Stable Strontium Isotopes (δ88/86Sr) As a Tracer of Sr Sources and Biogeochemical Cycling in Two Catchments Draining Fiordland, New Zealand

Abstract:

To understand how Sr isotopes behave during chemical weathering and biogeochemical cycling, we analyzed the stable Sr isotope composition (δ^88/86Sr) of rivers, rocks, sediments, plants, and soils from the Cleddau and Hollyford catchments in Fiordland, New Zealand. We leached rocks, sediments, and soils to isolate relatively soluble Sr sources. δ^88/86Sr values were measured using an ⁸⁷Sr-⁸⁴Sr double-spike MC-TIMS method, which was optimized according to the Monte Carlo error model described in Lehn et al. (2013). The long-term, external reproducibility of the method is ±0.020‰ (2σ_SD) based on repeated measurements of NBS-987 [δ^88/86Sr = 0.000 ± 0.004‰ (2σ_SEM), n=77] and IAPSO seawater [δ^88/86Sr = 0.396 ± 0.005‰ (2σ_SEM), n=54]. Although the study site receives abundant rainfall (6700mm/yr), atmospheric inputs of Sr are negligible. We find that δ^88/86Sr values can distinguish between silicate- versus carbonate- derived riverine Sr sources when traditional tracers, such as radiogenic Sr isotope (⁸⁷Sr/⁸⁶Sr) and molar Ca/Sr ratios, are equivocal. Moreover, rivers draining gabbro bedrock in the Cleddau and Upper Hollyford catchments have higher δ^88/86Sr values (0.368‰) as compared to bulk silicate rock (0.162 – 0.284‰) and sediment (0.286‰). In the Lower Hollyford catchment, tributary rivers draining volcanic and sedimentary rocks also have higher δ^88/86Sr values (0.328‰) as compared to bulk silicate rock (0.177‰) and sediment (0.260‰). We examine several hypotheses to explain the elevated riverine δ^88/86Sr values, including end-member mixing, fractionation during chemical weathering, and plant uptake. We attribute the riverine δ^88/86Sr values to mixing with the soil pore water pool, which is isotopically heavy due to preferential uptake of ⁸⁶Sr by plants.