Radiogenic and stable Sr isotope ratios (87Sr/86Sr, d88/86Sr) as tracers of riverine cation sources and biogeochemical cycling in the Milford Sound region of Fiordland, New Zealand

Abstract

This study reports radiogenic Sr isotope ratios (87Sr/86Sr), stable Sr isotope ratios (?88/86Sr), and major ion concentrations for river, rock, sediment, soil, and plant samples collected from the Cleddau and Hollyford catchments in the Milford Sound region of Fiordland, New Zealand. The catchments primarily drain gabbro, but some tributaries access limestone and volcanogenic sediments. The goal of the study was to understand controls on riverine ?88/86Sr values in a landscape with multiple factors that may influence chemical weathering, including dense vegetation, high rainfall, and abundant, freshly-eroded Holocene fluvio-glacial and landslide debris.

Rivers draining gabbro have higher ?88/86Sr values than bedrock, by as much as ?0.14‰, and the ?88/86Sr values strongly correlate with molar Ca/Sr ratios (R2 = 0.69). Leaching of rocks and sediment reveals no evidence for the preferential dissolution of minerals having high ?88/86Sr values and Ca/Sr ratios. In-stream Sr isotope fractionation seems unlikely because comparison against 87Sr/86Sr and Ca/Sr ratios demonstrates that riverine ?88/86Sr values conservatively trace water-mass mixing. The riverine data are best explained by the input of soil water, which is distinct from potential bedrock end-members (i.e., silicates and carbonates) based on ?88/86Sr but indistinguishable in terms of Ca/Sr and 87Sr/86Sr. While strontium isotope fractionation during secondary mineral formation and pedogenesis is possible, clay mineral formation is minor and most soils are poorly developed. Instead, soil water ?88/86Sr values more likely reflect plant uptake. Plant samples yielded a wide range of ?88/86Sr values, but on average, they are lower than those for bedrock, consistent with the expectation that plants preferentially incorporate lighter Sr isotopes. Mass-balance constraints, together with 87Sr/86Sr ratios, indicate that soil water ?88/86Sr values are ?0.30‰ higher than bedrock ?88/86Sr values, and mixing calculations show that the plant-fractionated soil water pool contributes ?27% of the riverine Sr. For tributaries accessing limestone and volcanogenic sediments, Ca/Sr and 87Sr/86Sr ratios appear consistent with two-component mixing between silicate and carbonate weathering, but ?88/86Sr values reveal a third contribution from soil water inputs, similar to gabbro catchments.

The results of this study suggest that Sr isotopes behave conservatively during water mass mixing and stream transport but non-conservatively in soil, where plant uptake can elevate soil water ?88/86Sr values relative to bedrock. Plant uptake, or related biogeochemical processes, such as ion-exchange on organic matter surfaces, also appear to modify soil water Ca/Sr ratios. Many studies use 87Sr/86Sr and Ca/Sr ratios to apportion riverine solutes between silicate and carbonate weathering, but Ca/Sr ratios may be non-conservative in densely vegetated areas. The stable Sr isotope tracer shows promise for resolving riverine cation sources, as well as effects from biological cycling.

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