Geochemical Journal, Vol. 44 (No. 6), pp. 451-460, 2010
Naoto Takahata, Taichi Tokukake, Kotaro Shirai, Shinzou Fujio, Kiyoshi Tanaka and Yuji Sano
Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo, Nakano-ku, Tokyo 164-8639, Japan
(Received June 4, 2009; Accepted June 4, 2010)
We measured helium isotopic ratios of 84 seawater samples from various depths collected in the western North Pacific Ocean and the western Philippine Sea. The 3He/4He ratios varied significantly from δ3He of 0.1% to 22.9%, where δ3He is defined as the percent deviation of the helium isotopic ratio relative to the atmospheric standard. Maximum δ3He > 20% was observed at mid-depth (2000-2500 m) in the western Philippine Sea and in the southern part (~10°N) of the western North Pacific, though not in the northern part (~30°N) at the same depth. Contour maps of the lateral δ3He distribution at mid-depth suggest that the helium-3 plume derived from the East Pacific Rise does not flow northward along the Izu-Ogasawara-Mariana Ridge but westward through the Caroline Basin or the Yap-Mariana Junction into the Philippine Sea. It then flows northward in the western Philippine Sea to a region adjacent to the Japanese Islands. Although these flows inferred from the δ3He distribution are roughly similar to those estimated from water properties such as isopycnal distributions, the δ3He distribution could reveal that deep-water circulation seems to be different at each depth (2000, 2500, 3000 m).
Key words: 3He/4He ratios, abyssal current, Philippine Sea, North Pacific, seawater