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

Abstract

AbstractRecent 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, https://doi.org/10.5194/bg-15-5991-2018; Hauzer et al., 2021, https://doi.org/10.1029/2021pa004218; Mezger et al., 2016, https://doi.org/10.1002/2016pa003052; 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; R2 = 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 δ18Osw record that is also thought to reflect SSS variability from the same core (Schmidt & Lynch‐Stieglitz, 2011, https://doi.org/10.1029/2011PA002157). 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 ∼2.2 across the last deglaciation, in agreement with modeling study estimates of salinity change across this period.