A multi-proxy approach to assess chemical weathering in the Southern French Alps since Marine Isotopic Stage 4

Abstract

Beginning by erosion at the source, through transport and intermediate storage, and to final deposition, sediment experiences various processes modifying its mineralogical composition. Among these processes, chemical weathering is governed by climatic conditions, with a cold and arid climate hindering dissolution or replacement of specific silicate mineral phases compared to a hot and humid setting. It is thus theoretically possible to utilise chemical weathering conditions reconstructed from the sedimentary record as a proxy to past climatic conditions. However, chemical weathering intensity, as determined from the final product of a sedimentary cycle, is strongly dependent on the duration of past exposure to weathering conditions, with shorter residence times in the sedimentary transport system resulting in lower chemical weathering intensities. To address these issues, this study interrogates both a modern river catchment (Var River, southeast France) and its offshore sedimentary equivalent (the Var turbidite system), spanning the Upper Pleistocene to Holocene. In the area, five main river tributaries drain various lithologies of the Southern French Alps carrying sediments from a mountainous landscape to the turbiditic system. This sediment delivery has been mainly controlled over the past ca. 70 kyrs by millennial-scale Dansgaard-Oeschger oscillations, with more frequent turbidite activity during the Last Glacial Maximum (LGM). Additionally, previous work on the area highlighted higher denudation rates during the LGM compared to pre- and post-LGM times. This project uses multiple proxies to reconstruct chemical weathering from both active river sandbars in the Var River catchment and from sediments collected in the adjacent turbiditic system. Bulk rock geochemistry data are used to calculate chemical weathering indices such as the traditional Chemical Index of Alteration and more recent alpha indices. Additionally, heavy minerals apatite and tourmaline are employed in the apatite-tourmaline index, a potential indicator of variations in past chemical weathering conditions. This dataset is compared to previously acquired neodymium isotope provenance data and palaeo-denudation rates (10Be). Whilst assessing the robustness of various chemical weathering techniques, this study also intends to shed light on the LGM impact on sediment delivery and chemical weathering in a mountainous landscape.