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Cordilleran Section - 113th Annual Meeting - 2017

Paper No. 3-7
Presentation Time: 11:05 AM

SURFACE LAYER ND ISOTOPIC COMPOSITION OF FE-MN CRUSTS COLLECTED FROM THE TAKUYO-DAIGO SEAMOUNT IN THE NORTHWEST PACIFIC REFLECTS AMBIENT SEAWATER


AMAKAWA, Hiroshi1, USUI, Akira2, IIJIMA, Koichi1 and SUZUKI, Katsuhiko1, (1)Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan, (2)Geology Department, Kochi University, 2-5-1 Akebono, Kochi, 780-8520, Japan, amakawa@jamstec.go.jp

Ferromanganese (Fe-Mn) crusts, which are widely distributed on the ocean floor, have been frequently used for deriving chemical compositions of seawater in the past by measuring various radiogenic isotopes, such as Be, Pb, Nd, Os, and Sr, because the chemical composition of hydrogenously formed Fe-Mn crusts is believed to directly reflect that of seawater. So far, however, the Fe-Mn crust samples were collected by dredging method, which might lead us to obtain samples separated from the substrate. Due to the potential diagenetic perturbation caused by a direct contact with the sediment, those samples are inappropriate for chemical analysis. Fortunately, recent progress in the use of remotely operated vehicle (ROV) enables us to collect the appropriate Fe-Mn crust samples for chemical analysis by in-situdeep-sea camera observations.

Here we report the first record of surface layer (outermost 0–1 mm) Nd isotopic composition of Fe-Mn crusts collected by ROV from various water depths (1000–5400 m) along the Takuyo-Daigo Seamount in the northwest Pacific, and compare our data with the seawater Nd data previously reported at the near by station, TPS 24 27-1 (24°17.2’N, 150°28.2’E: Piepgras and Jacobsen, 1988).

The samples were collected during three cruises: RV Natsushima NT09-02, RV Kairei KR15-E01, and RV Kairei KR16-01. The topmost surface of each Fe-Mn crust was carefully scraped with a ceramic knife.

The sample was leached with either 2.5 M HCl or 1M HCl with H2O2. Nd was separated and purified from the leachate, and its isotopic composition was determined with the NEPTUNE Plus housed at JAMSTEC, Yokosuka.

We found that the depth profile of ferromanganese crusts is similar to the vertical seawater profile reported for TPS 24 271-1 station. This fact indicates that the surface layers of the Fe-Mn crust reliably reflect seawater values. We conclude that these samples are suitable for determining time-series Nd isotopic data to study ocean circulation of intermediate to deep water masses in the past northern Pacific.

Session No. 3

T13. Marine Minerals, Key Resources for the Twenty-First Century I
Tuesday, 23 May 2017: 8:30 AM-11:50 AM
Room 318B (Hawai‘i Convention Center)
Geological Society of America Abstracts with Programs. Vol. 49, No. 4
doi: 10.1130/abs/2017CD-292511
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