Revisiting diagenesis on the Ontong‐Java plateau: Evidence for authigenic crust precipitation in Globorotalia tumida

The calcite tests of foraminifera lie in marine sediments for thousands to millions of years, before being analyzed to generate trace element and isotope paleoproxy records. These sediments constitute a distinct physio-chemical environment from the conditions in which the tests formed. Storage in sediments can modify the trace element and isotopic content of foraminiferal calcite through diagenetic alteration, which has the potential to confound their paleoceanographic interpretation. A previous study of Globorotalia tumida from the Ontong Java Plateau, western equatorial Pacific, found that preferential dissolution of higher-Mg chamber calcite and the preservation of a low-Mg crust on the tests significantly reduced whole-test Mg/Ca and Sr/Ca. Here we revisit specimens with a combination of synchrotron X-ray computed tomography (sXCT) and electron probe microanalyses to reevaluate the nature of their diagenetic alteration. The dissolution of higher-Mg calcite with depth was directly observed in the sXCT data, confirming the inference of the previous study. The sXCT data further reveal a thickening of the chemically and structurally distinct calcite crust with depth. We propose that these crusts have a diagenetic origin, driven by the simultaneous dissolution of high-Mg chamber calcite and precipitation of low-Mg crust from the resulting modified pore water solution. While the breadth of the study is limited by the nature of the techniques, the observation of both dissolution and reprecipitation of foraminiferal calcite serves to demonstrate the action of two simultaneous diagenetic alteration processes, with significant impacts on the resulting paleoproxy signals.