Diagenetic dolomite in planktonic foraminifera on the Australian Northwest Shelf

Planktonic foraminiferal shells are widely used to investigate past oceanographic and climatic variations via their trace elements and stable isotopes. However, these geochemical methods may be compromised by the presence of diagenetic high-Mg calcite. In this study, dolomite crystals are observed in planktonic foraminifera from International Ocean Discovery Program Site U1464 sediments on the Australian Northwest shelf, a shallow marine environment (<300 m). This study investigates the formation of the dolomite using new scanning electron microscopy, X-ray diffraction, stable isotope (carbon and oxygen), Mg/Ca and rare earth element data, combined with shipboard geochemistry and available literature. This study finds that foraminiferal dolomite formation was probably associated with early diagenesis, microbial and ageing processes. Two potential diagenetic processes (seafloor diagenesis and post-depositional diagenesis) are proposed, and both indicate that the favourable redox condition of dolomite formation is low oxygen, which may activate methanogenic catalysis to facilitate dolomite formation. It is postulated that high-Mg calcite may be the precursor of foraminiferal dolomite in this case, and that dissolution-precipitation may be the ageing process of dolomite formation. In contrast to deeper, more normal pelagic environments, this study shows that the marine environment can facilitate dolomite precipitation by higher fluxes of organic matter to the seafloor.