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Depositional Age of a Fossil Whale Bone from São Paulo Ridge, South Atlantic Ocean, Based on Os Isotope Stratigraphy of a Ferromanganese Crust
Author(s) -
Nozaki Tatsuo,
Takaya Yutaro,
Toyofuku Takashi,
Tokumaru Ayaka,
Goto Kosuke T.,
Chang Qing,
Kimura Junichi,
Kato Yasuhiro,
Suzuki Katsuhiko,
Augustin Adolpho Herbert,
Kitazato Hiroshi
Publication year - 2017
Publication title -
resource geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.597
H-Index - 43
eISSN - 1751-3928
pISSN - 1344-1698
DOI - 10.1111/rge.12138
Subject(s) - geology , crust , ferromanganese , oceanic crust , seafloor spreading , geochemistry , diagenesis , ridge , continental crust , seawater , hydrothermal circulation , paleontology , manganese , oceanography , tectonics , subduction , chemistry , organic chemistry
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 ( 187 Os/ 188 Os 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. 187 Os/ 188 Os 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.