Geochemical insights into spatial and temporal evolution of sediment at catchment scale (Egoutier stream, France)

International audience; The transfer and storage of chemical elements in particulate matter are controlled by physical, chemical and biological processes. Their dynamics are well understood, especially in environments that show strong tectonic and/or geomorphic pressures, and are frequently reconstructed over centuries or decades to evaluate climate change effects for example. However, observations carried out at high spatial and temporal resolutions are less common, particularly in lowland areas. The present study aimed to better understand the links between earth surface processes, such as soil weathering and erosion, by combining pedology, geochemistry and hydrology at a small catchment scale. It focuses on the transfer of selected chemical elements (Ca, K, Ti, Fe, Mn, Rb and Sr) associated with particulate erosion and transport. The first (topsoil) and last (subsoil) soil horizons from different soil profiles and stream bed-load sediments of the Egoutier catchment (8 km2, Loiret, France) were studied. The objective was to investigate the relationship between the weathering and erosion processes. The soils are developed on ancient alluvium as evidenced by their physical and chemical properties, which clearly displayed a weathering profile from bedrock to topsoil layers. Ca, Fe and Rb showed migration dynamics from the topsoils to the subsoils, whereas Mn appeared immobilized within topsoils. Enrichments of all investigated elements increased in bed-load sediments. Topsoil erosion is the main source of sediment. However, Fe and Rb enrichments also highlight the remobilization of subsoils during periods of enhanced precipitation. Furthermore, Fe and Rb enrichments show a selective erosion of clay minerals. K and Sr contents evidence mineral break-up processes favoring mechanical erosion of all mineral phases. Moreover, their enrichments can be interpreted as the result of higher erosion rates and deposition during high rainfall events. The same conclusion can be drawn for Mn and Ca although their spatial variabilities depend on contributions from runoff in urban areas, as rainwaters from anthropized areas supply Ca mineral phases and diluted Mn depleted bearing phases from topsoil layers.