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

Planktic foraminifera shell chemistry response to seawater chemistry: Pliocene–Pleistocene... 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/Casw) 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/Casw. We find a nonlinear Mg/Catest–Mg/Casw relationship and a lowering of the Mg/Ca–temperature sensitivity at lower than modern Mg/Casw from 9.0% °C−1 at Mg/Casw = 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/Casw ratio of ∼4.3 mol mol−1. This Mg/Casw 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/Casw is the same as at present. Correcting existing benthic foraminifera datasets for Mg/Casw 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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Earth and Planetary Science Letters Elsevier

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

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Publisher
Elsevier
Copyright
Copyright © 2016 Elsevier Ltd
ISSN
0012-821X
eISSN
1385-013X
D.O.I.
10.1016/j.epsl.2016.01.013
Publisher site
See Article on Publisher Site

Abstract

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/Casw) 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/Casw. We find a nonlinear Mg/Catest–Mg/Casw relationship and a lowering of the Mg/Ca–temperature sensitivity at lower than modern Mg/Casw from 9.0% °C−1 at Mg/Casw = 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/Casw ratio of ∼4.3 mol mol−1. This Mg/Casw 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/Casw is the same as at present. Correcting existing benthic foraminifera datasets for Mg/Casw 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.

Journal

Earth and Planetary Science LettersElsevier

Published: Mar 15, 2016

References

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