Characterising the stable (?88/86Sr) and radiogenic (87Sr/86Sr) isotopic composition of strontium in rainwater

Abstract

Rainwater plays a major role in determining the transfer of elements and pollutants to soils and hydrological catchments, with compositional variations reflecting the extent of mixing between marine (sea-salt), terrestrial (dust and vegetation) and anthropogenic aerosols. While the radiogenic strontium isotope system (87Sr/86Sr) is often used to help constrain the different sources of atmospheric Sr, the degree of variability in the stable strontium isotope system (?88/86Sr) remains unresolved. In order to determine how atmospheric Sr affects the ?88/86Sr composition of water entering the hydrological cycle this study analysed a suite of rainwater samples collected over the course of a year in central Paris. Rainwater ?88/86Sr compositions were found to range from 0.13‰ to 0.32‰, with 87Sr/86Sr ratios varying between 0.70796 and 0.71093. No seasonality was observed in ?88/86Sr, whereas 87Sr/86Sr ratios were observed to be generally more radiogenic in the summer and less radiogenic in the winter. Strontium enrichment factors significantly above seawater implied a predominantly terrestrial (non-sea-salt) origin of Sr in rainwater, with the ?88/86Sr and 87Sr/86Sr composition of this terrestrial fraction consistent with a mixture of Sr derived from carbonate dust and agricultural fertiliser. Evidence for a ~ 0.1‰ decrease in the ?88/86Sr composition of the River Seine as it passes through Paris suggests that anthropogenic pollutants may contribute to significant regional variations in the composition of Sr in the hydrological cycle. Together these results demonstrate that there is significant potential for both temporal and spatial variations in the ?88/86Sr and 87Sr/86Sr composition of rainwater that need to be taken into consideration when studying ?88/86Sr variations in localised catchments.

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