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

Abstract

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 (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.

A low climate threshold for south Greenland Ice Sheet demise during the Late Pleistocene

“…S7B). Compared with N. pachyderma (s) (36), N. incompta calcifies at or above the pycnocline (above 150 m), preferring shallower and warmer surface waters (37,38). Based on sediment trap studies, N. incompta blooms during late summer (July to September) at high latitudes (38) and follows closely our MAT summer SST estimates through the 4 previous interglacial plateaus (SI Appendix, Fig.…”

“…To convert our N. incompta-derived Mg/Ca data to temperature estimates, we further use the calibration developed by Morley et al (37), and we used the equation Mg/Ca = 0.40e (0.09T) after correcting the Mg/Ca for the ½CO 2− 3 ion effect. The latter correction was done on B/Ca data obtained for these samples following the approach proposed by Morley et al (37).…”

“…To convert our N. incompta-derived Mg/Ca data to temperature estimates, we further use the calibration developed by Morley et al (37), and we used the equation Mg/Ca = 0.40e (0.09T) after correcting the Mg/Ca for the ½CO 2− 3 ion effect. The latter correction was done on B/Ca data obtained for these samples following the approach proposed by Morley et al (37). Morley et al (37) report a large calibration error on the order of ±2.78°C, which they attribute to the limited data available to estimate the relationship between Mg/Ca and CO 3 ion concentration for values <200 μmol kg −2 .…”

“…The latter correction was done on B/Ca data obtained for these samples following the approach proposed by Morley et al (37). Morley et al (37) report a large calibration error on the order of ±2.78°C, which they attribute to the limited data available to estimate the relationship between Mg/Ca and CO 3 ion concentration for values <200 μmol kg −2 . However, based on the comparison of downcore data between the 2 foraminifera species and the MAT results, we estimate our error in these records to be on the order of <±0.6°C.…”

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

“…Additional confounding variables that may inhibit robust N . pachyderma calibrations include volcanic ash (Meland et al, ; Morley et al, ), mixing of glacial and Holocene foraminifera at low sedimentation rate sites (Meland et al, ), variations in the proportion of low‐Mg outer crust, which can comprise up to 90% of calcite (Davis et al, ; Lohmann, ) and up to seven different N . pachyderma genotypes (Darling et al, ).…”