Oxygen isotope and elemental ratios in waters and bivalves as tracers of hydrological change in the modern and past waters of the Coorong Lagoons, South Australia

Abstract

As European land clearance and the installation of engineering controls of water flow (such as locks and barrages) have altered many estuarine ecosystems from their natural state, the range of natural variability of hydrological conditions is often poorly understood. As the instrumental record of climate and environmental monitoring in Australia is short (approximately 120 years), palaeoenvironmental archives inform our understanding of past conditions and can be useful in informing management and conservation efforts in venerable hydrological ecosystems. The geochemistry of bivalve shells preserved in sediments provides one possible archive of past hydrological conditions, though modern calibration studies are necessary prior to palaeoenvironmental applications. This thesis investigates the controls on the composition of trace elements (Sr, Mg and Ba) and stable oxygen isotope ratios (δ18O) in the waters of the Coorong Lagoons, South Australia, alongside the incorporation of these elements and stable isotope ratios into shells of the bivalve Arthritica helmsi. The findings of these studies of modern populations were then applied to shells from the sediments of the Coorong South Lagoon to reconstruct hydroclimate variability for the past 1750 years. The trace elemental ratios Mg/Ca, Sr/Ca and Ba/Ca in waters of the Coorong Lagoons were significantly correlated to salinity. In particular, the relationship between Mg/Ca was found to be consistent across different hydrological regimes and could therefore be a target as a salinity proxy in carbonate archives. However, the partitioning of trace elemental ratios into A. helmsi carbonate was not found to correlate with the temperature, salinity, pH, or elemental concentrations of water, indicating that biological effects likely control the incorporation of elements this species. The δ18O of contemporary waters was mostly controlled by evaporation, though water mixing also had an influence. Oxygen isotope ratios in A. helmsi were significantly related to both the temperature and δ18O of waters resulting in the development of a temperature-dependent fractionation equation. A δ18O record spanning 1750 years was developed from shells from sediments of the Coorong South Lagoon. This record indicates that the Coorong South Lagoon has been a highly evaporated closed system for at least the past 1750 years and that the marine dominated modern North Lagoon is not an analogue for past conditions in the South Lagoon. Furthermore multi-decadal periods of high and low precipitation/evaporation balance indicated by the δ18O record are consistent with regional hydroclimate reconstructions suggesting a common driver for hydroclimate in southeastern Australia. The outcomes of this thesis are a significant contribution to the knowledge of bivalve geochemistry as a proxy for palaeoenvironmental change, as well as past hydroclimate in the Coorong South Lagoon and surrounding region, with potential implications for modern hydrological management.