Reliability of foraminiferal Na/Ca as a direct paleo-salinity proxy in various planktonic species from the eastern tropical North Atlantic

<p><span>Foraminiferal Na/Ca in planktonic and benthic foraminifers is a promising new method to assess directly past seawater salinities, which complements existing approaches (e.g., paired shell Mg/Ca and δ</span><sup><span>18</span></sup><span>O, shell Ba/Ca). Recent culture and field calibration studies have shown a significant positive relationship of Na incorporation into foraminiferal calcite shells with increasing salinity [1, 2], as confirmed by our culture study of <em>Trilobatus sacculifer </em>[3]. However, we note that the sensitivity of Na/Ca in response to salinity changes is species-specific and regional dependent, whereas temperature could be excluded as a secondary influencing factor [2, 3, 5]. Na/Ca values vary from 1–3 mmol/mol for the same salinity within and between foraminiferal species, suggesting a dominant biological control. </span></p><p><span>To further evaluate the robustness of Na/Ca for its application as a reliable proxy, we here examine possible secondary controls on foraminiferal Na/Ca with new data for commonly used species for paleoreconstructions (<em>Globigerinoides elongatus</em>, <em>G. ruber </em>(pink), <em>Orbulina universa</em>, <em>Globigerina bulloides</em>, <em>Neogloboquadrina dutertrei</em>) collected by plankton tows in the eastern tropical North Atlantic during R/V Meteor cruise M140. We performed laser ablation ICP-MS measurements on single foraminiferal shells from depth-resolved plankton tows in 20 m net-intervals from locations where salinity was essentially constant, </span>while seawater <span>pH and total alkalinity differed</span> by ~0.5 and 100 µmol/kg, respectively. Plankton tow samples <span>provide new insights </span>in<span>to </span>the <span>possible effects of </span>natural variations in <span>carbonate system parameters on Na incorporation into calcite tests with increasing water depth. The comparison of living foraminifers to sedimentary shells gives further information about the preservation state of Na/Ca in calcite shells over time, whereas fossil shells have mostly undergone gametogenesis during their life-time, or were affected post mortem by early diagenesis (sedimentation) processes. Those foraminifers were collected from surface sediments (M65-1) located in proximity to plankton tows. Our results show that all measured species, either from plankton tows or buried in the sediment, are within the Na/Ca range of previous studies [1-5], which increases the confidence for a robust Na/Ca to salinity proxy. However, the offset of ~2-5 mmol/mol between living foraminifers collected in surface waters (0-20 m) and fossil assemblages of the same species could be related to spine loss at the end of a foraminiferal life cycle [4]. In addition, the usage of inconsistent test sizes could further influence the foraminiferal Na/Ca signal. Our results reveal significant (R = </span><span>-</span><span>0.97, <em>p</em><0.03</span><span>) decreasing Na/Ca values with increasing test sizes between </span><span>180-250 µm </span><span>for <em>G. ruber </em>(pink, white), <em>N. dutertei </em>and <em>T. sacculifer</em>, whereas values increase again with larger size classes >355 µm (R = 0.87, <em>p</em><0.02).</span><span> </span></p><p><span>[1] Wit et al. (2013) Biogeosciences <strong>10</strong>, 6375-6387. [2] Mezger et al. (2016) Paleoceanography <strong>31</strong>, 1562-1582. [3] Bertlich et al. (2018) Biogeosciences </span><strong>15</strong>, 5991–6018.<span>[4] Mezger et al. (2019) Biogeosciences <strong>16</strong>, 1147-1165, 2019. [5] Allen et al. (2016) Geochim. Cosmochim. Acta <strong>193</strong>, 197-221.</span></p>