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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 01 Aug 2019

Submitted as: research article | 01 Aug 2019

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Earth Surface Dynamics (ESurf).

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

Apolline Mariotti1, Pierre-Henri Blard1, Julien Charreau1, Carole Petit2, Stéphane Molliex3,1, and Team ASTER4 Apolline Mariotti et al.
  • 1CRPG, CNRS - Université de Lorraine, UMR 7358, 54500 Vandoeuvre-lès-Nancy, France
  • 2Geoazur, Université de Nice, 06905 Sophia Antipolis, France
  • 3Laboratoire Géosciences Océan, Institut Universitaire Européen de la Mer, 29280 Plouzané, France
  • 4CEREGE, Laboratoire LN2C, CNRS Université Aix Marseille, 13090 Aix-en-Provence, France

Abstract. Marine sedimentary archives are well dated and often span several glacial cycles; cosmogenic 10Be 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 µm) than typically analyzed in 10Be studies, and few studies have measured 10Be concentrations in quartz grains smaller than 100 μm or assessed the impacts of mixing, grain size and interannual variability on the 10Be concentrations of such fine-grained sediments. Here, we analyzed the in situ cosmogenic 10Be concentrations of quartz grains in the 50–100 and 100–250 μm size fractions of sediments from the Var basin (southern French Alps) to test the reliability of denudation rates derived from 10Be 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 10Be concentrations. Both analyzed size fractions returned similar 10Be concentrations in downstream locations, notably at the Var's outlet, where concentrations ranged from (4.02 ± 0.78) × 104 to (4.40 ± 0.64) × 104 atoms gquartz−1. By comparing expected and observed 10Be concentrations at three major river junctions, we interpret that sediment mixing is efficient throughout the Var basin. We resampled four key locations one year later, and despite variable climatic parameters during that period, interannual 10Be concentrations were in agreement within uncertainties, except for one upper subbasin. The 10Be-derived denudation rates of Var subbasins range from 0.10 ± 0.01 mm yr−1 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 µm) return accurate denudation rates and are thus suitable targets for future 10Be applications, such as studies of paleo-denudation rates using offshore sediments.

Apolline Mariotti et al.
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Status: final response (author comments only)
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Apolline Mariotti et al.
Apolline Mariotti et al.
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Short summary
This work is the first assessment of the suitability of the in situ 10Be method to determine denudation rates from fine (50–100 μm) detrital quartz at the watershed scale. This method is used worldwide to determine denudation rates from sand grained sediments (250 μm–1 mm). We show that in the Var catchment, fine-grained sediments (50–100 μm) are suited to the 10Be method, which is vital for future applications of 10Be in sedimentary archives such as offshore sediments.
This work is the first assessment of the suitability of the in situ 10Be method to determine...