Geological and microbial anomalies in the extinct submarine volcano, Shiribeshi Seamount, in the eastern margin of the Japan Sea

Abstract

The manned submersible Shinkai 2000 investigated yellow patches on the near‐summit slope of Shiribeshi Seamount in the Japan Sea. Yellowish patches are often associated with seepage, and the possibility of seepage at Shiribeshi Seamount was tested by the following four lines of observation: (i) high subsurface temperature was measured at a ring‐like patch, although no increase in subsurface temperature was observed at other patches; (ii) high gamma ray (γ‐ray) intensity from the thorium series was recorded in the patch zone; (iii) the yellowish deposit was composed of calcite, quartz and amorphous iron compound, as seen at the yellowish patches in other seeps and volcanoes; (iv) lipid phosphate, a measure of microbial abundance, in sediments of the ring‐like patch was determined, and the recorded microbial abundance was higher inside the patch than outside it. The four lines of observation are explained consistently by postulating that the seepage of warm fluid contained Fe and γ‐ray sources. A hydrothermal origin of the yellow patches is not ruled out for the extinct but young (0.9 Ma) arc volcano.

Estimation of in situ distribution of carbonate produced from cold-water octocorals on a Japanese seamount in the NW Pacific

“…However, there was a report that there was a steep change in the direction of the current at > 600 to 500 m on the northern slope of the seamount, though the current velocity was low, ca. 0.1 knots (5 cm s -1 ) (Ogawa et al 2002). The different coral distribution between the slopes (Fig.…”

“…The relative height of the seamount is approximately 3000 m on the western side and 1600 m on the eastern side, and its diameter is ca. 10 km at a depth of 1800 m (Ogawa et al 2002). The shallowest depths at the summit of the seamount are about 120 to 200 m, and there is a buried volcanic crater which has a major fault zone extending north-northwest to south-southeast.…”