Marie-Claire Pierret

Transfer of rare earth elements (REE) from natural soil to plant systems: implications for the environmental availability of anthropogenic REE

Background and aimsRare Earth Elements (REE) are widely used to trace natural geochemical processes. They are also increasingly used by man (electronics industry, medicine, agriculture) and therefore considered as emerging pollutants. The present study documents REE mobility in non-polluted natural soil-plant systems in order to characterize their environmental availability for future anthropogenic pollution.MethodsThe study is based on a field approach in non-polluted natural sites with contrasting geological environments (limestone, granite, and carbonatite) and highly variable REE contents.ResultsREE concentrations in soils do not directly reflect bedrock concentrations, but depend largely on pedogenetic processes and on the mineralogy of bedrock and soil. The soils of all sites are with respect to bedrock enriched in heavy REE. The REE uptake by plants is not primarily controlled by the plant itself, but depends on the concentration and the speciation in the soil and the adsorbed soil water pool.ConclusionsREE uptake by plant roots are linked with those of Fe. Roots absorb preferentially the light REE. Before translocation, REE are retained by the Casparian strip leading to much lower concentrations in the aerial parts. The transport of the REE within the xylem is associated with the general nutrient flux.

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

Abstract About 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.

Crystallization conditions of fundamental particles from mixed-layer illite-smectite of bentonites based on isotopic data (K-Ar, Rb-Sr and δ18O)

Rb-Sr and oxygen isotope studies, in addition to K-Ar isotopic determinations published previously, are reported on diagenetic and hydrothermal fundamental particles (particle thickness of 0.03 to 0.05 nm and particle ab size of 0.02–0.05 µm) of the East-Slovak Basin. The combined data set allows us to ascertain the crystallization conditions of the illite material from two bentonite units collected at two basinal sites located ~20 km apart, and characterized by prolonged diagenetic conditions induced by progressive burial. A bentonite rock characterized by a short hydrothermal event from the Zempleni mountains to the SW of the East-Slovak basin is also studied.For the two first sites, the δ18O values increase in one case and decrease in the other, when the size of the diagenetic fundamental particles from bentonite samples increases. These variations suggest that temperature increased in one and decreased in the second of the two samples collected in the basin, while the particles were growing. In the case of the hydrothermal bentonite, the δ18O values of the different size-fractions consisting of fundamental particles remain about constant, suggesting constant temperature and fluid chemistry.The Rb-Sr dates of the fundamental particles of the three bentonite rocks were systematically higher than the corresponding K-Ar ages. The 87Sr/86Sr ratios, which are initially involved in the particle nucleation, appeared higher than that of contemporaneous sea-water. In all cases, the initial 87Sr/ 86Sr ratio decreases when particle size increases, which implies supply of external Sr into the bentonite units. This external Sr seems to have had an 87Sr/86Sr ratio close or identical to that of the contemporaneous sea water. This means that Sr, probably of sea-water origin, progressively diffused from host shales into the bentonite units, during burial diagenesis. In turn it favors the suggestion made previously about diffusion of K from shales into the bentonite layers during illitization of the smectite from these units.

Geochemical tracing and modeling of surface and deep water–rock interactions in elementary granitic watersheds (Strengbach and Ringelbach CZOs, France)

From the study of the Strengbach and Ringelbach watersheds we propose to illustrate the interest of combining the geochemical tracing and geochemical modeling approaches on surface and deep borehole waters, to decipher the diversity of the water flow and the associated water–rock interactions in such elementary mountainous catchments. The results point to a clear geochemical typology of waters depending on the water circulations (deep vs. hypodermic) within the substratum.

A soil column model for predicting the interaction between water table and evapotranspiration

Lateral waterfluxes are not realistically taken into account in soil column models, although they influence the dynamic evolution of the vertical soil moisture profile. By neglecting these fluxes, the modeling of the soil-vegetation-atmosphere continuum is incomplete, and the feedbacks between these three compartments cannot be fully simulated. These fluxes have an importance in the different fields where soil column models are used: hydrology, hydrometeorology, biogeochemical cycles, ecology, and soil weathering. This paper introduces a novel Hydrological Hillslope-based Soil Column model (H2SC) that simulates the temporal evolution of the water table depth and evapotranspiration fluxes and their interaction. The interconnected processes are infiltration, evapotranspiration, vertical soil water movements, and the nonexplicitly modeled lateral fluxes flowing through the soil column. These lateral fluxes are modeled as a drainage function built from physically based equations that describe a simplified hillslope hydrology. This drainage function can be easily implemented in any soil column model without penalizing computational times. The H2SC model was validated on numerical experiments where a 2-D hillslope simulation performed with an integrated hydrologic model was compared with simulations using the H2SC 1-D model. Each of the H2SC simulations represents a specific location of a soil column along the hillslope. Different climate forcings, soil properties, and geometric shapes of the hillslope were tested. The model was then applied at the locations of two piezometers in the Strengbach catchment, France. The model reproduced the temporal evolution of the water table level fairly well for both the numerical experiments and for the real test case.