Natural strontium isotope composition as a tracer of weathering patterns and of exchangeable calcium sources in acid leached soils developed on loess of central Belgium

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Abstract
Summary: The natural Sr isotope composition of acid leached soils developed on loess, under beech forest, in central Belgium was used as a tracer of soil forming processes, in conjunction with physico‐chemical and quantitative mineralogical investigations. Attention was focused on weathering and exchange processes, with special emphasis on the origin of the current soil exchangeable fraction and the influence of the atmospheric deposition and biological cycling on the calcium exchangeable pool (Sr acts as a proxy for Ca). The determination of 87 Sr/ 86 Sr ratios was made on the bulk soil, on the clay‐ and silt‐size soil separates, on 0.1 m HCl extracts, on the labile pool, on the soil solution and on the bulk precipitation. The acid leached soil profiles are characterized by a sequence of weathering processes that is highlighted by both mineralogical and isotopic changes. From the calcareous unweathered loess (pH 7.5) to the uppermost soil horizons (pH < 4.0) the evolution of the 87 Sr/ 86 Sr isotope ratio clearly reflects: (i) the selective weathering of Ca‐plagioclase (small 87 Sr/ 86 Sr ratio) and the increasing proportion of resistant K‐ and Rb‐rich minerals (large 87 Sr/ 86 Sr ratio) in the uppermost soil horizons; and (ii) a downward translocation of clay minerals with a large isotopic ratio, a physical breakdown of muscovite and a non‐congruent chemical weathering of K‐feldspar. The influence of organic restitutions or atmospheric deposition is not significant. The comparison between the Sr isotopic signature of the soil solution, and the exchangeable and HCl‐extractible soil fractions provides information about cation exchange efficiency, soil–water interaction and the origin of the exchangeable pool.

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