Incorporation of Ca isotopes in carapaxes of marine ostracods

Abstract

We studied Mg/Ca, Sr/Ca and Ca isotope ratios of ostracod carapaxes from surface sediments retrieved during research cruises to the Southern and Equatorial Eastern Atlantic. Collected specimens originate from water depth between 30 and about 5000 mbsl and cover bottom water temperatures between 0 and 23 °C. Analysed taxa include Buntonia sp., Cytherella sp., Henryhowella sp., Krithe sp., Poseidonamicus sp., and Ruggieria sp.Calcium isotope ratios of the studied specimens are in the overall range of other marine calcifiers and show a variability of about 0.75‰ over a temperature range of ~20 °C. There is no overall linear relation between temperature and Ca isotope fractionation, but some species show a maximum δ44/40Ca at intermediate temperatures. Mg/Ca and Sr/Ca agree well with values of previously published carapaxes, demonstrating large inter- and intra-species variability and different responses to bottom water temperatures. Magnesium is highly variably distributed within the carapax, with Mg enrichment in the inner part of the shell of most taxa. This enrichment is independent from the incorporation of Sr, which shows less variability within the carapax. The Ca isotope vs. Sr/Ca systematic in the ostracods deviates from inorganic calcite and foraminifers. The ostracod array is most likely controlled by either mixing of material originating from a precursor phase and newly grown calcite or Rayleigh type fractionation during precipitation from a restricted fluid reservoir, indicating varying degrees of Ca and Sr consumption during calcite precipitation. The different slope of the Ca isotope vs. Sr/Ca relation shown by the ostracods compared to those of inorganic calcite and foraminifers opens a new approach for the determination of Ca isotope and Sr/Ca ratios of past ocean water.