PRACTICALITY OF DETERMINING TRACE ELEMENT/CA RATIOS IN FORAMINIFERAL CALCITE IN THIN SECTION BY LA-ICP-MS

Analysis of foraminifera is a common practice in palaeoceanographic studies, as foraminiferal calcite provides trace element/calcium (TE/Ca) ratios that can be used for palaeothermometry. Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) allows for analysis of individual foraminifera in situ. Past studies have used LA-ICP-MS to sample large isolated foraminifera chambers from internal or external surfaces. However, such extractions are time-consuming and often impractical for highly indurated lithologies that may contain well-preserved foraminifera. Here we assess the practicality of LA-ICP-MS for determining TE/Ca ratios from individual foraminifera in thin section. Challenges to this approach include high variation in chamber wall thickness and that foraminiferal chambers develop micron-scale compositional banding during outward growth, requiring that laser sampling includes the full chamber wall thickness to get representative whole shell compositions. A significant advantage of the approach is the ability to use petrographic inspection to identify best-preserved chamber walls for LA-ICP-MS sampling. Optimal LA-ICP-MS measurement parameters were developed from analytical tests performed on well preserved foraminifera in thin sections made from core top foraminiferal splits (Ontong Java plateau, California margin). We then applied these parameters to analyze benthic and planktonic foraminifera in thin sections from a core of the Monterey Formation, which spans global cooling following the mid-Miocene climatic optimum. We find that curved line scans with square apertures provide highest quality data for sampling of chamber walls as thin as 10 µm. Average TE/Ca ratios obtained from best-preserved foraminifera chambers are within the expected range of foraminiferal calcite based on literature compilations. Further work aims to assess the practicality of determining water mass characteristics from benthic and planktonic foraminifera and to determine if the post-mid-Miocene cooling trend can by resolved by Mg/Ca palaeothermometry.