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Combined Effects of Gametogenic Calcification and Dissolution on δ18O Measurements of the Planktic Foraminifer Trilobatus sacculifer

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.