Influences on calcite Sr/Ca records from Ceara Rise and other regions: distinguishing ocean history and calcite recrystallization

Abstract

Strontium/calcium ratios in deep-sea carbonate sediments have been used as recorders of paleoceanographic Sr/Ca and as indicators of diagenetic alteration of calcite sediments. We evaluate the relative impacts of ocean history and calcite recrystalli-zation on bulk calcite Sr/Ca records from sediments from the Ceara Rise (CR) and the equatorial PaciÞc. We present Sr/Ca and Mg/Ca data from bulk calcite analyses of samples from the range of depths drilled at the Þve sites in the Ocean Drilling Program (ODP) Leg 154 Ceara Rise depth transect. The Sr/Ca and Mg/Ca records appear to vary more consistently with age than with sediment depth, indicating that they more strongly represent records of oceanic Sr/Ca and Mg/Ca ratios, rather than depth-controlled diagenetic alteration. We compare Sr/Ca records from the CR with bulk calcite records from the Ontong Java Plateau (OJP) in the western equatorial PaciÞc (Leg 130, Sites 803-807) and the eastern equatorial PaciÞc (EEqP; ODP Leg 138, Sites 844-846, 851, and 853), and with a planktonic foraminifer record. All Sr/Ca records show similar trends, with Sr/Ca values increasing to the present and large ßuctuations superimposed on this trend in the Neogene. We use quantitative estimates of cel-estite (SrSO 4) saturation in pore waters and a Sr-exchange model of calcite diagenesis to evaluate the potential inßuence of authigenic SrSO 4 precipitation on the bulk calcite Sr/Ca from the CR, OJP, and EEqP. We conclude that celestite does not precipitate in the EEqP sites and probably does precipitate at most or all CR and OJP sites. We further conclude that the precipitation of SrSO 4 at some sites and not others accounts for only a small fraction of the differences in contemporaneous Sr/Ca values in calcite.