NOAA/WDS Paleoclimatology - Micron-scale Intrashell Carbon Isotope and trace element data in the Cultured Planktic Foraminifer O. universa

In this study, we present experimental results from the planktic foraminifer Orbulina universa, cultured in the laboratory. We demonstrate that the d13C of shell calcite precipitated in 13C-labeled seawater for 24 h can be resolved and accurately measured using Secondary Ion Mass Spectrometry (SIMS). Specimens maintained at 20 °C were transferred from ambient seawater (d13CDIC = +1.3‰) into seawater with d13CDIC = +51.5‰ and enriched [Ba2+] for 24 h. Specimens were then transferred into ambient seawater with elevated [87Sr] for 6–9 h of calcification, followed by a transfer back into unlabeled ambient seawater until gametogenesis. This technique produced O. universa shells with calcite layers of distinct geochemical signatures. We quantify the spatial positions of trace element labels in the shells using laser ablation ICP-MS depth profiling. Using fragments from the same shells, we quantify intrashell d13Ccalcite using SIMS with a 6 or 8 µm spot (2 SD range ±0.5‰ to 1.7‰). Measured d13Ccalcite values in O. universa shell layers precipitated in ambient seawater are within 2‰ of predicted d13Ccalcite values. In 13C-labeled bands of calcite, 6 µm SIMS spot measurements are within 2‰ of predicted d13Ccalcite values, whereas 8 µm SIMS spots yield intermediate, mixed values. The spatial agreement between trace element and carbon isotope data suggests that 13C and cation tracers are synchronously incorporated into shell calcite. These results demonstrate the ability of SIMS d13C measurements to resolve ~10 µm features in foraminifer shell calcite using a 6 µm spot, and highlight the potential of this technique for addressing questions about ecology, biomineralization, and paleoceanography.