The impact of water-rock interaction and vegetation on calcium isotope fractionation in soil- and stream waters of a small, forested catchment (the Strengbach case)

This study aims to constrain the factors controlling the calcium isotopic compositions in surface waters, especially the respective role of vegetation and water-rock interactions on Ca isotope fractionation in a continental forested ecosystem. The approach is to follow changes in space and time of the isotopic composition and concentration of Ca along its pathway through the hydro-geochemical reservoirs from atmospheric deposits to the outlet of the watershed via throughfalls, percolating soil solutions and springs. The study is focused on the Strengbach catchment, a small forested watershed located in the northeast of France in the Vosges mountains. The delta Ca-44/40 values of springs, brooks and stream waters from the catchment are comparable to those of continental rivers and fluctuate between 0.17 and 0.87 parts per thousand. Soil solutions, however, are significantly depleted in lighter isotopes (delta Ca-44/40: 1.00-1.47 parts per thousand), whereas vegetation is strongly enriched (delta Ca-44/40: -0.487. to +0.197 parts per thousand). These results highlight that vegetation is a major factor controlling the calcium isotopic composition of soil solutions, with depletion in "light" calcium in the soil solutions from deeper parts of the soil compartments due to preferential 40 Ca uptake by the plants rootsystem. However, mass balance calculations require the contribution of an additional Ca flux into the soil solutions most probably associated with water-rock interactions. The stream waters are marked by a seasonal variation of their 844140 Ca, with low 844140 Ca in winter and high 844140 Ca in spring, summer and autumn. For some springs, nourishing the streamlet, a decrease of the delta Ca-44/40 value is observed when the discharge of the spring increases, with, in addition, a clear covariation between the delta Ca-44/40 and corresponding H4SiO4 concentrations: high delta Ca-44/40 values and low H4SiO4 concentrations at high discharge; low delta Ca-44/40 values and high H4SiO4 concentrations at low discharge. These data imply that during dry periods and low water flow rate the source waters carry a Ca isotopic signature from alteration of soil minerals, whereas during wet periods and high flow rates admixture of significant quantities of Ca-40 depleted waters (vegetation induced signal) from uppermost soil horizons controls the isotopic composition of the source waters. This study clearly emphasizes the potential of Ca isotopes as tracers of biogeochemical processes at the water-rock-vegetation interface in a small forested catchment. (c) 2009 Elsevier Ltd. All rights reserved.