Impact of salinity on element incorporation in two benthic foraminiferal species with contrasting magnesium contents
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. Incorpo...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-04-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/15/2205/2018/bg-15-2205-2018.pdf |
| Summary: | 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
(<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>A. lessonii </i>than in <i>A. tepida</i>, coinciding
with an offset in their Mg content ( ∼ 35 mmol mol<sup>−2</sup> versus
∼ 2.5 mmol mol<sup>−1</sup> 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 (0.077 versus 0.064 mmol mol<sup>−1</sup> change per salinity unit). In
addition, Mg ∕ Ca and Sr ∕ Ca are positively correlated with salinity
in cultured <i>A. tepida</i> but show no correlation with 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. 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 <i>A.
tepida</i>. In addition, Na banding is much less prominent in this species than
it is in <i>A. lessonii</i>. 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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| ISSN: | 1726-4170 1726-4189 |