A coupled radiogenic (87Sr/86Sr) and stable (δ88/86Sr) strontium isotope approach to reconstruct past changes in water mixing and salinity in the Coorong lagoon, South Australia

Abstract

Coastal lagoons represent an interface between terrestrial and marine ecosystems that are sensitive to natural and human-induced environmental changes. The Coorong lagoons in South Australia are part of the terminal hydrological system of the Murray Darling Basin catchment. This study explores novel geochemical analytical approaches to trace palaeo-hydrological changes in water-source mixing and to provide plausible palaeo-salinity variations within the Coorong Lagoons, specifically radiogenic (87Sr/86Sr) and stable (δ88/86Sr) strontium isotopic signatures, along with elemental ratios (such as Mg/Sr), to observe these changes by proxy. Modern and fossil carbonate archives of the micro-bivalve Arthritica helmsi and water chemistries of the Coorong Lagoon can provide important clues to reconstruct past hydrological changes to the system such as European settlement. Fossil water geochemical signatures are determined from fossil shells, calibrated through a comparison between modern shells and waters to infer past water characteristics such as palaeo-salinity. The acquired data and modelling suggest that there were two main sources of waters contributing to the Coorong South Lagoon; (i) seawater derived from the Southern Ocean and (ii) fresh/brackish water derived mostly from Salt Creek. Together these two water sources can explain most of the variability observed in 87Sr/86Sr, δ88/86Sr and Mg/Sr data in studies of carbonate shells and inferred palaeo-lagoon waters. This multi-proxy approach allowed an estimate of minimum plausible palaeo-salinity range in the Coorong South Lagoon, without the influence of carbonate precipitation and/or evaporation. The results indicate palaeo-salinities in the South Lagoon as low as 10-15 PSU (practical salinity units). Finally, the results of this study also confirmed that over the last ~3,275 years the waters in the Coorong South Lagoon were never ‘typical’ ocean waters, but rather complex mixtures of continental freshwaters with variable seawater inputs.