Biogeosciences, 15, 2205-2218, 2018
https://doi.org/10.5194/bg-15-2205-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
16 Apr 2018
Impact of salinity on element incorporation in two benthic foraminiferal species with contrasting magnesium contents
Esmee Geerken1, Lennart Jan de Nooijer1, Inge van Dijk1,a, and Gert-Jan Reichart1,2 1Department of Ocean Systems, NIOZ-Royal Netherlands Institute for Sea Research, and Utrecht University, Den Burg, the Netherlands
2Faculty of Geosciences, Utrecht University, Utrecht, the Netherlands
acurrently at: UMR CNRS 6112 LPG-BIAF, University of Angers, 49035 Angers, France
Abstract. 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 sea water carbonate chemistry. Better constraints on the role of salinity on these proxies will therefore improve their reliability. Using a controlled growth experiment spanning a salinity range of 20 units and analysis of element composition on single chambers using laser ablation-Q-ICP-MS, we show here that Na ∕ Ca correlates positively with salinity in two benthic foraminiferal species (Ammonia tepida and Amphistegina lessonii). The Na ∕ Ca values differ between the two species, with an approximately 2-fold higher Na ∕ Ca in A. lessonii than in A. tepida, coinciding with an offset in their Mg content ( ∼  35 mmol mol−2 versus  ∼  2.5 mmol mol−1 for A. lessonii and A. tepida, respectively). Despite the offset in average Na ∕ Ca values, the slopes of the Na ∕ Ca–salinity regressions are similar between these two species (0.077 versus 0.064 mmol mol−1 change per salinity unit). In addition, Mg ∕ Ca and Sr ∕ Ca are positively correlated with salinity in cultured A. tepida but show no correlation with salinity for A. lessonii. Electron microprobe mapping of incorporated Na and Mg of the cultured specimens shows that within chamber walls of A. lessonii, Na ∕ Ca and Mg ∕ Ca occur in elevated bands in close proximity to the primary organic lining. Between species, Mg banding is relatively similar, even though Mg content is 10 times lower and that variation within the chamber wall is much less pronounced in A. tepida. In addition, Na banding is much less prominent in this species than it is in A. lessonii. Inter-species differences in element banding reported here are hypothesized to be caused by differences in biomineralization controls responsible for element uptake.
Citation: Geerken, E., de Nooijer, L. J., van Dijk, I., and Reichart, G.-J.: Impact of salinity on element incorporation in two benthic foraminiferal species with contrasting magnesium contents, Biogeosciences, 15, 2205-2218, https://doi.org/10.5194/bg-15-2205-2018, 2018.
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