NOAA/WDS Paleoclimatology - Micron-scale Intrashell Oxygen Isotope Data in the Cultured Planktic Foraminifer O. universa

In this study, we show that the rate of shell precipitation in the extant planktic foraminifer Orbulina universa is sufficiently rapid that 12 h calcification periods in 18O-labeled seawater can be resolved and accurately measured using Secondary Ion Mass Spectrometry (SIMS) for in situ d18O analyses. Calcifying O. universa held at constant temperature (22°C) were transferred every 12 h between ambient seawater (d18Ow = -0.4‰ VSMOW) and seawater with enriched barium and d18Ow = +18.6‰ VSMOW, to produce geochemically distinct layers of calcite, separated by calcite precipitated with an ambient geochemical signature. We quantify the position of the Ba-labeled calcite in the shell wall of O. universa via laser ablation ICP-MS depth profiling of trace element ratios, and then measure intrashell d18Ocalcite in the same shells using SIMS with a 3 µm spot and an average precision of 0.6‰ (±2 SD). Measured d18Ocalcite values in O. universa shell layers are within ±1.1‰ of predicted d18Ocalcite values. Elemental and oxygen isotope data show that LA-ICP-MS and SIMS measurements can be cross-correlated within the spatial resolution of the two analytical techniques, and that d18Ocalcite and elemental tracers appear to be precipitated synchronously with no measurable spatial offsets. These results demonstrate the capability of SIMS to resolve daily growth increments in foraminifer shells, and highlight its potential for paleoceanographic and biomineralization applications on microfossils.