Insights into palaeo-hydrology of the Coorong Lagoon, South Australia, based on Strontium Isotope Tracers (87Sr/86Sr and _88/86Sr) in fossil carbonates

Yuexiao Shao1,2, Zara Woolston1, Juraj Farkaš1,2, Briony Chamberlayne1, John Tibby3 Deborah Haynes1, Jonathan Tyler1

1Department of Earth Sciences, School of Physical Sciences, University of Adelaide, Australia; 2Metal Isotope Group (MIG), University of Adelaide, Australia; 3Department of Geography, Environment and Population, School of Social Sciences, University of Adelaide, Australia

The Coorong lagoon, as part of the wetland system at the terminus of the River Murray, is recognised not only for its ecological importance but also for its unique geomorphology and salinity gradient that ranges from fresh/brackish (< 35 PSU) in the North Lagoon to hypersaline (> 70 PSU) in the South Lagoon. The lagoon hydrology is controlled by seawater-continental water mixing processes that are traceable via the radiogenic Strontium (Sr) isotopes (87Sr/86Sr ratios). The hypersaline South Lagoon, being more geomorphologically restricted, is known for high degree of evaporation, which leads to ongoing calcium carbonate precipitation, which also acts as a sink for dissolved inorganic carbon (DIC) [1]. These processes involving carbonate formation and a local inorganic carbon cycling are traceable via the novel stable Sr isotope (δ88/86Sr), which is particularly sensitive to mass-dependent isotope fractionation linked to carbonate precipitation / dissolution. Importantly, the South Lagoon has seen dramatic hydrological and ecological changes over the last ~200 years (since the European settlement), which is evident from geochemical and diatom records of the Coorong sediment cores [2], and has implications for water resource management and future strategies for the recovery of local ecosystem to more natural conditions[3]. In order to reconstruct paleo-hydrology of the Coorong before and after the European settlement, this study calibrated the δ88/86Sr fractionation between recent aragonitic bivalve shells (Arthritica helmsi species) and local water in the modern Coorong lagoon, and a constant difference of δ88/86Sr between the shells and the local water Δ88/86Sr (δ88/86Srsolid – δ88/86Srwater) = -0.92‰ was discovered. Such calibrations, coupled with 87Sr/86Sr and δ88/86Sr analyses of fossil A. helmsi shells from a sediment core in the South Lagoon, are complemented by radiocarbon dating and elemental concentration data, to better constrain (i) variability in the mixing of water sources in modern Coorong and over the last ~2500 years; and (ii) to reconstruct palaeo-salinity changes and associated carbonate precipitation/dissolution processes. Primary results based on 87Sr/86Sr of shells indicated the source of water in the South Lagoon were never purely marine; however, according to δ88/86Sr of the shells, the the South Lagoon in the past ~2500 years was probably less evaporated than it has been in recent times (i.e., post European settlement).

[1] Shao et al (2018), Geochimica et Cosmochimica Acta 239, 90-108.

[2] McKirdy et al. (2010) Organic Geochemistry 41, 96-110.

[3] Brookes et al. (2018). Goyder Institute for Water Research Technical Report Series No. 18/04, Adelaide, South Australia. ISSN: 1839-2725


3rd year PhD student in University of Adelaide, mainly interested in alkaline metal isotopes in coastal systems as tracers of water source mixing, carbonate dynamics, and redox states, and further application of these tracers in reconstructing the paleo-climate of the local coastal system.

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