Elemental concentrations of waters and bivalves in the fresh to hypersaline Coorong Lagoons, South Australia: Implications for palaeoenvironmental studies

摘要

Element-to-calcium ratios of bivalve shells are potentially useful proxies for environmental change, provided the relationship between the environmental variable and element ratio are calibrated using modern specimens. In this study we investigate the utility of trace elemental ratios in the estuarine micromollusc Arthritica helmsi as (palaeo)environmental proxies. Sr/Ca, Mg/Ca and Ba/Ca ratios were measured in waters (n = 137) and live bivalves (n = 125) were collected along a salinity gradient from fresh to hypersaline in the Coorong Lagoon and Lake Alexandrina, at the terminus of the Murray River, South Australia. Water Mg, Sr and Ca exhibited linear relationships with salinity, while Ba showed no relationship. Mg/Ca and Sr/Ca in water both showed positive logarithmic responses to increasing salinity, while the response of Ba/Ca was best explained by a negative power function. The Sr concentration and Sr/Ca of water collected between 2016 and 2018 were elevated compared to a previous study conducted between 2007 and 2008, possibly due to a higher river flow regime in the more recent period. The range of Sr/Ca, Mg/Ca and Ba/Ca measured in A. helmsi were in agreement with previous studies, as were the range of partition coefficients. However, the incorporation of Sr/Ca, Mg/Ca and Ba/Ca did not correlate with water elemental ratios, temperature, salinity or pH and are therefore likely to be more heavily influenced by biological processes. As a consequence, while the elemental composition of other carbonate fossils within the Coorong system may hold potential to reconstruct past climate and environmental change, the trace element geochemistry of A. helmsi aragonite shells, and possibly other similar micro-bivalve molluscs, should be treated with caution as a palaeoenvironmental tracer.