Reliability of foraminiferal Na/Ca as a direct paleo-salinity proxy in various planktonic species from the eastern tropical North Atlantic
Authors: Bertlich Jacqueline , Nürnberg Dirk , Hathorne Ed , Siccha Michael , Groeneveld Jeroen , Meilland Julie , Kucera Michal 
Published: March 2020

&lt;p&gt;&lt;span&gt;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 &amp;#948;&lt;/span&gt;&lt;sup&gt;&lt;span&gt;18&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;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 &lt;em&gt;Trilobatus sacculifer&amp;#160;&lt;/em&gt;[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&amp;#8211;3 mmol/mol for the same salinity within and between foraminiferal species, suggesting a dominant biological control.&amp;#160;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;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 (&lt;em&gt;Globigerinoides elongatus&lt;/em&gt;,&amp;#160;&lt;em&gt;G. ruber&amp;#160;&lt;/em&gt;(pink), &lt;em&gt;Orbulina universa&lt;/em&gt;, &lt;em&gt;Globigerina bulloides&lt;/em&gt;, &lt;em&gt;Neogloboquadrina dutertrei&lt;/em&gt;) 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, &lt;/span&gt;while seawater &lt;span&gt;pH and total alkalinity differed&lt;/span&gt;&amp;#160;by ~0.5 and 100 &amp;#181;mol/kg, respectively. Plankton tow samples&amp;#160;&lt;span&gt;provide new insights &lt;/span&gt;in&lt;span&gt;to &lt;/span&gt;the &lt;span&gt;possible effects of &lt;/span&gt;natural variations in &lt;span&gt;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 = &lt;/span&gt;&lt;span&gt;-&lt;/span&gt;&lt;span&gt;0.97, &lt;em&gt;p&lt;/em&gt;&lt;0.03&lt;/span&gt;&lt;span&gt;) decreasing Na/Ca values with increasing test sizes between &lt;/span&gt;&lt;span&gt;180-250 &amp;#181;m &lt;/span&gt;&lt;span&gt;for&amp;#160;&lt;em&gt;G. ruber&amp;#160;&lt;/em&gt;(pink, white), &lt;em&gt;N. dutertei&amp;#160;&lt;/em&gt;and&amp;#160;&lt;em&gt;T. sacculifer&lt;/em&gt;, whereas values increase again with larger size classes &gt;355 &amp;#181;m (R = 0.87, &lt;em&gt;p&lt;/em&gt;&lt;0.02).&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;[1] Wit et al. (2013) Biogeosciences &lt;strong&gt;10&lt;/strong&gt;, 6375-6387. [2] Mezger et al. (2016) Paleoceanography &lt;strong&gt;31&lt;/strong&gt;, 1562-1582. [3] Bertlich et al. (2018) Biogeosciences &lt;/span&gt;&lt;strong&gt;15&lt;/strong&gt;, 5991&amp;#8211;6018.&lt;span&gt;[4] Mezger et al. (2019) Biogeosciences &lt;strong&gt;16&lt;/strong&gt;, 1147-1165, 2019. [5] Allen et al. (2016) Geochim. Cosmochim. Acta &lt;strong&gt;193&lt;/strong&gt;, 197-221.&lt;/span&gt;&lt;/p&gt;