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

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 δ¹¹B 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 ¹¹B-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, δ¹¹B 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 dislocation of coarser grains in which they were encapsulated. On a broader scale, the large B isotopic fractionation during water–rock interactions greatly facilitates the quantification of matter exchange between soil solutions and solids and the determination of global weathering rates. The data from the present study demonstrate that the determination of B isotopes in different particle size fractions can reveal the relative importance of source and process effects during weathering.