Past temperature and δ18O of surface ocean waters inferred from foraminiferal Mg/Ca ratios

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 (δ18O) of calcite from planktonic foraminifera has been shown to reflect both surface temperature and seawater δ18O, itself an indicator of global ice volume and salinity. In addition, magnesium/calcium (Mg/Ca) ratios in foraminiferal calcite show a temperature dependence 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 and alkenone saturation. Moreover, combining Mg/Ca and δ18O data from the same faunal assemblage, we show that reconstructed surface water δ18O 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.

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