Geochemical and microtextural properties of pyritized foraminiferal tests: results from ODP hole 763A, Southeastern Indian Ocean

Abstract The pyritization of microfossils serves as a key indicator of paleoenvironmental conditions, yet the controlling factors on pyrite morphology and composition remain poorly constrained. This study encountered different pyrite morphology (framboids and patches) from the foraminiferal tests retrieved from the marine sediment samples of Ocean Drilling Program Hole 763A, southeastern Indian Ocean, during different geological time slices. We hypothesize that distinct microenvironments and controlling factors might have influenced the morphology. Detailed investigations of the morphology, mineralogy and geochemistry of Fe-S coatings within and on foraminiferal tests suggest the dynamics of paleoredox conditions during the Middle Pleistocene and Upper Miocene. The comprehensive geochemical overview and the presence of Ni and Ba associated with Fe-S coatings and sediments suggest deoxygenation of deep-sea sediments driven by climatic shifts rather than hydrothermal activities. The stable anoxic deep ocean setting during the Middle Pleistocene, evidenced by increased organic matter flux and ocean stratification, contributed to low bottom-water oxygen levels. The geochemical evidence from the Upper Miocene samples indicates predominantly oxidising conditions, as shown by the altered reddish-yellow foraminiferal tests, which are mainly composed of calcite. However, localized reducing conditions are evidenced by patches of pyrite associated with foraminiferal shells, suggesting the presence of transitional redox conditions within the oxidising sediments during the Upper Miocene.