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

David Evans, Bridget S. Wade, Michael Henehan, Jonathan Erez, Wolfgang Müller
2016 Climate of the Past  
<p><strong>Abstract.</strong> Much of our knowledge of past ocean temperatures comes from the foraminifera Mg<span class="thinspace"></span>/<span class="thinspace"></span>Ca palaeothermometer. Several nonthermal controls on foraminifera Mg incorporation have been identified, of which vital effects, salinity, and secular variation in seawater Mg<span class="thinspace"></span>/<span class="thinspace"></span>Ca are the most commonly considered. Ocean carbonate chemistry is also known to influence
more » ... known to influence Mg<span class="thinspace"></span>/<span class="thinspace"></span>Ca, yet this is rarely examined as a source of uncertainty, either because (1) precise pH and [CO<sub>3</sub><sup>2−</sup>] 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 and Mg<span class="thinspace"></span>/<span class="thinspace"></span>Ca for the surface-dwelling planktic species <i>Globigerinoides ruber</i> and compare our results to data compiled from existing studies. We find a coherent relationship between Mg<span class="thinspace"></span>/<span class="thinspace"></span>Ca and the carbonate system and argue that pH rather than [CO<sub>3</sub><sup>2−</sup>] is likely to be the dominant control. Applying these new calibrations to data sets for the Paleocene–Eocene Thermal Maximum (PETM) and Eocene–Oligocene transition (EOT) enables 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 pH-adjusted Mg<span class="thinspace"></span>/<span class="thinspace"></span>Ca and <i>δ</i><sup>18</sup>O data sets for the PETM are within error of no salinity change and that the amount of precursor cooling over the EOT has been previously underestimated by ∼ 2<span class="thinspace"></span>°C based on Mg<span class="thinspace"></span>/<span class="thinspace"></span>Ca. Finally, we present new laser-ablation data of EOT-age <i>Turborotalia ampliapertura</i> from St. Stephens Quarry (Alabama), for which a solution inductively coupled plasma mass spectrometry (ICPMS) Mg<span class="thinspace"></span>/<span class="thinspace"></span>Ca record is available (Wade et al., 2012). We show that the two data sets 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<span class="thinspace"></span>/<span class="thinspace"></span>Ca<sub>sw</sub>, the coherent picture of the relationship between Mg<span class="thinspace"></span>/<span class="thinspace"></span>Ca and pH that we outline here represents a step towards producing accurate and quantitative palaeotemperatures using this proxy.</p>
doi:10.5194/cp-12-819-2016 fatcat:oe3omcnhjvgj7j7pjqp44tnufe