Calibrating Trilobatus sacculifer Na/Ca Ratios From Atlantic Core-Tops as a Proxy for Sea Surface Salinity

Recent studies found a positive correlation between seawater salinity and Na/Ca ratios in foraminiferal calcite, suggesting a potential new paleosalinity proxy (Bertlich et al., 2018, ; Hauzer et al., 2021, ; Mezger et al., 2016, ; Wit et al., 2013). To test this new proxy in an open-ocean setting, Na/Ca ratios from the surface-dwelling planktic foraminifera Trilobatus sacculifer were measured from a suite of nine sediment core-tops spanning from the North Atlantic subtropical gyre to the South Atlantic subtropical gyre across a salinity range of 1.6. Results show a positive correlation between habitat salinity (HS) and T. sacculifer Na/Ca (Na/Ca (mmol/mol) = 0.516 (HS (+/- 0.17)) - 12.934; R-2 = 0.81, p < 0.005). We also find no relationship between T. sacculifer Na/Ca and sample size or habitat temperature, indicating Na/Ca is predominantly controlled by salinity. Additionally, we generate the first downcore record of sea surface salinity (SSS) in the Florida Straits over the last deglaciation based on T. sacculifer Na/Ca variability. Results show agreement between our Na/Ca record and a previously published deglacial delta O-18(sw) record that is also thought to reflect SSS variability from the same core (Schmidt & Lynch-Stieglitz, 2011, ). Both records indicate an abrupt increase in SSS during the Younger Dryas (11.7-12.9 kyr). Converting the Florida Straits T. sacculifer Na/Ca ratios to SSS using our new Atlantic core-top calibration indicates a maximum salinity change of similar to 2.2 across the last deglaciation, in agreement with modeling study estimates of salinity change across this period.