Paired analyses of oxygen isotope and elemental ratios within individual shells of benthic foraminifera genus Uvigerina

We compare different methodologies (in situ and bulk) for obtaining oxygen isotope (δ18O) and elemental ratios in the benthic foraminifera genus Uvigerina from ODP Site 1015, (California Margin), to assess what new information can be obtained with high-resolution in situ techniques. Specimens were prepared in epoxy mounts and exposed in cross-section such that multiple high-resolution analyses could be completed on the same shells using both Secondary Ionization Mass Spectrometry (SIMS) and Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS). We also measured elemental ratio data using LA-ICP-MS depth-profile measurements on whole, uncast foraminifera and elemental ratio and δ18O with standard bulk techniques using the same species of benthic foraminifera from the same sediment sub-samples. Comparison of the data collected by the different methodologies indicates that there is a consistent offset of ~0.9 ± 0.1‰ between SIMS and bulk analysis of δ18O in these samples. The in situ laser data studied in epoxy mount is correlated with the foraminifera bulk measurements for both Mg and Sr, whereas the in situ depth-profile laser measurements of Mg and Sr from whole foraminifera are less correlated with the bulk measurements. We also observe that the intra-shell variability for each of the proxies is larger than the analytical error and does not follow chamber number. We propose that the δ18O, Mg, and Sr variability within and between single specimens at this site is linked to some combination of measurement bias, vital effects, and variable environmental conditions in the pore water where the tests were precipitated. This information can in turn be related to the regional setting of the site.