Calcium and strontium isotope constraints on water mixing, carbonate fluxes and fish migration in Coorong Lagoon, South Australia

Abstract

To better understand the water chemistry and sources of water inputs to Coorong Lagoon, the terminal estuary of Australia’s largest river system (i.e. Murray), we established water mixing models using Sr and Ca isotopes (i.e. measuring 87Sr/86Sr and δ44/40Ca signatures) and investigated the sensitivity of these isotope proxies to salinity changes along the lagoon (ranging from fresh to hypersaline, i.e., 0 to ~120 PSU). Additionally, analyses of fish otoliths of the smallmouth hardyhead (Atherinosoma microstoma) are used as integrated average values of local water signatures over one-year (i.e. the fish’s lifespan). Basically, the lagoon waters were considered to be a result of brackish/fresh water – seawater mixing, with the strong effect of evaporation, which caused hypersalinity in the south part of the lagoon. From the water mixing models, the North Lagoon is strongly controlled by seawater input from Southern Ocean except when temporary groundwater input events occurred; the South Lagoon is mostly hypersaline, as a result of seawater – groundwater mixing and evaporation, and inputs from both components were significant, confirmed by Sr isotope signatures in waters and otoliths. The effect of water evaporation, which leads to oversaturation of carbonate minerals (i.e. calcite and aragonite) in lagoon waters, has also an impact on the Ca isotope composition of local waters, as these yielded systematically heavy δ44/40Ca signatures due to removal of light Ca isotopes into CaCO3. Our modelling of Ca isotope data suggests that about 35 - 40% of Ca in the South Lagoon waters has been removed as CaCO3, which in turn has implications for understanding local carbon cycling in the lagoon. Finally, the results of this study also provide a basis for future applications of Sr and Ca isotopes that aim to reconstruct ancient environmental conditions and paleo-salinity changes in the Coorong lagoon based on sediment core record and/or fossil archives.