Sr/Ca in Calcitic Tests of Benthic Foraminifera - A Potential Water-Depth Proxy?

Abstract

Well-preserved low-Mg calcite tests of modern benthic foraminifera from a depth transect off SW Norway show decreasing Sr/Ca with increasing water depth (r = –0.84). The Mg/Ca ratios also decrease with increasing water depth, although in a curvilinear fashion. Moreover, the inter- and intra-species scatter is apparently larger than for Sr/Ca. A majority of the analysed specimens have Mn/Ca < 200 μmol/mol, and show Sr/Ca, Mg/Ca that fall within published ranges for benthic foraminiferal calcite, indicating that pristine biogenic calcite has been analysed. The uniform temperature and salinity of bottom waters below ~800 m moreover indicate that neither temperature nor salinity can explain the observed Me/Ca trends. The Sr/Ca and Mg/Ca depth trends cannot be explained by dissolution since bottom waters are generally supersaturated with respect to calcite, and dissolution is only observed in a few specimens. Moreover, a regular increase in sedimentary organic content with depth needed to invoke a decrease in porewater pH and concomittant calcite dissolution is not observed. We tentatively ascribe the decreasing Sr/Ca and Mg/Ca trends to depthrelated physiological phenomena. The relatively small intra- and interspecies scatter at a given water depth, as well as the linear decrease in Sr/Ca ratios with increasing water depth observed in modern oceans, may render the Sr/Ca in calcitic benthic foraminifera a potential water-depth proxy for fossil deposits.