Over the last two decades, non-marine ostracods have been intensively studied with respect to the stable isotopic composition (d13C, d18O) of their calcified valves, however, few data for marine taxa have been published so far. Here, we provide new data from recent and Pleistocene near-surface sediments of the Gulf of Taranto (recent) and the southern Aegean Sea (15 ka, both in the Mediterranean Sea) helping to improve our understanding of ostracod stable isotopes in palaeoceanography. Results are compared to those of certain benthic foraminiferal taxa for which the living habitat and the isotopic disequilibrium from ambient sea-water are known. In addition, monospecific size fractions of three common ostracod species (Bairdia conformis, Bosquetina tarentina, Henryhowella sarsii) have been studied from the Gulf of Taranto in order to test the existence of a size-dependent change in the d13C and d18O composition of the valve calcite. Our results reveal positive average species-specific deviations from the d18O of calcite equilibrium for two ostracod taxa (B. conformis 0.31‰, H. sarsii 0.33‰), while B. tarentina shows a negative off-set of - 0.41‰. These values are substantially lower than those reported for most non-marine ostracods, which usually yield positive “vital effects” for d18O of more than 2‰. Furthermore, B. tarentina is thereby the only ostracod taxon so far for which a negative “vital effect” has been reported, apart from the non-marine Eucypris mareotica. d13C values cover a much larger range than d18O with a negative off-set from sea-water by - 1.8 to - 5‰. Size-fraction data show no systematic change, although a statistically-significant positive covariance between d13C and d18O has been observed, consisting of a slope similar to a kinetically controlled fractionation effect as has previously been documented for asymbiotic planktic foraminifera and corals. The documented interspecific offsets can be primarily attributed to differences in diet for d13C and for both d13C and d18O to fractionation processes related to different modes of calcification.
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