Salinity Effect on Trace Element Incorporation in Cultured Shells of the Large Benthic Foraminifer <i>Operculina</i><i>a</i><i>mmonoides</i>
Hagar Hauzer, David Evans, Wolfgang Müller, Yair Rosenthal, Jonathan Erez
Abstract
Abstract 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, https://doi.org/10.1016/j.epsl.2018.06.004 ). 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 CaCO 3 identified by a 135 Ba‐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 (∼1.4%/psu) compared to the changes in this ratio caused by varying seawater calcium concentrations (Ca sw ) with a sensitivity of ∼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 (∼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.