Salinity Effect on Trace Element Incorporation in Cultured Shells of the Large Benthic Foraminifer Operculina ammonoides

The ratio of sodium to calcium in the shells of foraminifera (Na/Ca-shell) has been experimentally calibrated as a proxy for past ocean Ca concentrations (Hauzer et al., 2018, ). In parallel, it has been suggested that Na/Ca-shell could be used as a proxy for paleo-salinity. In this study, we determined the extent to which foraminiferal Na/Ca (and other elements) change with salinity for the shallow-dwelling large benthic foraminifer Operculina ammonoides, an extant relative of the abundant Eocene Nummulites. The culture experiment was conducted under four salinities between 33 and 43 psu. Shell chemistry was measured by LA-ICPMS with the newly precipitated CaCO3 identified by a Ba-135-spike added to the experimental seawater. Na/Ca-shell, Mg/Ca-shell and Li/Ca-shell in O. ammonoides increased slightly with salinity, while Sr/Ca-shell showed no resolvable change. The change in Na/Ca-shell due to salinity was small (similar to 1.4%/psu) compared to the changes in this ratio caused by varying seawater calcium concentrations (Ca-sw) with a sensitivity of similar to 5%/(mmol kg(-1)) Ca-sw. Moreover, the change in salinity in most regions of the past open oceans is minor compared to the large secular variations in Ca-sw during the Phanerozoic (10-40 mmol kg(-1)). Thus, if at all, paleo-salinity may be reconstructed based on Na/Ca-shell only for samples younger than Ca-sw residence time (similar to 1 Myr). Furthermore, both regional and global changes in ocean salinity over geological time do not pose a significant complication for the use of Na/Ca-shell as a proxy for past changes in seawater calcium concentrations.

Automated summary

Experimental results show that the ratio of sodium to calcium in foraminifera shells changes slightly with salinity, but this change is small compared to variations in seawater calcium concentrations. Therefore, Na/Ca-shell may not be a suitable proxy for paleo-salinity.