Clay mineral response to hyperthermal events in coastal settings: new insights from Eocene deposits in the Paris Basin.

Clay mineral assemblages can provide valuable insights into weathering conditions in marine or lacustrine environments, but littoral and fluvial environments corresponding to transit zones can be questioned about their ability to record a climate signal [1]. These environments, influenced by climate and sea-level fluctuations, as well as rapid palaeogeographical changes, present optimal conditions for the neoformation, transport, and sedimentation of clays. Understanding the mechanisms affecting clay minerals in coastal and fluvial environments is therefore crucial for interpreting the mineralogical signal in the sediments of distal basins in terms of climate in a source-to-sink approach. In this study, we combine more than 1 400 clay mineralogy (X-Ray Diffraction and electron microscopy) and organic carbon isotope analyses to investigate the impacts of the Eocene hyperthermal events in the coastal settings of the Paris Basin. We propose the first high-resolution synthesis of the climatic and weathering evolution of the Paris Basin during the early-to-middle Eocene, providing valuable insights on the impacts of hyperthermal events on clay mineralogy in coastal settings. Our results first provide evidence of the local transformation of smectite to kaolinite within soils during the Paleocene-Eocene Thermal Maximum (PETM). This process, evidenced by interstratified kaolinite-smectite, reflects tropical conditions in the Paris Basin during the PETM favouring strong chemical weathering. During the Early Eocene Climatic Optimum (EECO), enhanced hydrological conditions evidenced by clay minerals are however hindered by differential settling mechanisms highlighted in estuarine environments. Kaolinite formed in soils during pronounced hydrolysing conditions and transported during high runoff events are only observed in the proximal part of the estuary while smectite and glauconite predominates in the more distal part. During the Middle Eocene Climatic Optimum (MECO), rapidly changing hydrolysing conditions successively favour either runoff as evidenced by detrital kaolinite, or chemical weathering, when smectites predominate. More arid conditions however promote complex interactions between microbial activity, dolomite formation, and the neoformation of fibrous clays in evaporative settings, especially during the Late Lutetian Thermal Maximum (LLTM).