17:15 〜 18:30
[MIS21-P03] Chemosynthesis, surface ruptures and possible methane eruption on the Kuroshima Knoll: a review from previous studies
キーワード：黒島海丘、炭酸塩チムニー、メタン、化学合成
Kuroshima Knoll is located about 40km south of Ishigaki Island in the southwest Ryukyu arc. A Calyptogena colony was first discovered during the survey on seafloor fractures on the knoll by use of a deep-tow camera system in 1996. After this expedition, a reconnaissance survey on the eastern edge of the summit of the knoll was carried out at 24° 07-08’ N, 124° 11-13’ E by use of ROV Dolphin-3K and DSV Shinkai2000 in 1997 (Cruise ID: NT97-14). The purpose of the expedition was to detect the distribution pattern of the chemosynthetic communities and relationship between their situation and the basement rocks of the knoll. The survey revealed a living white Calyptogena colony (chemosynthetic communities) in the survey area (depth: 812-682m). Dense colonies of Calyptogena (both living and dead) were observed. A living tube worm is also found in this place. Doming structures and sedimentary mounds which were apparently formed recently together with brecciated rocks were also observed. These suggest that compressional stresses are predominant in the eastern part of the summit. Pipe-like calcareous sandstone “chimneys” (both standing and lying) were also observed near the mounds. It is estimated that the top of the Kuroshima Knoll is composed of the Miocene to Early Pleistocene Shimajiri Group overlain by the Pleistocene Ryukyu Group and that they are covered with recent calcareous sediments. The whole survey area seems to be suffered from fracture and brecciation. It is estimated that cold water seep and gas eruption have occurred since late Pleistocene era, accompanied with rock fracturing. Since then, calcareous sandstone chimneys might have been formed. (Matsumoto et al., 1998, JAMSTEC J. Deep Sea Res.).
Thus, a series of research cruises up to 2001 suggested the existence of methane hydrate under the seafloor of the knoll. After those expeditions, total 33 dives by ROV Dolphin-3K, ROV Hyper-Dolphin, DSV Shinkai2000 and DSV Shinkai6500 were operated from 2002 to 2004 for reconnaissance survey in the vicinity of 24° 07’48” N 124° 11’30” E (depth: 686-622m). Based on the video images from these dives, a seafloor mapping on the knoll was carried out in order to detect the area of the communities of chemosynthetic fauna and methane eruption. Calcareous sandstones, dead Calyptogena and living Bathymodiolus colonies spread over the wide area on the knoll. 35 methane bubble gushing were also observed. The area where methane may be existing beneath the seafloor of the knoll was estimated to be about 40,000 square meters in the survey area (Matsumoto and Aoki, JpGU2014, MIS22-P05).
It is necessary to examine if the sub-seafloor beneath the knoll is within the temperature and pressure conditions for methane hydrate stability. The temperature and depth records at 17 ARGO float stations around the knoll (there are no ARGO float stations just above the knoll) were examined to draw a temperature-depth profile. The result shows that the temperature at the summit (water depth : 640m) is too high (6 degrees Celsius in autumn, 9 degrees Celsius in spring) for methane hydrate to exist stably throughout the year (less than 7 degrees Celsius at pressure of 6MPa, according to the methane hydrate stability curve for bottom water with a salinity of 3.46%, Tishchenko et al., 2005). The water depth of the seafloor should be 750m or more if a possible sub-seafloor methane hydrate is stable. The observed bubbles are gushing directly from the methane that was formed sub-seafloor beneath the knoll.
Thus, a series of research cruises up to 2001 suggested the existence of methane hydrate under the seafloor of the knoll. After those expeditions, total 33 dives by ROV Dolphin-3K, ROV Hyper-Dolphin, DSV Shinkai2000 and DSV Shinkai6500 were operated from 2002 to 2004 for reconnaissance survey in the vicinity of 24° 07’48” N 124° 11’30” E (depth: 686-622m). Based on the video images from these dives, a seafloor mapping on the knoll was carried out in order to detect the area of the communities of chemosynthetic fauna and methane eruption. Calcareous sandstones, dead Calyptogena and living Bathymodiolus colonies spread over the wide area on the knoll. 35 methane bubble gushing were also observed. The area where methane may be existing beneath the seafloor of the knoll was estimated to be about 40,000 square meters in the survey area (Matsumoto and Aoki, JpGU2014, MIS22-P05).
It is necessary to examine if the sub-seafloor beneath the knoll is within the temperature and pressure conditions for methane hydrate stability. The temperature and depth records at 17 ARGO float stations around the knoll (there are no ARGO float stations just above the knoll) were examined to draw a temperature-depth profile. The result shows that the temperature at the summit (water depth : 640m) is too high (6 degrees Celsius in autumn, 9 degrees Celsius in spring) for methane hydrate to exist stably throughout the year (less than 7 degrees Celsius at pressure of 6MPa, according to the methane hydrate stability curve for bottom water with a salinity of 3.46%, Tishchenko et al., 2005). The water depth of the seafloor should be 750m or more if a possible sub-seafloor methane hydrate is stable. The observed bubbles are gushing directly from the methane that was formed sub-seafloor beneath the knoll.