Depositional Age of a Fossil Whale Bone from São Paulo Ridge, South Atlantic Ocean, Based on Os Isotope Stratigraphy of a Ferromanganese Crust

Tatsuo Nozaki, Yutaro Takaya, Takashi Toyofuku, Ayaka Tokumaru, Kosuke T. Goto, Qing Chang, Jun Ichi Kimura, Yasuhiro Kato, Katsuhiko Suzuki, Adolpho Herbert Augustin, Hiroshi Kitazato

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    Abstract

    Whale carcasses (whale falls) deposited on the deep seafloor are associated with a distinctive biotic community. A fossil whale bone recovered from São Paulo Ridge, South Atlantic Ocean, during cruise YK13–04 Leg 1 of R/V Yokosuka was covered by a ferromanganese (Fe–Mn) crust approximately 9 mm thick. Here, we report an age constraint for this fossil bone on the basis of Os isotopic stratigraphy (187Os/188Os ratio) of the Fe–Mn crust. Major- and trace-element compositions of the crust are similar to those of Fe–Mn crusts of predominantly hydrogenous origin. Rare earth element concentrations in samples of the crust, normalized with respect to Post-Archean average Australian Shale, exhibit flat patterns with positive Ce and negative Y anomalies. These results indicate that the Fe–Mn crust consists predominantly of hydrogenous components and that it preserves the Os isotope composition of seawater at the time of its deposition. 187Os/188Os ratios of three Fe–Mn crust samples increased from 0.904 to 1.068 in ascending stratigraphic order. The value of 1.068 from the surface slice (0–3 mm depth in the crust) was identical to that of present-day seawater within error (~1.06). The value of 0.904 from the basal slice (6–9 mm) equaled seawater values from ca. 4–5 Ma. Because it is unknown how long the bone lay on the seafloor before the Fe–Mn crust was deposited, the Os stratigraphic age of ca. 5 Ma is a minimum age of the fossil. This is the first application, to our knowledge, of marine Os isotope stratigraphy for determining the age of a fossil whale bone. Such data may offer valuable insights into the evolution of the whale-fall biotic community.

    Original languageEnglish
    Pages (from-to)442-450
    Number of pages9
    JournalResource Geology
    Volume67
    Issue number4
    DOIs
    Publication statusPublished - 2017 Oct 1

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    Keywords

    • ferromanganese crust
    • Os isotope stratigraphy
    • South Atlantic Ocean
    • São Paulo Ridge
    • whale fall

    ASJC Scopus subject areas

    • Geology
    • Geochemistry and Petrology

    Cite this

    Nozaki, T., Takaya, Y., Toyofuku, T., Tokumaru, A., Goto, K. T., Chang, Q., Kimura, J. I., Kato, Y., Suzuki, K., Augustin, A. H., & Kitazato, H. (2017). Depositional Age of a Fossil Whale Bone from São Paulo Ridge, South Atlantic Ocean, Based on Os Isotope Stratigraphy of a Ferromanganese Crust. Resource Geology, 67(4), 442-450. https://doi.org/10.1111/rge.12138