Impact of salinity on element incorporation in two benthic foraminiferal species with contrasting Magnesium contents

Abstract

<p><strong>Abstract.</strong> Accurate reconstructions of seawater salinity could provide valuable constraints for studying past ocean circulation, the hydrological cycle and sea level change. Controlled growth experiments and field studies have shown the potential of foraminiferal Na/Ca as a direct salinity proxy. Incorporation of minor and trace elements in foraminiferal shell carbonate varies, however, greatly between species and hence extrapolating calibrations to other species needs validation by additional (culturing) studies. Salinity is also known to impact other foraminiferal carbonate-based proxies, such as Mg/Ca for temperature and Sr/Ca for seawater carbonate chemistry. Better constraints on the role of salinity on these proxies will improve their reliability. Using a controlled growth experiment spanning a salinity range of 20 units and analysis of single chamber element composition using laser ablation-ICP-MS, we here show that Na/Ca correlates positively with salinity in two benthic foraminiferal species (<i>Ammonia tepida</i> and <i>Amphistegina lessonii</i>). The Na/Ca values differ between the two species, with an approximately 2-fold higher Na/Ca in <i>Amphistegina</i> than in <i>Ammonia</i>, which coincides with an offset in their Mg content (~ 35 mmol/mol versus ~ 2.5 mmol/mol for <i>A. lessonii</i> and <i>A. tepida</i>, respectively). Despite the offset in average Na/Ca values, the slopes of the Na/Ca-salinity regressions are similar between these two species. In addition, Mg/Ca and Sr/Ca are positively correlated with salinity in cultured <i>A. tepida</i>, but do not show a correlation to salinity for <i>A. lessonii</i>. Electron microprobe mapping of incorporated Na and Mg of the cultured specimens shows that within chamber walls of <i>A. lessonii</i>, Na/Ca and Mg/Ca occur in elevated bands in close proximity to the primary organic lining. For specimens of <i>A. tepida</i>, Mg-banding shows a similar pattern to that in <i>A. lessonii</i>, albeit that variation within the chamber wall is much less pronounced. Also Na-banding is much less prominent in this species. The less prominent banding and lower Mg and Sr contents of <i>A. tepida</i> are likely related to the absence of an inter-element correlation within experimental conditions.</p>

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

“…In this culture experiment, O. ammonoides data are characterized by a significant linear increase in Mg/ Ca shell and Na/Ca shell with increasing salinity (Figures 5a and 5c), a nonsignificant increase in Li/Ca shell (Figure 5b) and no change in Sr/Ca shell . Increases of El/Ca with increasing salinity have been reported before, in low-Mg calcitic foraminifera, for Na, Mg and Sr in both planktonic and benthic species (Allen et al, 2016;Bertlich et al, 2018;Dissard et al, 2010;Dueñas-Bohórquez et al, 2009;Geerken et al, 2018;Kısakürek et al, 2008;Lea et al, 1999;Nürnberg et al, 1996;Wit et al, 2013). A detailed comparison of our data with literature values for each element is given below.…”

“…This is also found in several field calibrations (e.g., Gray et al, 2018). The experimental work of Geerken et al (2018) for A. lessonii did not show this correlation. We note that the lowest Mg/Ca shell values (Figure 6c) and lowest slopes (Figure 6e) are for cultured A. tepida, even though the slopes of Na and Sr are closer to those of the planktonic species (Figure 6e).…”

“…Recently, an additional culture experiment with the arctic planktonic species N. pachyderma showed again a weak correlation between Na/Ca shell and salinity (Bertlich et al, 2021). For A. tepida, the two experiments (Geerken et al, 2018;Wit et al, 2013) showed a tight correlation with salinity, but the slope of Wit et al (2013) is over 2× higher than that of Geerken et al (2018) (0.215 and 0.076 respectively, Figure 6a and Table 3). We note that Wit et al (2013) modified the salinity by adding major ions to the seawater and kept the carbonate chemistry relatively constant.…”

“…We note that the lowest Mg/Ca shell values (Figure 6c) and lowest slopes (Figure 6e) are for cultured A. tepida, even though the slopes of Na and Sr are closer to those of the planktonic species (Figure 6e). In addition, the differences between the three A. tepida experiments (Dissard et al, 2010;Geerken et al, 2018;Wit et al, 2013) are emphasized when plotted as relative sensitivity in Figure 6f for all measured elements (values in Table 3). This species is well adapted to very low salinities or brackish water (reported to be 7 psu with optimal growth at 13 psu, Bradshaw, 1957).…”

“…However, further work by Mezger et al (2019) demonstrated a significant difference in Na concentrations between shell and spine of spinose planktonic foraminifera and concluded that coretop specimens lacking spines were characterized by a low sensitivity of Na/Ca shell to salinity. In addition to the Na/Ca shell , previous studies have shown that Mg/Ca shell and Sr/Ca shell also correlate with salinity (e.g., Allen et al, 2016;Dissard et al, 2010;Geerken et al, 2018;Kısakürek et al, 2008;Lea et al, 1999;Nürnberg et al, 1996;Wit et al, 2013). Although Mg/Ca shell has also been shown to be influenced by salinity, its use as a salinity proxy is limited because the sensitivity of Mg/Ca to salinity changes is significantly lower than to temperature (Gray et al, 2018).…”