Stable isotope data and calcification temperature from planktic foraminifera in sediment core BOFS31/1K

Determining the past record of temperature and salinity of ocean surface waters is essential for understanding past changes in climate, such as those which occur across glacial-interglacial transitions. As a useful proxy, the oxygen isotope composition (delta18O) of calcite from planktonic foraminifera has been shown to reflect both surface temperature and seawater delta18O, itself an indicator of global ice volume and salinity (Shackleton, 1974; Rostek et al., 1993, doi:10.1038/364319a0). In addition, magnesium/calcium (Mg/Ca) ratios in foraminiferal calcite show a temperature dependence (Nürnberg, 1995, doi:10.2113/gsjfr.25.4.350; Nürnberg et al., 1996, doi:10.1016/0016-7037(95)00446-7; Lea et al., 1999, doi:10.1016/S0016-7037(99)00197-0) due to the partitioning of Mg during calcification. Here we demonstrate, in a field-based calibration experiment, that the variation of Mg/Ca ratios with temperature is similar for eight species of planktonic foraminifera (when accounting for Mg dissolution effects). Using a multi-species record from the Last Glacial Maximum in the North Atlantic Ocean we found that past temperatures reconstructed from Mg/Ca ratios followed the two other palaeotemperature proxies: faunal abundance (CLIMAP, 1981; Mix et al., 1999, doi:10.1029/1999PA900012) and alkenone saturation (Müller et al., 1998, doi:10.1016/S0016-7037(98)00097-0 ). Moreover, combining Mg/Ca and delta18O data from the same faunal assemblage, we show that reconstructed surface water delta18O from all foraminiferal species record the same glacial-interglacial change-representing changing hydrography and global ice volume. This reinforces the potential of this combined technique in probing past ocean-climate interactions.