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 (delta O-18) 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 similar to 4.3 mol mol(-1). This Mg/Ca-sw implies Pliocene ocean temperature 0.9-1.9 degrees C higher than previously reported and, by extension, ESL similar to 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. Crown Copyright (C) 2016 Published by Elsevier B.V. All rights reserved.