A User Guide for Choosing Planktic Foraminiferal Mg/Ca‐Temperature Calibrations.

Although foraminiferal magnesium/calcium (Mg/Ca) measurements are now widely used for reconstructing sea surface temperature (SST), there is uncertainty about the fidelity of different calibrations of this proxy. Whereas single‐variable calibrations suggest temperature sensitivity of about 9–10% per °C, multivariable calibrations suggest ∼6% per °C, with additional dependence on salinity and pH. Here, we apply five calibrations to six published Mg/Ca records of Globigerinoides ruber, a planktic foraminifer typically used for reconstructing low latitude SST during the Pleistocene. Reconstructed SST by the different calibrations, spanning the past 250,000 years, can be offset by a few degrees, possibly reflecting variable dissolution or hydrographic effects. However, for 4 out of 5 calibrations, the reconstructed temperature anomalies yield estimates that are consistent within the calibrations' uncertainty (<±1 °C), despite the fundamental differences in temperature sensitivity among the equations. We further propose a new seawater Mg/Ca record for the late Neogene and show that the same consistency holds for longer time scales (∼4 My) independently of the choice of the calibration or which seawater Mg/Ca record is used. These comparisons attest to the robustness of the calibrations despite all the confounding nonthermal effects, and offer an empirical basis for researchers and reviewers to judge the records without any prejudice about which calibration is the "best" and evaluate their uncertainties. Plain Language Summary: The ratio of magnesium to calcium (Mg/Ca) in the shells of foraminifera (single‐celled marine organisms) is sensitive to temperature, and therefore is used for reconstructing seawater temperatures. However, salinity and the pH exert additional influence. Recently, multivariable equations, which account for these nonthermal effects, have been developed to replace the simpler calibrations that account only for temperature variability. By applying the different calibrations to the same Mg/Ca records in several cores, we find that the inconsistencies among the calibrations are mostly below ±1 °C, despite the additional effects of salinity and pH on Mg/Ca‐derived temperatures. We further show that these results may be applicable for at least the past 4 million years. Key Points: We assess the performance of different planktic foraminiferal Mg/Ca calibrations for reconstructing sea surface temperaturesWe propose a new seawater Mg/Ca record for the late NeogeneWe assess uncertainties in Pliocene Mg/Ca‐temperature reconstruction [ABSTRACT FROM AUTHOR]