Calibrating Non-Thermal Effects on Planktic Foraminiferal Mg/Ca for Application Across the Cenozoic

Foraminiferal Mg/Ca has proven to be a powerful paleothermometer for reconstructing past sea-surface temperature, which, among other applications, is a critical parameter for boron isotope reconstructions of past surface ocean pH and PCO2. However, recent laboratory culture studies indicate seawater pH and the total dissolved inorganic carbon content (DIC) may both exert a significant additional control on foraminiferal Mg/Ca, likely influencing paleotemperature records as a result of seawater chemistry evolution on geologic timescales. In addition, the seawater Mg/Ca composition (Mg/Ca-sw) has been shown to reduce the sensitivity of foraminiferal Mg/Ca to temperature and possibly its sensitivity to the carbonate system as well. Here we present new Mg/Ca data from laboratory culture experiments with living planktic foraminifera-Globigerinoides ruber (p), Trilobatus sacculifer, and Orbulina universa-grown under a range of different pH and/or seawater DIC conditions and in low Mg/Ca-sw to mimic the chemical composition of the Paleocene ocean. We also conducted targeted [Ca] experiments to help define Mg/Ca-calcite-Mg/Ca-sw relationships for each species and conducted new pH experiments with G. bulloides. We find that pH effects on foraminiferal Mg/Ca are reduced or absent at Mg/Ca-sw = 1.5 mol/mol in all three species, and that T. sacculifer is generally insensitive to variable DIC and pH, making it the ideal species for Mg/Ca SST reconstructions back to 20 Ma. We apply our new T. sacculifer calibration to a Middle Miocene Mg/Ca record and provide recommendations for interpreting Mg/Ca records from extinct species. Plain Language Summary Reconstructing sea surface temperatures in the geologic past is an important endeavor because it allows us to compare modern climate change to natural variability. The magnesium/calcium ratio (Mg/Ca) of fossilized foraminifera shells is known to record the seawater temperature in which the foraminifer grew. However, shell Mg/Ca can be biased by other variables, like the total amount of carbon in seawater and its pH. In addition, the Mg/Ca ratio of the ocean itself was much lower in the past. In this study, we grew living foraminifera in the laboratory with simulated "ancient" seawater chemistry to test the influence of these confounding factors on shell Mg/Ca. We find that seawater carbon chemistry (like pH) has less of an effect on Mg/Ca in "ancient" seawater chemistry. We also find that one species, Trilobatus sacculifer, is least sensitive to these variables, making it the best choice for reconstructing sea surface temperatures.