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Geochemical Journal, Vol. 38 (No. 6), pp. 527-534, 2004

Discovery of a new hydrothermal venting site in the southernmost Mariana Arc: Al-rich hydrothermal plumes and white smoker activity associated with biogenic methane

Toshitaka Gamo,1,2 Harue Masuda,3 Toshiro Yamanaka,4,5 Kei Okamura,1 Junichiro Ishibashi,6,4 Eiichiro Nakayama,7 Hajime Obata,7 Kiminori Shitashima,8 Yoshiro Nishio,6 Hiroshi Hasumoto,1 Masaharu Watanabe,1 Kyohiko Mitsuzawa,9 Nobukazu Seama,10 Urumu Tsunogai,6,2 Fumitaka Kouzuma2 and Yuji Sano1

1Ocean Research Institute, The University of Tokyo, Nakano-ku, Tokyo 164-8639, Japan
2Graduate School of Science, Hokkaido University, N10 W8, Sapporo 160-0810, Japan
3Graduate School of Science, Osaka City University, Sugimoto, Osaka 558-8585, Japan
4Graduate School of Science, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan
5Institute of Geoscience, University of Tsukuba, Tsukuba 305-8571, Japan
6Graduate School of Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan
7University of Shiga Prefecture, Hikone 522-8533, Japan
8Central Research Institute of Electric Power Industry, Abiko, Chiba 270-1194, Japan
9Department of Deep Sea Research, JAMSTEC, Yokosuka 237-0061, Japan
10Research Center for Inland Seas, Kobe University, Kobe 657-8501, Japan

(Received June 25, 2003; Accepted April 19, 2004)

Abstract: This paper reports a series of studies leading to the discovery of a submarine hydrothermal field (called Nakayama Field) at an arc seamount (12°43´ N, 143°32´ E) in the southernmost part of the Mariana Trough, western Pacific Ocean. We first detected hydrothermal plumes characterized by water column anomalies of temperature, light transmission, Mn, Fe, Al, O2, CH4, and δ13C of CH4 above the summit caldera of the seamount. Then deep-tow camera surveys confirmed the existence of hydrothermal activity inside the caldera, and an ROV dive finally discovered white smoker-type fluid venting associated with vent fauna. A high concentration of aluminum in the plume and white smoker-type emissions imply acidic hydrothermal activity similar to that observed at the DESMOS Caldera in the eastern Manus Basin, Papua New Guinea. Anomalously low δ13C (CH4) of -38‰of a vent fluid sample compared to other arc hydrothermal systems along the Izu-Bonin and Mariana Arcs suggests an incorporation of biogenic methane based on a subsurface microbial ecosystem.
Keywords: submarine hydrothermal activity, southern Mariana Trough, hydrothermal plume chemistry, aluminum anomaly, biogenic methane


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