Ocean Bottom Gamma-Ray Anomaly Around Methane Seeps Related to Gas Hydrate- Bearing Zone in The Eastern Margin of Japan Sea and Off Southwest Taiwan
Abstract
JAMSTEC has conducted the ocean bottom gamma-ray measurement using ROVs and Submersibles since 1997. Gamma-ray spectrometer utilizes 3-inch spherical NaI(Tl) scintillator and the signal processor including DA converter in a pressure case. After processing data, we get total count rate (intensity value: count per second (cps)) of gamma ray and contents of K, U-, and Th-series radionuclides. The sensor was equipped to the side of the sample basket or foot of ROVs and submersibles, and always touches the seafloor when ROVs completely landed. Their results are posted on JAMSTEC website as a database. On the basis of past achievements, we present the results of the ocean bottom gamma-ray measurement at the methane seep sites related to gas hydrate off Joetsu in the eastern margin of Japan Sea and off southwest Taiwan. Off Joetsu: A number of mounds, large pockmarks (20 - 50 m deep and 200 - 500 m across), gas plumes, and gas hydrate are found at water depth of 900 - 1000 m in the Umitaka Spur and the Joetsu Knoll. Gamma-ray intensity values are 50 - 70 cps in normal muddy seafloor. On the other hand, the intensity values are 100 - 200 cps around methane venting sites, bacteria mats, and 'collapsed hydrate zone' which has an undulating, rugged seafloor with carbonate nodules and gravels. Contents of each radionuclide are also high. Low U/Th ratio suggests that there is less contribution of Rn accompanied with a recent fault activity. Off southwest Taiwan: Large, dense chemosynthetic communities, associated with carbonate pavements, were discovered at water depth of about 1100 - 1200 m on the top of the Formosa Ridge. Gamma-ray intensity values in normal muddy seafloor (120 - 150 cps) are higher than those around Japan. Since Th-series radionuclide easily absorbs other particles, it is commonly included in surface sediments. This may cause higher content of Th-series radionuclide in normal muddy seafloor. On the other hand, anomaly of gamma-ray intensity (200 - 300 cps) shows a methane seep activity from subsurface. It is hard to pinpoint the location and occurrence of seeps and fault without biotic activities. The ocean bottom gamma-ray measurement is one of the effective tools for the exploration of seeps and faults, though their values are different among areas and are not quantitative.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMOS33A1317M
- Keywords:
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- 3000 MARINE GEOLOGY AND GEOPHYSICS;
- 3004 Gas and hydrate systems;
- 3094 Instruments and techniques