Boron isotopes in different grain size fractions: Exploring past and present water-rock interactions from two soil profiles (Strengbach, Vosges Mountains)

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Lemarchand, D., Cividini, D., Turpault, Marie-Pierre, Chabaux, F.
• Other Authors: Université de Strasbourg (UNISTRA), Centre National de la Recherche Scientifique (CNRS), Unité de recherche Biogéochimie des Ecosystèmes Forestiers (BEF), Institut National de la Recherche Agronomique (INRA), EC2CO-Cytrix program (CNRS); Region Alsace (REALISE)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2012
• Subjects: N-E FRANCE; WEATHERING PROCESSES; LITHIUM ISOTOPES; MACKENZIE BASIN; CATCHMENT; GEOCHEMISTRY; BEHAVIOR; SMECTITE; ACFOREST SOILS; SEPARATION; [SDV]Life Sciences [q-bio]; Mackenzie Basin
• Online Access: https://hal.inrae.fr/hal-02643676; https://doi.org/10.1016/j.gca.2012.09.009

International audience In the present study, we test the ability of B isotopes to trace past and present weathering reactions in the case of two forest soils formed by a polyphasic sequence involving early hydrothermal alteration of the bedrock and pedogenesis. We provide B chemical and isotopic compositions in two 2-m soil profiles sampled in the Strengbach watershed (Vosges, France). The two soils belong to different soil series, an ochreous podzol (Haplorthod) and a brown acidic soil (dystrochrept), developed on the same granitic bedrock but differently affected by hydrothermal alteration. Separated granulometric fractions and bulk soil samples have been analyzed. Coarse particles are mostly composed of primary minerals and show a clear anti-correlation between delta B-11 and Mg/Al, reflecting various grades of the early hydrothermal alteration. The finest particles (clay-size fractions) deviate from this "hydrothermal" trend and are diagnostic of pedogenic processes occurring after B exchange with the surrounding B-11-rich soil solution. Examination of the B distribution in different particle size fractions also indicates that B is transferred from the coarse sand fraction in deeper soil layers to the clay fraction in the upper layers. The intermediate size fractions are found to play only a secondary role in the soil-forming reactions. The two soils show distinct B isotope profiles, which are consistent with the different pedogenic processes that govern their formation. In the podzolic soil, delta B-11 can be explained by a simple model in which pedogenesis occurs by continuous dissolution of primary minerals and precipitation of secondary ones. In the brown acid soil, a more complex model that includes the precipitation of clay minerals in the deep horizon and their gradual dissolution in the upper soil layers accounts for the observed results. Additionally, residual hydrothermal illites are more resistant to weathering and are found in the clay fraction where they were directly inherited from the ...