Radiocarbon (14C) 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 14C 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 14C 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 δ18O 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 14C 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 14C ages derived from their ostensibly well-preserved shells can lead to discrepancies in the timing of Quaternary climate events and ocean circulation reconstructions.