Comparison of δ18O analyses on individual planktic foraminifer (Orbulina universa) shells by SIMS and gas-source mass spectrometry.

The oxygen isotope (δ 18 O) compositions of final chamber fragments of individual shells of the planktic foraminifer Orbulina universa were measured in situ via secondary ion mass spectrometry (SIMS) and by traditional gas-source mass spectrometry (GSMS) entailing acid digestion of sampled calcite. The paired SIMS-GSMS analyses were performed on final chamber fragments of fossil shells taken from the top of a sediment core (Holocene) as well as shells grown in laboratory culture. Multiple iterations of SIMS-GSMS analyses were conducted on final chamber fragments treated with a variety of cleaning protocols. The series of paired analyses yielded an average SIMS-GSMS δ 18 O offset (Δ 18 O SIMS-GSMS ) of −0.9 ± 0.1‰ (±2 SE). The volume of material analyzed in 10-μm SIMS spots is ~10 5 times smaller than that analyzed by GSMS; hence, the extent to which these Δ 18 O SIMS-GSMS values represent real differences in analyte vs. instrumental factors remains unclear. Possible contributing factors to the SIMS-GSMS δ 18 O difference include sample-standard mismatch by SIMS, differences in standardization of SIMS and GSMS, and non-calcite contaminants in samples. Although the two datasets are consistently offset, SIMS values reproduce inter-shell δ 18 O variability delineated by shell fragment GSMS values. This strong positive covariance proved useful for bringing the two datasets into agreement (i.e. Δ 18 O SIMS-GSMS  = 0), and confirms that SIMS-based foraminifer δ 18 O values record changes in calcification temperature and/or δ 18 O of seawater. Whether shells of foraminifer taxa with differing microcrystalline structures, chemical composition, and/or preservation histories register a similar Δ 18 O SIMS-GSMS value is a subject of ongoing testing. [ABSTRACT FROM AUTHOR]