The influence of salinity on Mg/Ca in planktic foraminifers - Evidence from cultures, core-top sediments and complementary δ¹⁸O

The Mg/Ca ratio in foraminiferal calcite is one of the principal proxies used for paleoceanographic temperature reconstructions, but recent core-top sediment observations suggest that salinity may exert a significant secondary control on planktic foraminifers. This study compiles new and published laboratory culture experiment data from the planktic foraminifers Orbulina universa, Globigerinoides sacculifer and Globigerinoides ruber, in which salinity was varied but temperature, pH and light were held constant. Combining new data with results from previous culture studies yields a Mg/Ca-sensitivity to salinity of 4.4±2.3%, 4.7±1.2%, and 3.3±1.7% per salinity unit (95% confidence), respectively, for the three foraminifer species studied here. Comparison of these sensitivities with core-top data suggests that the much larger sensitivity (27±4% per salinity unit) derived from Atlantic core-top sediments in previous studies is not a direct effect of salinity. Rather, we suggest that the dissolution correction often applied to Mg/Ca data can lead to significant overestimation of temperatures. We are able to reconcile culture calibrations with core-top observations by combining evidence for seasonal occurrence and latitude-specific habitat depth preferences with corresponding variations in physico-chemical environmental parameters. Although both Mg/Ca and δ¹⁸O yield temperature estimates that fall within the bounds of hydrographic observations, discrepancies between the two proxies highlight unresolved challenges with the use of paired Mg/Ca and δ¹⁸O analyses to reconstruct paleo-salinity patterns across ocean basins. The first step towards resolving these challenges requires a better spatially and seasonally resolved δ¹⁸O_sw archive than is currently available. Nonetheless, site-specific reconstructions of salinity change through time may be valid.