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.

Trace and minor elements incorporated in foraminiferal shells are among the most used proxies for reconstructing past environmental conditions. A prominent issue concerning these proxies is that the inter-specimen variability in element composition is often considerably larger than the variability associated with the environmental conditions for which the proxy is used. Within a shell of an individual specimen the trace and minor elements are distributed in the form of bands of higher and lower concentrations. It has been hypothesized that differences in specimen-specific element banding patterns cause the inter-specimen and inter-species variability observed in average element composition, thereby reducing the reliability of proxies. To test this hypothesis, we compared spatial distributions of Mg, Na, Sr, K, S, P and N within chamber walls of two benthic foraminiferal species ( Amphistegina lessonii and Ammonia tepida ) with largely different average Mg content. For both species the selected specimens were grown at different temperatures and salinities to additionally assess how these parameters influence the element concentrations within the shell wall. Our results show that Mg, Na, Sr and K are co-located within shells, and occur in bands that coincide with organic linings but extend further into the calcite lamella. Changes in temperature or salinity modulate the element-banding pattern as a whole, with peak and trough heights co-varying rather than independently affected by these two environmental parameters. This means that independent changes in peak or trough height do not explain differences in average El/Ca between specimens. These results are used to evaluate and synthesize models of underlying mechanisms responsible for trace and minor element partitioning during calcification in foraminifera.

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