Micron-scale intrashell oxygen isotope variation in cultured planktic foraminifers

In this study, we show that the rate of shell precipitation in the extant planktic foraminifer Orbulina universa is sufficiently rapid that 12h calcification periods in ¹⁸O-labeled seawater can be resolved and accurately measured using secondary ion mass spectrometry (SIMS) for in situ δ¹⁸O analyses. Calcifying O. universa held at constant temperature (22°C) were transferred every 12h between ambient seawater (δ¹⁸O_w=-0.4‰ VSMOW) and seawater with enriched barium and δ¹⁸O_w=+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 δ¹⁸O_calcite in the same shells using SIMS with a 3μm spot and an average precision of 0.6‰ (±2 SD). Measured δ¹⁸O_calcite values in O. universa shell layers are within ±1.1‰ of predicted δ¹⁸O_calcite 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 δ¹⁸O_calcite 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.