NOAA/WDS Paleoclimatology - Measurements of d18O on the Planktic Foraminifer Trilobatus sacculifer from different ocean basins

Oxygen isotope ratios (d18O) 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 d18O compositions. Here we demonstrate how secondary ion mass spectrometry (SIMS) can be used to measure intra-shell d18O heterogeneity by performing in situ analyses on micrometer-scale (3-10 μm) domains within individual shells of the extant, mixed layer species Trilobatus sacculifer. SIMS measurements on shells taken from Holocene-aged sediments at three sites in different ocean basins confirm that the d18O 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 pre-gametogenic (PREGAM) calcite. Examination of shells in cross-section reveals that many have suffered varying degrees of internal dissolution, which further skews whole-shell d18O compositions toward higher values by preferentially removing low d18O PREGAM calcite. The results of this study echo the calls of earlier studies cautioning that spatiotemporal changes in the proportion of high d18O GAM calcite should be considered when assessing T. sacculifer d18O records generated via conventional isotope ratio mass spectrometry.