Observed dissolved gallium in the tropical and subtropical waters in the Western Pacific Ocean

The concentration data of Gallium (Ga) observed in the Western Pacific section spanning from 2°S to 20°N along 142°E are reported for the first time. The spatial distribution of Ga in the surface, subsurface, intermediate, and deep layers is presented, and its influencing factors are discussed. We find that terrestrial input from Papua New Guinea constitutes a significant source of surface Ga in this region, while the distribution of Ga concentrations is regulated by water mass mixing. The elevated concentrations of dissolved Ga in the subsurface are derived from the North Pacific Subtropical Mode Water, implying that the latter exerts significant influence on the spreading of dissolved Ga. The Ga concentration in intermediate waters is mainly governed by the Antarctic Intermediate Water (AAIW). A strong correlation (R² > 0.8) is found between Ga concentration and salinity at depths of deep waters, and the non-conservative behavior of Ga in deep waters is difficult to detect in this region. Profile structures of Ga concentration inside the Philippine Sea basin are different from that outside the basin, indicating that the Ga concentration may reflect the upwelling of deep water within the basin. We further compared the concentration data of Ga with those from the GS01, GP16, GP15, IOC2002, and VERTEX cruises in terms of distribution, behavior and potential usage. We suggest that Ga exhibits promising potential as a new tracer for monitoring intermediate and deep water masses, and Ga concentration can serve as a good indicator for the AAIW and Upper Circumpolar Deep Water.