Planktic foraminifera shell chemistry response to seawater chemistry: Pliocene-Pleistocene seawater Mg/Ca, temperature and sea level change

Foraminifera Mg/Ca paleothermometry forms the basis of a substantial portion of ocean temperature reconstruction over the last 5 Ma. Furthermore, coupled Mg/Ca-oxygen isotope (δ¹⁸O) measurements of benthic foraminifera can constrain eustatic sea level (ESL) independent of paleo-shoreline derived approaches. However, this technique suffers from uncertainty regarding the secular variation of the Mg/Ca seawater ratio (Mg/Ca_sw) on timescales of millions of years. Here we present coupled seawater-test Mg/Ca-temperature laboratory calibrations of Globigerinoides ruber in order to test the widely held assumptions that (1) seawater-test Mg/Ca co-vary linearly, and (2) the Mg/Ca-temperature sensitivity remains constant with changing Mg/Ca_sw. We find a nonlinear Mg/Ca_test-Mg/Ca_sw relationship and a lowering of the Mg/Ca-temperature sensitivity at lower than modern Mg/Ca_sw from 9.0% °C^-1 at Mg/Ca_sw = 5.2 mol mol^-1 to 7.5±0.9%°C^-1 at 3.4 mol mol^-1. Using our calibrations to more accurately calculate the offset between Mg/Ca and biomarker-derived paleotemperatures for four sites, we derive a Pliocene Mg/Ca_sw ratio of ~4.3 mol mol^-1. This Mg/Ca_sw implies Pliocene ocean temperature 0.9-1.9 °C higher than previously reported and, by extension, ESL ~30 m lower compared to when one assumes that Pliocene Mg/Ca_sw is the same as at present. Correcting existing benthic foraminifera datasets for Mg/Ca_sw indicates that deep water source composition must have changed through time, therefore seawater oxygen isotope reconstructions relative to present day cannot be used to directly reconstruct Pliocene ESL.