| Abstrakt: |
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 (Ammonia tepida and Amphistegina lessonii). The Na/Ca values differ between the two species, with an approximately 2-fold higher Na/Ca in Amphistegina than in Ammonia, which coincides with an offset in their Mg content (~35mmol/mol versus ~2.5mmol/mol 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. In addition, Mg/Ca and Sr/Ca are positively correlated with salinity in cultured A. tepida, but do not show a correlation to 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. For specimens of A. tepida, Mg-banding shows a similar pattern to that in A. lessonii, 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 A. tepida are likely related to the absence of an inter-element correlation within experimental conditions. [ABSTRACT FROM AUTHOR] |
