Concentration of dissolved lead in the upper Northwestern Pacific Ocean

Abstract

During a cruise in 2016, samples of clean seawater were collected, and the first high-quality dataset of total dissolvable Pb concentration ([TDPb]) was developed for the upper 750 m of the low-latitude Northwestern Pacific Ocean (NWPO). The distribution of [TDPb] was influenced by the low-latitude NWPO current system. The surface [TDPb] ranged from 22 to 60 pmol/kg, with two significant maxima occurring in the North Equatorial Current (NEC) and the Kuroshio Current (KC). The vertical [TDPb] ranged from 16 to 70 pmol/kg. The [TDPb] profile to the north of 13°N, where the NEC occupys, was characterized by a significant sub-surface maximum, and this feature extended eastward to the North Equatorial Countercurrent (NECC) and possibly, to the Mindanao Current (MC). In the area south of 5°N, which is affected by currents originating from the eastern equator and the southern hemisphere, [TDPb] was greatly reduced and its distribution was uniform vertically. The vertical Pb maximum characteristic was continuous throughout the North Pacific subtropical gyre at σθ = 25.2–26.2, which was associated with high surface Pb at 30°N–40°N in Northwestern Pacific. The lateral transport of Pb in the NEC along 130°E amounted to approximately 16.8–22.5 Gg/yr, which could be considered to be the convergence budget of the advection transport of the North Pacific. With the bifurcation of the NEC, approximately 4.5–9.0 Gg/yr of Pb was distributed into the northward-flowing KC which rejoins the subtropical circulation of the North Pacific, while another 7.8–18.0 Gg/yr of Pb was distributed into the southward-flowing MC, which joins the tropical circulation of the Pacific or enters the Indian Ocean via the Indonesian through-flow (ITF). The distribution pattern of Pb in the low-latitude western Pacific is influenced by the western boundary current system and possible changes associated with the climate change.