Climate-driven fluxes of organic-bound uranium to an alpine lake overthe Holocene

Pierre Lefebvre (1) , Pierre Sabatier (2) , Arnaud Mangeret (3) , Alkiviadis Gourgiotis (3) , Pierre Le Pape (1) , Anne-Lise Develle (2) , Pascale Louvat (4) , Olivier Diez (3) , Jean-Louis Reyss (2) , Jérôme Gaillardet (4) , Charlotte Cazala (3) , Guillaume Morin (5)

1 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie
2 EDYTEM - Environnements, Dynamiques et Territoires de Montagne
3 IRSN/PSE-ENV/SEDRE/LELI - Laboratoire de recherche sur le devenir des pollutions de sites radioactifs
4 IPGP (UMR_7154) - Institut de Physique du Globe de Paris
5 IMPMC_MINENV - Minéralogie Environnementale [IMPMC]

Uranium (U) isotopic signatures and concentration in sediments are widely used as paleo-redox proxies, as the behavior of U is often controlled by bottom water oxygenation. Here, we investigated the processes controlling U accumulation in the sediments of Lake Nègre (Mediterranean Alps, South-East France) over the past 9200 years. Exceptionally high natural U concentrations (350-1250 μg.g-1) allowed the measurement of U along with other elements by high-resolution X-Ray Fluorescence core-scanning. Weathering and erosion proxies (Ti content, Zr/Al and K/Ti ratios) indicate that sedimentary inputs were controlled by Holocene climatic variations. After a period of low erosion during the Holocene Climatic Optimum, a major regime shift was recorded at 4.2 kyr BP when terrigenous fluxes consistently increased until present with high sensitivity to centennial-scale climatic events. Sedimentary organic matter (OM) inputs were dominated by terrigenous OM from the catchment soils until 2.4 kyr BP, as attested by carbon to nitrogen (C/N) and bromine to organic carbon (Br/TOC) ratios. From 2.4 kyr BP to present, lake primary production and soils equally contributed to sedimentary OM. Uranium fluxes to the sediments were well correlated to terrigenous OM fluxes from 7 kyr BP to present, showing that U supply to the lake was controlled by U scavenging in the soils of the watershed followed by transport of U bound to detrital organic particles. Higher U/OM ratios before 7 kyr BP likely reflect the development of the upstream wetland. The fluctuations of U sedimentary inputs appear to be independent of bottom water oxygenation, as estimated from constant Fe/Mn ratios and δ238U isotopic signatures, and rather controlled by the production, erosion and sedimentation of terrigenous OM. This finding confirms that the use of U (and potentially other metals with high affinity to OM) concentrations alone should be used with caution for paleo-redox reconstructions.

Domaines: Sciences de l'environnement; Géochimie

Pierre Lefebvre, Pierre Sabatier, Arnaud Mangeret, Alkiviadis Gourgiotis, Pierre Le Pape, et al.. Climate-driven fluxes of organic-bound uranium to an alpine lake overthe Holocene. Science of the Total Environment, 2021, 783, pp.146878. ⟨10.1016/j.scitotenv.2021.146878⟩. ⟨hal-03211623⟩