Determination of Intra-Test Variability of Trace Elements in Foraminifera by Laser Ablation Inductively Coupled Plasma Mass Spectrometry

Analysis of trace elements in fossil foraminiferal tests is widely used by palaeoceanographers to reconstruct past environmental conditions in the oceans. However, there remain a number of problems associated with the interpretation of these trace element data. Firstly, there is a lack of understanding as to the controls on the uptake of trace elements into foraminiferal tests. Secondly, fossil test chemistry may be influenced by post-depositional diagenesis, and thirdly, the impact of chemical cleaning techniques, used to remove contaminant detrital and authigenic mineral phases from the test surface, on test chemistry is uncertain. In order to address these problems, information is required as to the variability of trace elements within individual foram tests. To this end, we have developed a technique for the analysis of trace elements in foraminiferal calcite by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). This technique is reproducible (<6 % for most elements), and has low detection limits. The accuracy of the technique is difficult to assess, but Mg/Ca, Sr/Ca and Li/Ca ratios for foraminiferal calcite are within the range reported elsewhere in the literature when normalised to a CaCO₃ standard. Analysis of different types of calcite within a foram test shows that Sr/Ca and Li/Ca of calcite pustules surrounding the aperture of Gr. tumida are distinctly different from the chamber and keel calcite. Successive analyses of the same Orbulina universa test throughout a chemical cleaning procedure show that the Mg/Ca ratio of the test falls significantly after removal of ferromanganese oxide phases, but is unchanged after oxidative cleaning. Rastering of relatively flat parts of G. sacculifer has enabled us to investigate changes in chemistry through the test wall. High Li/Ca on the outermost part of the test can be attributed to contaminant phases, but Li/Ca is relatively constant at >5 μm depth. The depth of this `contaminant' layer is significantly shallower if the foram is glassy, rather than opaque.