In situ 14C-10Be disequilibrium suggests a recent and major denudation event of French Massif Central, despite slow tectonic deformation.
Résumé
Geomorphologists classically compute denudation rates from in situ cosmogenic 10Be concentrations. A major assumption is that denudation rates remain steady during the 10Be integration time scale. But early 14C-10Be data we presented last year at this conference suggested that this is hardly tested in environments slowly deformed by tectonics, with integration time covering thousands of years, and erosion rates from 10 to 100 mm/ky.Here, we extended our 14C-10Be dataset to test recent and substantial shifts in denudation. 14C is more sensitive than 10Be to recent and short-term changes in denudation, because of a shorter half-life (5,700 y versus 1.4 My). Studies (Hippe, 2017; Mudd, 2017; Skov et al., 2019; Hippe et al., 2021) have discussed this application of coupled 14C - 10Be measurements.We carried out in situ 14C measurements on river sand which has available 10Be date (Desormeaux et Al., 2021). The studied mountain range is called Massif Central and is west of the European Alp foreland, in southern France. Elevation is ~700 m on average, with an elevated low-relief surface and a steep escarpment along the Cevennes Fault bordering the Alp foreland. The area has a homogeneous lithology rich in quartz. Past glaciations were of limited extent. There is little space for sediment storage, thin soils, no dams, and presently limited anthropic activity. Massif Central is only impacted by slow tectonic deformation. Landslides are very rare but erosion processes are active.Our new 14C results combined with Desormeaux et al., 2021's 10Be data confirm the substantial 14C-10Be disequilibrium. 14C apparent denudation rates are several times higher than 10Be denudation rates. We explore four end-members which could explain such a disequilibrium. This exploration suggests that only major and recent events in denudation could produce such a disequilibrium, and that the landscape we presently see is rather transient than steady.