大西洋中央海嶺TAGの熱水活動と長期観測:MODE'94 TAG Cruiseの結果

Abstract

大西洋中央海嶺のTAG熱水マウンド海域で計15回の潜航を行い、以下のことが明らかとなった。(1)鉛直型長期温度測定装置(Giant Kelp)、長期熱流量観測装置(Daibutsu)、海底地震計(OBSH)、長期海底観察装置(Manatee)を潜水調査船「しんかい6500」でTAG熱水マウンドの上に設置することに成功した。(2)TAGマウンドのブラックスモーカー、ホワイトスモーカーから熱水の採集に成功し、1990年に観測された値とは異なったシリカと硫化水素の値が得られた。またホワイトスモーカーの熱水のアルカリ度と水素イオン濃度(pH)の値は中央海嶺で今までに得られているどの値よりも低く、このことはTAGマウンドの地下で硫化物が大量に沈殿していることを示唆する。(3)熱水マウンドを作る硫化物とマウンドの基盤を作る玄武岩の枕状溶岩の深度分布がマウンドの東と南の斜面では浅く、北と西の斜面では深いことが分った。(4)TAGマウンドの南北の重力プロファイルが得られ硫化物と枕状溶岩で密度が明瞭に違い、重力の値としても明瞭に異なることが明らかになった。地殻熱流量の値としてはTAGで今までに観測された中で最も高い、100W/平方メートルを超える値が得られた。12日間海底に設置されていた海底地震計は海嶺の軸上で数多くの微小地震を観測した。(5)地形、重力、地磁気に関して延べ4000マイルにわたる広域の観測が行われ、大西洋中央海嶺で今まで重力の観測値のなかった地域が完全に埋められた。また様々な機械を設置するに当たって「よこすか」から組み立てた機械が投入されそれらの位置決めが行われた。

During this cruise 15 dives were successfully accomplished by the 'Shinkai 6500'. The following briefly summarize the results of this cruise. Three Giant Kelps were deployed in a triangular pattern around the upper Laputa each separated from the others by approximately 5 m intervals to monitor the black smoker for up to half a year. A high temperature probe was also deployed inside one of the black smoker vents near the Kelp. A long-term heat flow monitoring system called 'Daibutsu' was deployed on the southern upper terrace. The long term observation system called 'Manatee' was deployed at the eastern pit for 16 days and obtained 30 minutes of intermittent video and 800 still photos as well as current and nephelometry data. The Trans Atlantic Geotraverse (TAG) mound consists of two relatively flat terraces, a lower and upper terrace. The depth of the boundary between basal pillow lava and the sulfide mound differs around the periphery, being deeper at the northern edge compared with the southern boundary. The edges of the mound consist mostly of sulfide talus overlying pillow lava and calcareous ooze. The lower part of the slope consists of huge blocks of sulfide, in situ outcrops of sulfide rock and crust in the middle part. Fluid samples from white smoker and black smoker are obtained at other sites to the east of Laputa and ambient seawater from the off-mound area. The pH and alkalinity of the white smoker fluid, were lower than any of all the data ever obtained. The black smoker fluids show the chemistry in-between real endmember and seawater. In situ measurement of Si and hydrogen sulfide contents of the seawater on the TAG mound were attempted twice, but only the Si concentration was measured with good results. Heat flow measurements were carried out at ten stations and one station east of the base of the Laputa recorded a value more than 100 W/sq m, which is the highest value so far measured at the TAG mound. Gravity measurements were performed at 12 stations across the TAG mound and surrounding seafloor in a north to south profile. Gravity data indicate a significantly greater mean density beneath the sulfide mound compared to the surrounding basalt pillows. The records obtained by the OBSH (Ocean Bottom Seismometer Hydrophone) installed on the TAG mound site are not retrievable onboard. Surface ship geophysical measurements, i.e., multibeam bathymetry, gravity and magnetics, were carried out between the dive operations and during the maintenance days of the 'Shinkai 6500' over approximately 4,000 miles on and off the ridge axis, covering the WMARK (Western Mid-Atlantic Ridge of Kane transform fault) and TAG areas.