Takaya Shimono

Abundant bacterial magnetite occurrence in oxic red clay

Magnetotactic bacteria (MTB) produce chains of intracellular magnetite and/or greigite crystals and respond to an ambient magnetic field. MTB are considered to be microaerophilic to anaerobic organisms that live at and below the oxic-anoxic transition zone of aquatic environments. On the basis of rock magnetic analyses, including first-order reversal curve diagrams and isothermal remanent magnetization component analyses, along with transmission electron microscopy, we demonstrate that bacterial magnetites (magnetofossils) dominate magnetic mineral assemblages throughout a 76 m thickness of red clay at Integrated Ocean Drilling Program Site U1365 in the South Pacific Gyre, as well as in subsurface red clay of the North Pacific Gyre, where the sediment column contains abundant dissolved oxygen and no oxic-anoxic transition zone exists. This implies that MTB inhabit red clay; this conflicts with widespread interpretations of MTB ecology, namely that they are microaerophilic, requiring low levels of oxygen to grow and produce magnetite, and that magnetotaxis is used to help them find optimal positions in a strong vertical chemical gradient. Most magnetofossils in the red clay have cubo-octahedral morphology. This supports the notion that magnetofossil morphology can be a paleoenvironmental indicator; the proportion of elongated magnetofossils increases in less oxic environments. Our results also have implications for red-clay paleomagnetism in that magnetofossils may cause much-delayed remanence acquisition if MTB can live at decimeter depths within red clay.

Environmental rock-magnetism of Cenozoic red clay in the South Pacific Gyre

Nonfossiliferous red clay can be used for elucidating long-range environmental changes, although such studies were limited so far because of the difficulty in precise age estimation and extremely low sedimentation rates. We conducted an environmental rock-magnetic study of Cenozoic red clay at the Integrated Ocean Drilling Program Site U1365 in the South Pacific Gyre. Magnetostratigraphy could be established only above ∼6 m below the seafloor (mbsf) (∼5 Ma). Below ∼6 mbsf, the ages of the cores were transferred from the published ages of nearby Deep Sea Drilling Project Site 596, which is based mainly on a constant Cobalt flux model, by intercore correlation using magnetic susceptibility and rare earth element content variation patterns. Rock-magnetic analyses including first-order reversal curve diagrams, the ratio of anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (SIRM), and IRM component analyses revealed that magnetic minerals consist mainly of biogenic magnetite and terrigenous maghemite, and that the proportion of the terrigenous component increased since ∼23 Ma. We consider that the increase reflects a growth of eolian dust flux associated with a northward shift of Australia and the site to an arid region of the middle latitudes. The increase of the terrigenous component accelerated after ∼5 Ma, which may be associated with a further growth of the Antarctic glaciation at that time. This is coeval with the onset of the preservation of magnetostratigraphy, suggesting that the primary remanent magnetization is carried by the terrigenous component.

Influence of sampling on magnetic susceptibility anisotropy of soft sediments: comparison between gravity and piston cores

Anisotropy of magnetic susceptibility (AMS) has been used extensively for determining mineral orientatrections. In terms of the sample coordinate, Kmax declinations in the three gravity cores are oriented along the core-splitting surface, whereas Kmax declinations in the three piston cores are perpendicular to the splitting surface. We attribute the artificial AMS to the stress created by the deformation of core liners when being split. When interpreting AMS data from sediment cores, it is necessary to investigate the influence of sampling using the sample coordinates. In this paper, we also report over-sampling and under-sampling of piston cores from a comparison of down-core magnetic susceptibility variations between piston and gravity cores. It is noteworthy that under-sampling as well as over-sampling can occur in the uppermost few meters of piston cores.