ABSTRACT

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).