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

Abstract

Marine sedimentary archives are well dated and often span several glacial cycles; cosmogenic 10 Be 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 10 Be studies, and few studies have measured 10 Be concentrations in quartz grains smaller than 100 µm or assessed the impacts of mixing, grain size, and interannual variability on the 10 Be concentrations of such fine-grained sediments. Here, we analyzed the in situ cosmogenic 10 Be 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 10 Be 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 10 Be concentrations. Both analyzed size fractions returned similar 10 Be concentrations in downstream locations, notably at the Var's outlet, where concentrations ranged from (4.02±0.78)×10 4 to (4.40±0.64)×10 4 atoms g −1 of quartz. By comparing expected and observed 10 Be 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 10 Be concentrations were in agreement within uncertainties, except for one upper subbasin. The 10 Be-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 µm) may return accurate denudation rates and are thus potentially suitable targets for future 10 Be applications, such as studies of paleo-denudation rates using offshore sediments.Published by Copernicus Publications on behalf of the European Geosciences Union. 1060 A. Mariotti et al.: Denudation systematics inferred from in situ cosmogenic 10 Be concentrations

Times Associated With Source-to-Sink Propagation of Environmental Signals During Landscape Transience

“…A measured parameter represents current conditions within the landscape only once this parameter is fully adjusted to new steady conditions (end of signal duration, D). For example, the 10 Be concentration in fluvially transported sediments are regularly applied as a proxy to estimate catchment averaged denudation rates (Balco and Stone, 2005;Charreau et al, 2011;Mandal et al, 2015;Puchol et al, 2017;Mariotti et al, 2019). A theoretical step change in a catchment averaged denudation rate causes an exponential adjustment in detrital 10 Be concentrations.…”

Hurricanes alter 10Be concentrations in tropical river sediment but do not change regional erosion rate estimates

“…At locations that experience frequent mass movements, such as steep, tectonically active, or heavily agricultural areas, temporal replicability is more frequently poor (Sosa Gonzalez et al, 2017). In contrast, 10 Be i concentrations in the Southern French Alps have been relatively consistent over millennia (Mariotti et al, 2019).…”

River incision, 10Be production and transport in a source-to-sink sediment system (Var catchment, SW Alps)

“…These analyses revealed a larger frequency of turbidite flows and larger Epsilon-Neodymium (ε) values during the Last Glacial Maximum (LGM), which is interpreted as reflecting more intense erosion, especially in the crystalline massif, and larger sediment production during glacial periods. In these cores, 10 Be MS (per gram of quartz) varies between 2e4 and 7e4 at.g −1 , which gives average denudation rates of 0.2 to indicate the location of data constraints provided by surface exposure dating of river polished surfaces (Saillard et al, 2014;Rolland et al, 2017Rolland et al, , 2020Petit et al, 2019;Cardinal et al, 2022), geochemical analyses of submarine sediments (Bonneau et al, 2017;Mariotti et al, 2021), and detrital 10 Be in river sands (Mariotti et al, 2019), respectively. For river polished surfaces, the numbers refer to the following (Cardinal et al, 2022;Rolland et al, 2020Rolland et al, , 2017Petit et al, 2019;Saillard et al, 2014).…”

“…and have been estimated to ~7.7e-4 and ~1.65 for the Var River, respectively. Modern average denudation rates estimates of the Var River catchment from 10 Be measurements in fluvial sediments range between 0.1 and 0.8 mm.yr −1 (Mariotti et al, 2019).…”