Environmental controls on mg/ca in neogloboquadrina incompta : a core-top study from the subpolar north atlantic

Magnesium/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 (delta O-18 and delta C-13) 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 delta O-18(c) 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 mu mol kg(-1), we find that temperature exerts the dominant control on Mg/Ca values, while at values < 200 mu mol 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