CALCIUM ISOTOPES IN THE MARINE-ENVIRONMENT AND THE OCEANIC CALCIUM CYCLE

A systematic investigation of Ca-44/Ca-40 ratios in the oceanic Ca cyc le indicates that the average Ca isotopic composition of input and out put 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, a s expected from the long residence time of Ca in the oceans. It also e xhibits the heaviest isotopic composition among all samples measured, consistent with recent results by Skulan et al. (1997), and must be ma intained at this value by strong biogenic isotope fractionation during the precipitation of CaCO3. Our measurements show that the riverine i nput is variable. The most radiogenic river waters we have measured (l owest 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 correlati on between Ca and Sr isotopic compositions based on available data. Th ere is also significant isotopic variability among samples of CaCO3 pr ecipitated from the present ocean, with coccolith oozes showing the mo st highly fractionated isotopic values relative to seawater. Among for aminifera, Ca isotopes exhibit species-dependent fractionation effects but also appear to display systematic temperature-dependent fractiona tion within inidividual species. Copyright (C) 1998 Elsevier Science L td.