Abstract
Abstract. Much of our knowledge of past ocean temperatures comes from the foraminifera Mg / Ca palaeothermometer. Several nonthermal 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 examined 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 and Mg / Ca 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 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 / Ca and δ18O 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 °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 inductively coupled plasma mass spectrometry (ICPMS) Mg / 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 / 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.
Highlights
The relationship between foraminifera Mg / Ca and temperature has been widely applied as a method of reconstructing the thermal evolution of the oceans across a range of timescales
Evans et al.: pH control on foraminifera Mg / Ca a substantial portion of our understanding of the response of ocean temperature, hydrology, and circulation to external forcing is derived from this proxy, several nonthermal controls have been identified, which complicate the translation of measured Mg / Ca ratios to temperature
Because it has been argued that there is a limited control on foraminifera Mg / Ca at high pH and/or [CO23−] (Russell et al, 2004) and there is no relationship between pH and
Summary
The relationship between foraminifera Mg / Ca and temperature has been widely applied as a method of reconstructing the thermal evolution of the oceans across a range of timescales (see, e.g., Mashiotta et al, 1999; Lear et al, 2000; Sosdian and Rosenthal, 2009; Wade et al, 2012). The potential effect of differential palaeo-carbonate chemistry on Mg / Ca-derived reconstructions is rarely noted for planktic foraminifera, a carbonate ion correction is routinely applied to some benthic foraminifera species (Sosdian and Rosenthal, 2009; Yu and Broecker, 2010) All of these studies indicate that the relationship between pH or [CO23−] and Mg / Ca is significant: on the order of a 30–60 % increase in Mg / Ca for a 0.4 unit pH decrease from 8.0 (total scale). This implies that absolute Mg / Ca-derived temperatures may be systematically biased when ocean pH or [CO23−] was significantly different to modern values, and relative Mg / Ca-derived temperature shifts over climatic events associated with a large perturbation of the carbonate system may be inaccurate
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