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

Abstract

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.

Micro‐CT Scanning of Tests of Three Planktic Foraminiferal Species to Clarify DissolutionProcess and Progress

“…For example, the outermost chamber of T. sacculifer comprises two different calcite structures with ontogenetic calcite on the inner side and gametogenic calcite on the outer side. SEM observation of the outermost chamber wall structure of T. sacculifer revealed gametogenic calcite on the outer side, an ontogenetic calcite layer on an intermediate layer of chamber wall, and an inner calcite layer on the inside of a primary organic sheet (Wycech et al, ). Furthermore, our XMCT scan results are consistent with cross section SEM images of T. sacculifer tests from several core top samples that showed selective dissolution of the ontogenetic calcite layer on an intermediate layer of the outermost chamber wall as suggested in Wycech et al ().…”

“…SEM observation of the outermost chamber wall structure of T. sacculifer revealed gametogenic calcite on the outer side, an ontogenetic calcite layer on an intermediate layer of chamber wall, and an inner calcite layer on the inside of a primary organic sheet (Wycech et al, ). Furthermore, our XMCT scan results are consistent with cross section SEM images of T. sacculifer tests from several core top samples that showed selective dissolution of the ontogenetic calcite layer on an intermediate layer of the outermost chamber wall as suggested in Wycech et al (). The fact that our XMCT scans revealed such selective dissolution of an intermediate calcite layer of the outermost chamber wall also in G. ruber tests suggests that test dissolution of these two species is characterized by selective dissolution of early developed calcite formed during the juvenile stage and the ontogenetic calcite layer on an intermediate layer of the outermost chamber wall, and preservation of gametogenic calcite on the outer side and the inner calcite layer on the inner side of the primary organic sheet.…”

Multiproxy Reduced‐Dimension Reconstruction of Pliocene Equatorial Pacific Sea Surface Temperatures

“…The Mg/Ca-based SSTs were calculated using the T. sacculifer calibration of Dekens et al (2002). This Mg/Ca-temperature calibration includes a dissolution correction based on water depth that is deemed appropriate for SST reconstruction since foraminifer dissolution is prevalent in the equatorial Pacific (e.g., Fehrenbacher & Martin, 2011;Mekik et al, 2007;Rongstad et al, 2017;Rosenthal et al, 2000;Thompson & Saito, 1974;Wycech et al, 2018). Although the degree of dissolution likely varied slightly through the study interval (Pälike et al, 2012), a temporal adjustment to ΔCO 3 2− has a negligible effect on the reconstructed SST trend (see Appendix A).…”

Oxygen isotope composition of the final chamber of planktic foraminifera provides evidence of vertical migration and depth-integrated growth

“…1, scenarios A and B), the calcite produced in such a way may be indistinguishable isotopically from older chambers. Whilst the size of this bias induced by GAM may have been overestimated in the literature, for instance using cultures, Hamilton et al (2008) showed that approximately 80 % of the shell material is pre-GAM; new evidence suggests that the δ 18 O between pre-GAM and GAM is ∼ 1 ‰ (Wycech et al, 2018). The same work suggests that GAM comprises 32 % to 44 % of T. sacculifer shells.…”