Denudation systematics inferred from in situ cosmogenic 10Be concentrations in fine (50-100 μm) and medium (100-250 μm) sediments of the Var River basin, southern French Alps

Marine sedimentary archives are well dated and often span several glacial cycles; cosmogenic Be-10 concentrations in their detrital quartz grains could thus offer the opportunity to reconstruct a wealth of past denudation rates. However, these archives often comprise sediments much finer (< 250 mu m) than typically analyzed in Be-10 studies, and few studies have measured Be-10 concentrations in quartz grains smaller than 100 mu m or assessed the impacts of mixing, grain size, and interannual variability on the Be-10 concentrations of such fine-grained sediments. Here, we analyzed the in situ cosmogenic Be-10 concentrations of quartz grains in the 50-100 and 100-250 mu m size fractions of sediments from the Var basin (southern French Alps) to test the reliability of denudation rates derived from Be-10 analyses of fine sands. The Var basin has a short transfer zone and highly variable morphology, climate, and geology, and we test the impact of these parameters on the observed Be-10 concentrations. Both analyzed size fractions returned similar Be-10 concentrations in downstream locations, notably at the Var's outlet, where concentrations ranged from (4.02 +/- 0.78) x 10(4) to (4.40 +/- 0.64) x 10(4) atoms g(-1) of quartz. By comparing expected and observed Be-10 concentrations at three major river junctions, we interpret that sediment mixing is efficient throughout the Var basin. We resampled four key locations 1 year later, and despite variable climatic parameters during that period, interannual Be-10 concentrations were in agreement within uncertainties, except for one upper subbasin. The Be-10-derived denudation rates of Var subbasins range from 0.10 +/- 0.01 to 0.57 +/- 0.09 mm yr(-1), and spatial variations are primarily controlled by the average subbasin slope. The integrated denudation rate of the entire Var basin is 0.24 +/- 0.04 mm yr(-1), in agreement with other methods. Our results demonstrate that fine-grained sediments (50-250 mu m) may return accurate denudation rates and are thus potentially suitable targets for future Be-10 applications, such as studies of paleo-denudation rates using offshore sediments.