On the fidelity of shell-derived δ18Oseawater estimates

Paired foraminiferal Magnesium/Calcium ratio and oxygen isotope analyses are widely used to estimate surface ocean δ18Oseawater, a robust proxy for surface salinity. We assess the fidelity of shell-derived δ18Oseawater estimates for the surface-dwelling foraminifer Globigerinoides ruber (white) using an Atlantic meridional coretop transect spanning basin-scale temperature and salinity gradients. Shell-derived and observed δ18Oseawater values are well correlated (r2 = 0.77), but a large systematic bias is observed. Shell Mg/Ca ratios are significantly elevated above values expected from observed and isotopic calcification temperatures in the saline subtropical gyres of both hemispheres. This temperature-independent shell Mg/Ca ratio variability, termed “excess Mg/Ca”, is highly correlated with surface salinity (r2 = 0.77), and the observed salinity dependence (27 ± 4%) is much higher than indicated by culture studies (6 ± 2%). Our coretop data are used to develop new Atlantic Basin temperature and salinity calibration equations that are accurate (± 1.1 °C and ± 0.20, respectively), precise (r2 = 0.82 and 0.81, respectively), and verifiable using previously published data. These results are valid for the relatively high salinities of the subtropical Atlantic (35.5–37.3). We discover that inclusion of other published data from lower salinity regions (< 35) indicates little or no excess Mg/Ca. Taken together, these results point to a strongly non-linear, positive salinity effect on shell Mg/Ca ratios that significantly affects the accuracy of SST and δ18Oseawater estimates in high salinity settings (> 35).