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SIMS measurements of intrashell δ13C in the cultured planktic foraminifer Orbulina universa


In this study, we present experimental results from the planktic foraminifer Orbulina universa, cultured in the laboratory. We demonstrate that the δ13C of shell calcite precipitated in 13C-labeled seawater for 24h can be resolved and accurately measured using Secondary Ion Mass Spectrometry (SIMS). Specimens maintained at 20°C were transferred from ambient seawater (δ13CDIC=+1.3‰) into seawater with δ13CDIC=+51.5‰ and enriched [Ba2+] for 24h. Specimens were then transferred into ambient seawater with elevated [87Sr] for 6-9h 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 δ13Ccalcite using SIMS with a 6 or 8μm spot (2 SD range ±0.5‰ to 1.7‰). Measured δ13Ccalcite values in O. universa shell layers precipitated in ambient seawater are within 2‰ of predicted δ13Ccalcite values. In 13C-labeled bands of calcite, 6μm SIMS spot measurements are within 2‰ of predicted δ13Ccalcite values, whereas 8μm 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 δ13C measurements to resolve ~10μm features in foraminifer shell calcite using a 6μm spot, and highlight the potential of this technique for addressing questions about ecology, biomineralization, and paleoceanography. © 2014 .

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