Radiocarbon ( 14 C) ages obtained from planktic foraminiferal calcite are a mainstay for reconstructing ocean-climate change and carbon cycle dynamics of the past 30 k.y., yet the effects of diagenesis on this vital chronometer are poorly constrained. Here, we address this shortcoming by comparing 14 C ages and trace element ratios (Mg/Ca, Mn/Ca) of planktic foraminifera with white, opaque shells deemed well preserved by traditional standards to those with exquisitely preserved translucent shells. Results support a diagenetic mechanism as opaque shells yield 14 C ages invariably older and trace element ratios consistently higher than those of translucent shells. Radiocarbon age offsets are particularly pronounced in mono-specific samples taken from stratigraphic horizons proximal to the δ 18 O maximum marking the Last Glacial Maximum (LGM) and the subsequent deglacial. Radiocarbon-based calendar ages of translucent shells from the two intervals are congruent with the established age ranges for these climate events, whereas those of co-occurring opaque shells overestimate the LGM and deglacial by 8-15 k.y. and 14-22 k.y., respectively. These results demonstrate that the use of translucent foraminifera enhances reproducibility and accuracy of 14 C ages by minimizing the deleterious effects of diagenesis. This study serves as a cautionary tale since white, opaque foraminifera are common in pelagic sediments, and 14 C ages derived from their ostensibly well-preserved shells can lead to discrepancies in the timing of Quaternary climate events and ocean circulation reconstructions.

Isotope ratio mass spectrometry Planktic foraminifera 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.

Oxygen isotope ratios (δ18O) measured from planktic foraminifer shells are commonly used to reconstruct past surface ocean conditions, yet the shells of many planktic foraminifers are an aggregate mixture of multiple carbonate phases with differing δ18O compositions. Here we demonstrate how secondary ion mass spectrometry can be used to measure intrashell δ18O heterogeneity by performing in situ analyses on micrometer‐scale (3–10 μm) domains within individual shells of the extant, mixed layer species Trilobatus sacculifer. Secondary ion mass spectrometry measurements on shells taken from Holocene‐aged sediments at three sites in different ocean basins confirm that the δ18O of gametogenic (GAM) calcite added to shells during the terminal (reproductive) stage of the life cycle is 1.0–1.4‰ higher than that of pregametogenic calcite. Examination of shells in cross section reveals that many have suffered varying degrees of internal dissolution, which further skews whole‐shell δ18O compositions toward higher values by preferentially removing low δ18O pregametogenic calcite. The results of this study echo the calls of earlier studies cautioning that spatiotemporal changes in the proportion of high δ18O GAM calcite should be considered when assessing T. sacculifer δ18O records generated via conventional isotope ratio mass spectrometry.