Calcium isotopes in the marine environment and the oceanic calcium cycle

A systematic investigation of Ca-44/Ca-40 ratios in the oceanic Ca cycle indicates that the average Ca isotopic composition of input and output are quite different, suggesting that the present ocean is not in a steady state. Our data show that sea water has uniform Ca-44/Ca-40, as expected from the long residence time of Ca in the oceans. It also exhibits the heaviest isotopic composition among all samples measured, consistent with recent results by Skulan et al. (1997), and must be maintained at this value by strong biogenic isotope fractionation during the precipitation of CaCO3. Our measurements show that the riverine input is variable. The most radiogenic river waters we have measured (lowest Ca-44/Ca-40) are from Ganges tributaries which also have high Sr-87/Sr-86, although there does not appear to be a systematic correlation between Ca and Sr isotopic compositions based on available data. There is also significant isotopic variability among samples of CaCO3 precipitated from the present ocean, with coccolith oozes showing the most highly fractionated isotopic values relative to seawater. Among foraminifera, Ca isotopes exhibit species-dependent fractionation effects but also appear to display systematic temperature-dependent fractionation within inidividual species. Copyright (C) 1998 Elsevier Science Ltd.