Helium isotopes of seawater in the Philippine Sea and the western North Pacific

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).