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<p>Marine sedimentary archives are well dated and often span several glacial cycles; cosmogenic <span class="inline-formula">¹⁰Be</span> 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 (<span class="inline-formula">&lt;250</span>&thinsp;<span class="inline-formula">µ</span>m) than typically analyzed in <span class="inline-formula">¹⁰Be</span> studies, and few studies have measured <span class="inline-formula">¹⁰Be</span> concentrations in quartz grains smaller than 100&thinsp;<span class="inline-formula">µ</span>m or assessed the impacts of mixing, grain size, and interannual variability on the <span class="inline-formula">¹⁰Be</span> concentrations of such fine-grained sediments. Here, we analyzed the in situ cosmogenic <span class="inline-formula">¹⁰Be</span> concentrations of quartz grains in the 50–100 and 100–250&thinsp;<span class="inline-formula">µ</span>m size fractions of sediments from the Var basin (southern French Alps) to test the reliability of denudation rates derived from <span class="inline-formula">¹⁰Be</span> 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 <span class="inline-formula">¹⁰Be</span> concentrations. Both analyzed size fractions returned similar <span class="inline-formula">¹⁰Be</span> concentrations in downstream locations, notably at the Var's outlet, where concentrations ranged from <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>(</mo><mn mathvariant="normal">4.02</mn><mo>±</mo><mn mathvariant="normal">0.78</mn><mo>)</mo><mo>×</mo><msup><mn mathvariant="normal">10</mn><mn mathvariant="normal">4</mn></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="91pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="aebcf34e8c69a9594bf1900d9f6c1bce"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="esurf-7-1059-2019-ie00001.svg" width="91pt" height="15pt" src="esurf-7-1059-2019-ie00001.png"/></svg:svg></span></span> to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>(</mo><mn mathvariant="normal">4.40</mn><mo>±</mo><mn mathvariant="normal">0.64</mn><mo>)</mo><mo>×</mo><msup><mn mathvariant="normal">10</mn><mn mathvariant="normal">4</mn></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="91pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="b28fe01a0209b602a275e5d0e96f5c22"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="esurf-7-1059-2019-ie00002.svg" width="91pt" height="15pt" src="esurf-7-1059-2019-ie00002.png"/></svg:svg></span></span>&thinsp;atoms&thinsp;g<span class="inline-formula">⁻¹</span> of quartz. By comparing expected and observed <span class="inline-formula">¹⁰Be</span> concentrations at three major river junctions, we interpret that sediment mixing is efficient throughout the Var basin. We resampled four key locations 1&thinsp;year later, and despite variable climatic parameters during that period, interannual <span class="inline-formula">¹⁰Be</span> concentrations were in agreement within uncertainties, except for one upper subbasin. The <span class="inline-formula">¹⁰Be</span>-derived denudation rates of Var subbasins range from <span class="inline-formula">0.10±0.01</span> to <span class="inline-formula">0.57±0.09</span>&thinsp;mm&thinsp;yr<span class="inline-formula">⁻¹</span>, and spatial variations are primarily controlled by the average subbasin slope. The integrated denudation rate of the entire Var basin is <span class="inline-formula">0.24±0.04</span>&thinsp;mm&thinsp;yr<span class="inline-formula">⁻¹</span>, in agreement with other methods. Our results demonstrate that fine-grained sediments (50–250&thinsp;<span class="inline-formula">µ</span>m) may return accurate denudation rates and are thus potentially suitable targets for future <span class="inline-formula">¹⁰Be</span> applications, such as studies of paleo-denudation rates using offshore sediments.</p>