Strontium isotope systematics in two glaciated crystalline catchments : Rhone and Oberaar Glaciers (Swiss Alps)

We studied the strontium (Sr) isotope composition of runoff and particulate material in two glaciated catchments, the Oberaar (OA) and the Rhone (RH) in the Swiss Alps. Both areas are contained within the crystalline rocks of the Aar Massif, but a zone of highly deformed Variscan basement gneisses and schists that may contain up to 9 wt% calcite is present in the Oberaar catchment. We analysed meltwaters and precipitation as well as bulk suspended sediment and local bedrock. Ca/Na ratios of 5.0–13.3 and 1.8–2.4 in Oberaar and Rhone meltwaters, respectively, indicate that meltwaters in both catchments are enriched in Ca relative to Na, compared to suspended sediments. 87Sr/86Sr ratios of Rhone meltwaters (0.7251–0.7258) are lower than those of Rhone bulk suspended sediment (0.7279–0.7306). The Rhone suspended sediment composition is interpreted as a mixture of Grimsel Granodiorite (0.7106–0.7161) and Central Aar Granite (0.7449). We explain the lower meltwater Sr ratio by the preferential dissolution of disseminated calcite, which has a relatively low 87Sr/86Sr ratio (estimated 0.720–0.730). This interpretation is supported by the enrichment of Ca in the meltwater compared to the suspended sediment. Early and nonstoichiometric cation release exerts another impact on the meltwater signal. Oberaar meltwaters have a higher 87Sr/86Sr isotopic ratio (0.7137–0.7174) than the Oberaar suspended sediment (0.7130–0.7148) which reflects the preferential dissolution of the calcite contained within the Variscan gneisses (estimated 0.7160 ± 0.001). The 87Sr/86Sr ratio measured in rain is 0.7104 (n = 4). An influence of atmospheric precipitation on Rhone meltwater 87Sr/86Sr ratio is not excluded. The high Sr isotopic signature of Oberaar meltwaters compared to that of the corresponding suspended sediment and also to the low Sr isotopic ratio of rainwater, however, suggests that the atmospheric contribution is not visible in the Oberaar meltwaters. The results discussed here suggest that the presence of disseminated calcite within the crystalline rocks of the Aar massif exerts a major impact not only on the meltwater major ion composition but also on its Sr isotope systematics.