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

The oxygen isotope (δ18O) 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 δ18O offset (Δ18OSIMS-GSMS) of −0.9 ± 0.1‰ (±2 SE). The volume of material analyzed in 10-μm SIMS spots is ~105 times smaller than that analyzed by GSMS; hence, the extent to which these Δ18OSIMS-GSMS values represent real differences in analyte vs. instrumental factors remains unclear. Possible contributing factors to the SIMS-GSMS δ18O 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 δ18O variability delineated by shell fragment GSMS values. This strong positive covariance proved useful for bringing the two datasets into agreement (i.e. Δ18OSIMS-GSMS = 0), and confirms that SIMS-based foraminifer δ18O values record changes in calcification temperature and/or δ18O of seawater. Whether shells of foraminifer taxa with differing microcrystalline structures, chemical composition, and/or preservation histories register a similar Δ18OSIMS-GSMS value is a subject of ongoing testing

The Asian monsoon over the past 640,000 years and ice age terminations

Oxygen isotope records from Chinese caves characterize changes in both the Asian monsoon and global climate. Here, using our new speleothem data, we extend the Chinese record to cover the full uranium/thorium dating range, that is, the past 640,000 years. The record's length and temporal precision allow us to test the idea that insolation changes caused by the Earth's precession drove the terminations of each of the last seven ice ages as well as the millennia-long intervals of reduced monsoon rainfall associated with each of the terminations. On the basis of our record's timing, the terminations are separated by four or five precession cycles, supporting the idea that the '100,000-year' ice age cycle is an average of discrete numbers of precession cycles. Furthermore, the suborbital component of monsoon rainfall variability exhibits power in both the precession and obliquity bands, and is nearly in anti-phase with summer boreal insolation. These observations indicate that insolation, in part, sets the pace of the occurrence of millennial-scale events, including those associated with terminations and 'unfinished terminations'.National Research Foundation (NRF)Accepted versionThis work was supported by China grants NBRP 2013CB955902, NSFC 41230524, 4157020432 and 41561144003, US NSF grants 0502535, 1103404, 0823554, 1003690, 1137693 and 1317693 and Singapore grant NRF-NRFF2011-08. We thank M. Siddall for help with analysis of the millennial-scale variability of the Antarctic temperature record and A. P. Roberts for converting the ice core chronology