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Revisiting carbonate chemistry controls on planktic foraminifera Mg / Ca: implications for sea surface temperature and hydrology shifts over the Paleocene–Eocene Thermal Maximum and Eocene–Oligocene Transition

Evans, D; Wade, BS; Henehan, M; Erez, J; Muller, W; (2015) Revisiting carbonate chemistry controls on planktic foraminifera Mg / Ca: implications for sea surface temperature and hydrology shifts over the Paleocene–Eocene Thermal Maximum and Eocene–Oligocene Transition. Climate of the Past , 11 pp. 3143-3185. 10.5194/cpd-11-3143-2015. Green open access

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Abstract

Much of our knowledge of past ocean temperatures comes from the foraminifera Mg / Ca palaeothermometer. Several non-thermal controls on foraminifera Mg incorporation have been identified, of which vital-effects, salinity and secular variation in seawater Mg / Ca are the most commonly considered. Ocean carbonate chemistry is also known to influence Mg / Ca, yet this is rarely considered as a source of uncertainty either because (1) precise pH and [CO32−] reconstructions are sparse, or (2) it is not clear from existing culture studies how a correction should be applied. We present new culture data of the relationship between carbonate chemistry for the surface-dwelling planktic species Globigerinoides ruber, and compare our results to data compiled from existing studies. We find a coherent relationship between Mg / Ca and the carbonate system and argue that pH rather than [CO32−] is likely to be the dominant control. Applying these new calibrations to datasets for the Paleocene–Eocene Thermal Maximum (PETM) and Eocene–Oligocene Transition (EOT) enable us to produce a more accurate picture of surface hydrology change for the former, and a reassessment of the amount of subtropical precursor cooling for the latter. We show that properly corrected Mg / Ca and δ18O datasets for the PETM imply no salinity change, and that the amount of precursor cooling over the EOT has been previously underestimated by ∼ 2 °C based on Mg / Ca. Finally, we present new laser-ablation data of EOT-age Turborotalia ampliapertura from St Stephens Quarry (Alabama), for which a solution ICPMS Mg / Ca record is available (Wade et al., 2012). We show that the two datasets are in excellent agreement, demonstrating that fossil solution and laser-ablation data may be directly comparable. Together with an advancing understanding of the effect of Mg / Casw, the coherent picture of the relationship between Mg / Ca and pH that we outline here represents a step towards producing accurate and quantitative palaeotemperatures using this proxy.

Type: Article
Title: Revisiting carbonate chemistry controls on planktic foraminifera Mg / Ca: implications for sea surface temperature and hydrology shifts over the Paleocene–Eocene Thermal Maximum and Eocene–Oligocene Transition
Open access status: An open access version is available from UCL Discovery
DOI: 10.5194/cpd-11-3143-2015
Publisher version: http://dx.doi.org/10.5194/cpd-11-3143-2015
Language: English
Additional information: © Author(s) 2015. This work is distributed under the Creative Commons Attribution 3.0 License.
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1474717
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