Role of Subsurface Brines in Salt Balance: The Case Study of the Caspian Sea and Kara Bogaz Bay

International audienc

Biogeochemical cycling of rare earth elements in surface soils

International audienc

A record of Late Quaternary continental weathering in the sediment of the Caspian Sea: evidence from U–Th, Sr isotopes, trace element and palynological data

International audienc

Local and seasonal climate change and its influence on the hydrological cycle in a mountainous forested catchment

International audienc

U-Th-Ra fractionation in soil horizons of forested ecosystem (Strengbach catchment, France)

International audienc

Lithium isotope systematic in the Strengbach catchment (Vosges, France)

International audienc

Lithium isotope systematics in a forested granitic catchment (Strengbach, Vosges Mountains, France)

International audienceOver the last decade it has become apparent that Li isotopes may be a good proxy to trace silicate weathering. However, the exact mechanisms which drive the behaviour of Li isotopes in surface environments are not totally understood and there is a need to better calibrate and characterize this proxy. In this study, we analysed the Li concentrations and isotopic compositions in the various surface reservoirs (soils, rocks, waters and plants) of a small forested granitic catchment located in the Vosges Mountains (Strengbach catchment, France, OHGE http://ohge.u-strasbg.fr). Li fluxes were calculated in both soil profiles and at the basin scale and it was found that even in this forested basin, atmospheric inputs and litter fall represented a minor flux compared to input derived from the weathering of rocks and soil minerals (which together represent a minimum of 70% of dissolved Li). Li isotope ratios in soil pore waters show large depth dependent variations. Average dissolved d7Li decreases from _1.1&to _14.4& between 0 and _30 cm, but is +30.7&at _60 cm. This range of Li isotopic compositions is very large and it encompasses almost the entire range of terrestrial Li isotope compositions that have been previously reported. We interpret these variations to result from both the dissolution and precipitation of secondary phases. Large isotopic variations were also measured in the springs and stream waters, with d7Li varying from +5.3& to +19.6&. d7Li increases from the top to the bottom of the basin and also covaries with discharge at the outlet. These variations are interpreted to reflect isotopic fractionations occurring during secondary phase precipitation along the water pathway through the rocks. We suggest that the dissolved d7Li increases with increasing residence time of waters through the rocks, and so with increasing time of interaction between waters and solids. A dissolution precipitation model was used to fit the dissolved Li isotopic compositions. It was found that the isotopic compositions of springs and stream waters are explicable by an isotopic fractionation of _5&to _14& (best fit _10.8&), in agreement with Li incorporation into clay. In soil solutions, it was found that isotopic fractionation during secondary precipitation is larger (at least _23&), suggesting a major role for different secondary phases, such as iron oxides that maybe incorporate Li with a higher isotope fractionation

Chemical and isotopic (87Sr/86Sr, δ18O, δD) constraints to the formation processes of Red-Sea brines

International audienceAbout twenty deeps filled with hot brines and/or metalliferous sediments, are located along the Red-Sea axis. These brines present a well-suited framework to study the hydrothermal activity in such a young ocean. The present study outlines the results of a geochemical approach combining major-, trace-element and isotopic (oxygen, hydrogen, strontium) analyses of brines in six of the deeps, to evaluate different processes of brine formation and to compare the evolution of each deep. Important heterogeneities in temperature, salinity, hydrographic structure and chemistry are recorded, each brine having its own characteristics. The intensity of hydrothermal circulation varies among the deeps and ranges from being strong (Atlantis II and Nereus) to weak (Port-Soudan) and even to negligible (Valdivia and Suakin) and it varies along the entire Red-Sea axis. These observations do not favour a unique formational model for all of the brines. For example, the brines of the Suakin deep appear to have been formed by an old sea water which dissolved evaporite beds, without significant fluid circulation and hydrothermal input, while others such as Atlantis II or Nereus Deeps appear to be dominated by hydrothermal influences. A striking feature is the absence of a relationship between the position of the deeps along the axis and their evolutionary maturity