Ostracod shell chemistry

Abstract

We demonstrate the feasibility of determining a continuous record of chemical changes occurring during marine transgressive/regressive phases, using the Mg/Ca and Sr/Ca ratios measured in some 300 single valves of the euryhaline ostracod Cyprideis from two cores from the Gulf of Carpentaria, Australia. The molar Mg and Sr distribution coefficients Kd[Mg] and KD[Sr] for modern Australian Cyprideis specimens (Chivas et al. 1986b), KD[Mg] at 25°C=(Mg/Ca)shell/(Mg/Ca)water=0.00458±0.00072KD[Sr]=(Sr/Ca)water/(Sr/Ca)water=0.475±0.057 are used here to reconstruct the palaeoenvironmental record of the gulf. The two cores studied show similar patterns during the past 40,000 yr for equivalent hydrologic phases recognized in the cores. The Mg/Ca ratios of the water inferred from ostracod shell chemistry indicate that Carpentaria was a fresh or slightly saline lake between ≈ 40,000 and ≈ 13,000 yr B.P., before being transgressed by the sea during sea level rise. Around 26,000 yr B.P., a phase of substantial precipitation of authigenic calcite caused a dramatic increase in the Sr/Ca and Mg/Ca ratios of the lake water, and this was registered by the ostracod shell chemistry. From ≈ 26,000 to ≈ 13,000 yr B.P., the lake remained fresh or slightly saline despite the Ca depletion of the water which caused the Sr/Ca and Mg/Ca of the ostracods to be unusually high. Show more