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金沢大学理工研究域地球社会基盤学系, Completely carbonated peridotites represent a window to study reactions of carbon-rich fluids with mantle rocks. Here, we present details on the carbonation history of listvenites close to the basal thrust in the Samail ophiolite. We use samples from Oman Drilling Project Hole BT1B, which provides a continuous record of lithologic transitions, as well as outcrop samples from listvenites, metasediments, and metamafics below the basal thrust of the ophiolite. 87Sr/86Sr of listvenites and serpentinites, ranging from 0.7090 to 0.7145, are significantly more radiogenic than mantle values, Cretaceous seawater, and other peridotite hosted carbonates in Oman. The Hawasina sediments that underlie the ophiolite, on the other hand, show higher 87Sr/86Sr values of up to 0.7241. δ13C values of total carbon in the listvenites and serpentinites range from −10.6‰ to 1.92‰. We also identified a small organic carbon component with δ13C as low as −27‰. Based on these results, we propose that during subduction at temperatures above >400°C, carbon-rich fluids derived from decarbonation of the underlying sediments migrated updip and generated the radiogenic 87Sr/86Sr signature and the fractionated δ13C values of the serpentinites and listvenites in core BT1B. © 2021. American Geophysical Union. All Rights Reserved.

Additional file 2 Representative orthogonal vector plots for the natural remanences. (a) Normal polarity sample. (b) Reversed polarity sample. (c-e) Samples whose polarities were indeterminate (see text for discussion). Solid circles show the horizontal projection, and open circles show the vertical projection. Colored markers show steps used for PCA analyses. Insets show decay of remanence intensity.

Additional file 1 Location and backtrack of the site. (Top) Location and backtrack path of Minamitorishima Island at 5 Myr intervals. The backtrack path was calculated using GPlates (Boyden et al. 2011) with the rotation model of Matthews et al. (2016). (Bottom) Detailed locations of the core sites.

金沢大学理工研究域地球社会基盤学系, 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. © 2017. American Geophysical Union. All Rights Reserved.

Additional file 1: Fig. S1. Photos of three piston cores from the western North Pacific Ocean. Fig. S2. Inclination of remanent magnetization and kARM/SIRM for core MR13-E02 Leg 2 PC01. Fig. S3. Inclination of remanent magnetization and kARM/SIRM for core KR14-02 PC04. Fig. S4. Age estimation of IODP Hole U1365A sediments, South Pacific Ocean. Fig. S5. Examples of FORC diagrams for two cores from the western North Pacific Ocean. Fig. S6. Examples of FORC diagrams for IODP Hole U1365A, South Pacific Ocean.

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