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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 7 | Copyright
Biogeosciences, 15, 2205-2218, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 16 Apr 2018

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 Esmee Geerken et al.
  • 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 NaCa 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 MgCa for temperature and SrCa 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 NaCa correlates positively with salinity in two benthic foraminiferal species (Ammonia tepida and Amphistegina lessonii). The NaCa values differ between the two species, with an approximately 2-fold higher NaCa in A. lessonii than in A. tepida, coinciding with an offset in their Mg content ( ∼ 35mmolmol−2 versus  ∼ 2.5mmolmol−1 for A. lessonii and A. tepida, respectively). Despite the offset in average NaCa values, the slopes of the NaCa–salinity regressions are similar between these two species (0.077 versus 0.064mmolmol−1 change per salinity unit). In addition, MgCa and SrCa 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, NaCa and MgCa 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.

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