11:30 AM - 11:45 AM
[MIS24-04] A cold seep site on the outer shelf –a case study in the lower Pleistocene Ofuna and Koshiba formations, Yokohama City-
★Invited Papers
Keywords:Cold seep, Quaternary, Outer shelf
Ancient cold seep activities are preserved in the outer shelf facies in the Lower Pleistocene Ofuna and Koshiba formations, and the following observations are obtained.
Authigenic carbonate depleted in 13C and sulfide depended bivalves composed mostly of lucinids occur massively and abundantly, as a result of active anaerobic oxidation of methane associated with cold seep including methane. Authigenic carbonate consists of aragonite, high-Mg calcite and dolomite, and the origin of dolomite is likely to be a dolomitization of high-Mg calcite.
Central part of the seep site
Abundant bivalve fragments and distinct erosional surfaces of beds are observed. These observations suggest the presence of small scale pockmarks.
Peripheral part of the seep site
The fluctuation of carbon stable isotope ratios of authigenic carbonate normal to the beddings show a very similar pattern to those of the dissolved inorganic carbon at the modern methane seep site. The most abundant horizon of the archaeal and bacterial biomarkers greatly depleted in 13C matches the lowest value horizon of the carbon stable isotope ratios of the authigenic carbonate. These observations suggest that an ancient MSI (Methane Sulfate Interface) horizon can be identified and indicate that the precipitation of authigenic carbonate at the seep peripheral can be explained by a snapshot model.
Age span and timing of the seep activity
Seep activity continued intermittently from 1.68Ma to 1.48Ma estimated from the age model based on the oxygen stable isotope fluctuation of foraminifera. Seep activity was synchronized with the glacial-interglacial cycle, and active in the glacial period. This result is consistent with the fact that few seep activities have been found on the current shelf. The trace of seep activity in the glacial time on the sea floor must have been covered by the strata due to the high sedimentation rate of the shelf.
Authigenic carbonate depleted in 13C and sulfide depended bivalves composed mostly of lucinids occur massively and abundantly, as a result of active anaerobic oxidation of methane associated with cold seep including methane. Authigenic carbonate consists of aragonite, high-Mg calcite and dolomite, and the origin of dolomite is likely to be a dolomitization of high-Mg calcite.
Central part of the seep site
Abundant bivalve fragments and distinct erosional surfaces of beds are observed. These observations suggest the presence of small scale pockmarks.
Peripheral part of the seep site
The fluctuation of carbon stable isotope ratios of authigenic carbonate normal to the beddings show a very similar pattern to those of the dissolved inorganic carbon at the modern methane seep site. The most abundant horizon of the archaeal and bacterial biomarkers greatly depleted in 13C matches the lowest value horizon of the carbon stable isotope ratios of the authigenic carbonate. These observations suggest that an ancient MSI (Methane Sulfate Interface) horizon can be identified and indicate that the precipitation of authigenic carbonate at the seep peripheral can be explained by a snapshot model.
Age span and timing of the seep activity
Seep activity continued intermittently from 1.68Ma to 1.48Ma estimated from the age model based on the oxygen stable isotope fluctuation of foraminifera. Seep activity was synchronized with the glacial-interglacial cycle, and active in the glacial period. This result is consistent with the fact that few seep activities have been found on the current shelf. The trace of seep activity in the glacial time on the sea floor must have been covered by the strata due to the high sedimentation rate of the shelf.