Can 14C-10Be detect transient patterns of denudation ? Application to the French Massif Central.

Denudation rates are routinely derived from concentrations of terrestrial in situ produced cosmogenic nuclides (TCN), particularly from ¹⁰Be concentrations in river sand. Denudation rates are calculated assuming that they remain steady throughout the integration time scale of the TCN. However, such an assumption is possibly unverified in settings with negligible tectonics, where rates typically range from 10 to 100 mm/ky. In those settings, the TCN conveys a signal that integrates denudation over a time span longer than a few thousand years. The signal may include periods when anthropogenic and climatic forcing on denudation was distinct from modern times. For instance, agricultural practices were limited before 6,000 years B.P. and climatic conditions were colder and drier before 10,000 years B.P. A variable forcing may produce variable and transient denudation rates. In that case, the assumption of steady denudation rates is invalid, and their derivation may introduce a bias.

To detect transient landscapes and resolve such a bias, we can take advantage of the different sensitivity of the ¹⁴C and ¹⁰Be TCNs to recent and short-term changes in surface denudation. In situ ¹⁴C is more sensitive than ¹⁰Be to such changes, because of a shorter half-life (5,700 y compared to 1.4 My). This potential application of coupled ¹⁴C - ¹⁰Be measurements has recently been discussed in several theoretical studies (Hippe, 2017; Mudd, 2017; Skov et al., 2019). Despite the improvement of ¹⁴C extraction lines and measurement facilities (Hippe et al., 2009; Lupker et al., 2019), sensitivity tests remain limited on natural cases (Hippe et al., 2012).

Here, we propose assessing this new application by in situ ¹⁴C - ¹⁰Be measurements on river sand from the Cevennes and the Monts Margeride within the Variscan Massif Central in France. With an average elevation of ~700 m, this mountain range presents an asymmetrical topography, composed of a low-relief surface reaching 1,700 m, and bordered by a gently sloping flank to the west and a steep escarpment to the southeast, along the Cevennes fault. This escarpment receives frequent and seasonal extreme precipitation events (300-700 mm in 48h) on its southeast flank.

The range is subject to very limited seismic activity and appears relevant for an application of the ¹⁴C-¹⁰Be couple. Basins are rich in quartz and have homogeneous lithology. The recent paleoclimatic context is well constrained, with substantial climatic variations but with limited Pleistocene glaciations (e.g. Fauquette et al., 1999; Magny et al., 2003; Mayewski et al., 2004). The Massif Central is subject to active erosion processes, without major contribution from stochastic events such as landslides. Denudation rates are in the range of the theoretical study of Skov et al. 2019 (Schaller et al. 2001; Molliex et al. 2016; Olivetti et al. 2016; Desormeaux et al., 2021) and several studies have suggested transient denudation patterns (Schaller et al. 2001; Olivetti et al. 2016). With our new measurements, we will verify whether the ¹⁴C-¹⁰Be couple has sufficient resolution to detect such transience in natural cases.