Environmental Controls on Mg/Ca in Neogloboquadrina incompta: A Core‐Top Study From the Subpolar North Atlantic

Abstract

AbstractMagnesium/calcium paleothermometry is an established tool for reconstructing past surface and deep‐sea temperatures. However, our understanding of nonthermal environmental controls on the uptake of Mg into the calcitic lattice of foraminiferal tests remains limited. Here we present a combined analysis of multiple trace element/calcium ratios and stable isotope (δ18O and δ13C) geochemistry on the subpolar planktonic foraminifera Neogloboquadrina incompta to assess the validity of Mg/Ca as a proxy for surface ocean temperature. We identify small size‐specific offsets in Mg/Ca and δ18Oc values for N. incompta that are consistent with depth habitat migration patterns throughout the life cycle of this species. Additionally, an assessment of nonthermal controls on Mg/Ca values reveals that (1) the presence of volcanic ash, (2) the addition of high‐Mg abiotic overgrowths, and (3) ambient seawater carbonate chemistry can have a significant impact on the Mg/Ca‐to‐temperature relationship. For carbonate‐ion concentrations of values > 200 μmol kg−1, we find that temperature exerts the dominant control on Mg/Ca values, while at values < 200 μmol kg−1 the carbonate‐ion concentration of seawater increases the uptake of Mg, thereby resulting in higher‐than‐expected Mg/Ca values at low temperatures. We propose two independent correction schemes to remove the effects of volcanic ash and carbonate‐ion concentration on Mg/Ca values in N. incompta within the calibration data set. Applying the corrections improves the fidelity of past ocean temperature reconstructions.