Characterising the stable (δ88/86Sr) and radiogenic (87Sr/86Sr) isotopic composition of strontium in rainwater

- We investigate variations in the stable (δ88/86Sr) and radiogenic (87Sr/86Sr) Sr composition of Parisian rainwater.
- Significant annual changes are observed in both δ88/86Sr and 87Sr/86Sr, with a slight seasonality in 87Sr/86Sr.
- High Sr enrichment factors indicate a minor marine (sea-salt) contribution to atmospheric Sr.
- The terrestrial (non-sea-salt) Sr component is attributed to mixing between carbonate dust and agricultural fertiliser.
- Anthropogenic contamination can significantly affect the δ88/86Sr and 87Sr/86Sr composition of the local hydrological cycle.

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 (δ88/86Sr) remains unresolved. In order to determine how atmospheric Sr affects the δ88/86Sr 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 δ88/86Sr 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 δ88/86Sr, 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 δ88/86Sr 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 δ88/86Sr 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 δ88/86Sr and 87Sr/86Sr composition of rainwater that need to be taken into consideration when studying δ88/86Sr variations in localised catchments.