land-use (Nankai) to least intensive human land use (Yongchun).The authors conclude that in-situ 10Be apparent erosion rates can be significantly affected by human land-use at the basin scale and suggest caution to researchers attempting to derive long term apparent erosion rates in watersheds which have undergone significant human land-use change, are not large enough to buffer the effects of land-use change on sediment composition, and are heavily disturbed.