Changing Abundance of Magnetofossil Morphologies in Pelagic Red Clay Around Minamitorishima, Western North Pacific

Recent investigations have discovered an unexpected abundance of magnetofossils in oxic pelagic red clays. These have potential to serve as paleoenvironmental tracers in otherwise nonfossiliferous sediments. Here, we report on variations in the abundance and morphology of magnetofossils in red clay from the western North Pacific. Magnetic measurements revealed that magnetofossils dominate the magnetic mineral assemblage of the sediments. An endmember analysis of isothermal remanent magnetization acquisition curves, supplemented by an analysis of S ratios, indicates that the magnetic assemblage can be unmixed into three endmembers, two corresponding to magnetofossils and one to terrigenous magnetic minerals. Direct counting of magnetofossil morphologies under a transmission electron microscope shows that the two magnetofossil endmembers differentiate equant magnetofossils and bullet‐shaped magnetofossils, respectively. The stratigraphic variation of the endmember contributions revealed that the equant magnetofossils are dominant for the most part, while an interval at around 7 m in core depth shows higher abundance of the bullet‐shaped magnetofossils. This may reflect enhanced organic carbon flux to the sediments. The organic carbon content is low throughout the sediments, and it does not show any change corresponding to the increase of bullet‐shaped magnetofossils, pointing at extensive remineralization of the organic carbon. On the basis of lithostratigraphic correlation to nearby drilling sites, we tentatively estimate the age of the bullet‐shaped magnetofossil increase as sometime between ∼75 and 25 Ma. These results suggest that environmental information can be obtained from magnetofossils in pelagic red clay. Plain Language Summary Ocean is important to understand global environmental change, and past marine environment is often estimated using fossils. However, abyssal sediments, which cover large part of world ocean floor, is generally non‐fossiliferous. We studied magnetic mineral magnetite produced by microbes in such deep sea sediments. Those magnetites can be considered as fossils (magnetofossils), and they should also record past environment. We have discovered a layer with enhancement of bullet‐shaped magnetofossils in abyssal western North Pacific. Our results demonstrate that abyssal environment may be variable in geological history, and changes can be recovered using magnetofossils. Key Points Depth variation of magnetofossil morphologies i