Quaternary incision dynamics of the western to central Alpine valleys from cave systems investigations

Plio-Quaternary global climate changes had major impacts on landscape dynamics and relief evolution worldwide. The Quaternary onset and intensification of the glaciation in the European Alps greatly reshaped the mountainous reliefs with deep glacial carving of the modern main valley systems. Quantifying this climate forcing on the long-term relief evolution is challenging because of the poor preservation of the surface geomorphic markers in a context of strong landscape rejuvenation. Previous studies have shown that a major incision phase occurred for the Aare and upper Rhône valleys (Switzerland) since the mid-Pleistocene transition (onset of the 100-ka glacial-interglacial cycles). But the dynamics of this incision phase remains poorly constrained in both time and space across the Alpine realms. Moreover, the Pliocene to Lower Pleistocene Alpine relief dynamics is still largely unknown. To fill this current knowledge gap, we study cave systems in karst environments which are widespread in the frontal part of the western to central Alps. Karst network development and associated cave sediment records are closely coupled with valley evolution and can be preserved for timescales of million years. They are therefore ideal proxies for quantifying long-term relief dynamics. This study focuses on 3 cave systems nearby the Isere valley (western French Alps) and 1 cave system at the head of the Sarine valley (central Swiss Alps). We apply a multi-method approach that combines 3D analysis of the cave networks with geochronological data on both the detrital sediments (26Al/10Be burial dating and paleomagnetism) and speleothems (U/Th and U/Pb dating). Our results show a significant development of the major Alpine cave systems during the Pliocene, in agreement with previous studies. The abandonment of the perched networks around the Isère valley highlights a first incision phase in the frontal part of the Alps at the Pliocene-Quaternary transition. The apparently later abandonment of the cave system in the upper Sarine valley (~1.8 Ma) suggests an apparent lag in the incision onset for the upper Alpine watersheds. The main incision phase of the Isère valley to its modern base level (i.e. not considering the overdeepened section) took place in the early Middle Pleistocene from ~800 ka up to 450 ka, therefore occurring over only few glacial cycles. Our results imply thus a rapid response time (i.e. few 100 ka) of the major Alpine glacial valleys physiography to the Plio-Quaternary climatic forcing.