Inner gorges incision history: A proxy for deglaciation? Insights from Cosmic Ray Exposure dating (10Be and 36Cl) of river-polished surfaces (Tinée River, SW Alps, France)

Abstract

10Be and 36Cl Cosmic Ray Exposure (CRE) dating performed on river polished surfaces of river gorges in a mountain-to-sea river system in the French SW Alps highlights transient erosional events involving incision rates >10 mm a−1. These events took place during the last two major deglaciation phases following (1) the Last Glacial Maximum (LGM) at 16–14 ka, (2) the Younger Dryas at 8–11 ka, and during the warm and humid Holocene climatic optimum at 4–5 ka. These periods of high incision rates (3–>30 mma−1) alternated with periods of low incision rates (<1 mm a−1), which probably correspond to a long-term equilibrium between incision and relative uplift. The Alpine river staircase shape profiles evidence local and transient responses that are ascribed to cumulate disequilibrium after the long-time-spanned glaciations. After each glaciation, rivers rush down to get closer to their equilibrium profile. Incision is amplified both by the sediment discharge due to the erosion of moraines and by landslides triggered by the glacier retreat.