Oxygen-isotope systematics of the nektic ostracod australocypris robusta

Individual specimens of the euryhaline non-marine nektic ostracod Australocypris robusta were reared from the penultimate growth stage to adulthood in controlled in-vitro experiments. Four different water temperatures (12°, 15°, 20°, 25°C), four different water salinities (15‰, 35‰, 45‰ and 65‰, each of sea-water like solute composition) encompassing 6 different water isotopic compositions (with δ¹⁸O_V-SMOW varying from -13.2 to +4.3%o) were used. We measured the δ¹⁸O values of both ‘bulk’ ostracods (groups of 2 to 6 valves weighing from 95 to 225 µg/analysis), and individual, commonly poorlycalcified, valves (to as little as 12 µg CaC0₃), and show that ^1SO/¹⁶O fractionation is independent of valve size or degree of calcification. The relationship between δ¹⁸O of the cultured valves and of the δ¹⁸O of their host waters is linear and strongly temperature-dependent. At 15-25°C, the ostracod calcite is about 0.7%o more enriched in ¹⁸O than inorganically precipitated calcite at the same temperatures. At 12°C, the ¹⁸O/¹⁶O fractionation more closely approaches that of inorganic calcite, indicating that the expected "vital effect" offset is reduced. We offer two possible testable explanations for this behavior; firstly, that isotopic fractionation can occur "out of equilibrium" in the lowest temperature range when the ostracods moulted close to their torpidity limit and, secondly, that the culture solution pH has exerted an influence on the δ¹⁸O value of the ostracod calcite. If the pH of lacustrine waters is a significant control on ostracod- calcite δ¹⁸O values, their paleoenvironmental interpretation will be more complicated than heretofore appreciated.