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

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.

Evaluating controls on planktonic foraminiferal geochemistry in the Eastern Tropical North Pacific

“…For example, Russell et al (2004) Evans et al (2016) confirms the trend of increasing Mg/Ca with decreasing pH for multiple species of foraminifera. Using their linear relationship between pH and excess Mg/Ca (Evans et al, 2016) indicates that up to 1.4 mmol/mol of "excess Mg/Ca" in our samples can be explained by the influence of pH. In the GoT, average excess Mg/Ca is 1.09-2.04 for G. ruber and 0.61-1.00 for G. bulloides.…”

“…Consistent throughout all of these calibration studies is an exponential Mg/Ca ratio:temperature sensitivity of 8-10% change in ratio per • C (Anand et al, 2003;Bolton et al, 2011;Dekens et al, 2002;Lea et al, 1999;Martínez-Botí et al, 2011;Mashiotta et al, 1999;McConnell and Thunell, 2005;Regenberg et al, 2009). The influences of secondary factors such as salinity and carbonate chemistry are still being evaluated, and in many cases appear to be species -and/or regionspecific (Arbuszewski et al, 2010;Dekens et al, 2002;Evans et al, 2016;Hertzberg and Schmidt, 2013;Hönisch et al, 2013;Kısakürek et al, 2008;Russell et al, 2004).…”

Considerations for Globigerinoides ruber (White and Pink) Paleoceanography: Comprehensive Insights From a Long‐Running Sediment Trap

“…Some core-top studies have suggested salinity sensitivity as high as 15%-27% in G. ruber Mg/Ca (Arbuszewski et al, 2010;Ferguson et al, 2008); however, the majority of culture studies (Kisakürek et al, 2008;Dueñas-Bohórquez et al, 2009;Hönisch et al, 2013), sediment trap (Gray et al, 2018), and reevaluations of those core-top data (Hertzberg & Schmidt, 2013;Hönisch et al, 2013;Khider et al, 2015) suggest that salinity sensitivity is on the order of 3%-6%. Studies evaluating the influence of pH on Mg/Ca in G. ruber (Allen et al, 2016;Evans et al, 2016;Gray et al, 2018;Kısakürek et al, 2008) converge on a 5%-10% increase in Mg/Ca per 0.1 unit decrease in pH.…”

Calibration and Validation of Environmental Controls on Planktic Foraminifera Mg/Ca Using Global Core‐Top Data

“…All our Ω shallow values are highly oversaturated with a mean value of 5.5 ± 0.5 (1σ) and only three sites with a value below 4, while complementary pH values almost entirely fall between 8.0 and 8.1. The sensitivity of G. ruber Mg/Ca to carbonate ion concentration should be reduced at these high saturation states (Allen et al, 2016;Evans et al, 2016) and may be difficult to resolve given the narrow range of Ω shallow values we considered. Similarly, the range of pH values we considered is far narrower than the 7.6-8.5 range used to infer a pH dependence in culture experiments (Allen et al, 2016;Evans et al, 2016;Kısakürek et al, 2008).…”

“…A power function was used to relate Ω shallow to Mg/Ca as suggested by a recent compilation of culture data Figure 3c). The relationship between Mg/Ca and pH at the time of calcification has been modeled using logistic and power functions but has also been suggested to be linear (Allen et al, 2016;Evans et al, 2016;Russell et al, 2004), and for simplicity we model Mg/Ca as a linear function of pH shallow (Figure 3d). Mg/Ca…”