Evidence for mass-dependent isotopic fractionation of strontium in a glaciated granitic watershed

Abstract

The stable isotope composition of strontium (expressed as δ 88/86Sr) may provide important constraints on the global exogenic strontium cycle. Here, we present δ 88/86Sr values and 87Sr/ 86Sr ratios for granitoid rocks, a 150 yr soil chronosequence formed from these rocks, surface waters and plants in a small glaciated watershed in the central Swiss Alps. Incipient chemical weathering in this young system, whether of inorganic or biological origin, has no resolvable effect on the 87Sr/ 86Sr ratios and δ 88/86Sr values of bulk soils, which remain indistinguishable from bedrock in terms of Sr isotopic composition. Although due in part to the chemical heterogeneity of the forefield, the lack of a resolvable difference between soil and bedrock isotopic composition indicates that these soils have thus far witnessed minimal net loss of Sr; a low degree of chemical weathering is also implied by bulk soil chemistry. The isotopic composition of Sr in streamwater is more radiogenic than median soil, reflecting the preferential weathering of biotite in the catchment; streamwater δ 88/86Sr values, however, are indistinguishable from bulk soil δ 88/86Sr values, implying that no resolvable fractionation of Sr isotopes takes place during release to the weathering flux in the Damma forefield. Analyses of plant tissue reveal that plants ( Rhododendron and Vaccinium) preferentially assimilate the lighter isotopes of Sr such that their δ 88/86Sr values are significantly lower than those of the soils in which they grow. Additionally, δ 88/86Sr values of foliar and floral tissues are lower than those of roots, contrary to observations for Ca, for which Sr is often used as an analogue in weathering studies. We suggest that processes that discriminate against Sr in favour of Ca, due to the different nutritional requirement of plants for these two elements, are responsible for the observed contrast.