Analysis of U-series Nuclides and in situ Be in a Weathering Profile from the Strengbach Catchment (Vosges Massif, France)

Abstract:

Recent studies underline the potential of U series nuclides to quantify the regolith production rate in weathering profiles (e.g.Chabaux et al, 2013; Ma et al., 2012). However, the quantification of regolith production rate in temperate climates encountered some limitations when analyzing only superficial samples in the first meter of soil (Rihs et al, 2011). Here, we propose to combine the analysis of U-Th-Ra disequilibrium with the cosmogenic in situ Be in a weathering profile extending from the top soil to the granitic fractured bedrock at 2 m of depth to better estimate both production and denudation rate of regolith.

The weathering profile is located on the top of the watershed and was sampled with a high spatial resolution (15 samples). Whole-rock data show trends of variation for major and trace element concentrations and for U-Th-Ra disequilibria, which are different in the upper part of the regolith (0-80 cm) and in the deeper part of the fractured saprolith and/or bedrock (100cm-200cm). U-series isotopes in the surface of the profile are difficult to interpret in terms of weathering rate, while the disequilibria in the deeper weathered bedrock show a smooth trend of variation. The modeling of the U-Th-Ra data in this deeper part of the profile, performed with a nuclide gain and loss model, leads to a regolith production rate of 35 ± 10 T/km²/year. In addition, a numerical optimization for nonlinear inverse problem has been performed to estimate the exposure age and the mean denudation rate at the summit from the Be data. The results show that the exposure age of the profile is about 20 000 years and the mean denudation rate is 37 ± 15 T/km²/year. The consistency between the regolith production rate and the soil denudation rate suggests therefore that in such a temperate context, the long-term mass balance of soil developed on granitic bedrock would be close to a steady-state.