Distinct basin-scale-distributions of aluminum, manganese, cobalt, and lead in the North Pacific Ocean

Abstract

Abstract Aluminum (Al), manganese (Mn), cobalt (Co), and lead (Pb) are key trace elements in seawater and thus significant in chemical oceanography research. However, although all of these elements are highly scavenged in the ocean, only a few studies focus on the intercomparison of their distributions. Here, we report the basin-scale and full-depth sectional distributions of these elements observed during three GEOTRACES Japan cruises in the North Pacific. We confirmed that a surface maximum of the dissolved (d) species is not a common feature for the four elements and that the d species have the lowest concentrations in the Pacific Deep Water (PDW) as compared to other oceans. The elements showed different speciations and distributions. The fraction of labile particulate (lp) species was calculated as the difference between the total dissolvable (td) species and d species. The lpM/tdM ratio, where M refers to an element, is highest for Al, at 0.66 ± 0.31 (average ± sd, n = 489), and lowest for Pb, at 0.02 ± 0.08 (n = 575). Further, the distribution of each element is uniquely related to ocean circulation. The tdAl concentration is high in the Equatorial Under Current (EUC), the North Equatorial Current (NEC), and the Lower Circumpolar Deep Water (LCDW). Manganese is supplied from reductive sources such as sediments on the continental shelves around the northern boundary. Cobalt is concentrated in the North Pacific Intermediate Water (NPIW) and in the Equatorial Pacific Intermediate Water (EqPIW) owing to the combined effects of supply from the continental shelves, biogeochemical cycling, and scavenging. Lead shows a subsurface maximum centered at ∼35°N and ∼200 m depth, implying an association with the formation of the Subtropical Mode Water (SMW) and the Central Mode Water (CMW). Although the subsurface Pb maximum in the Atlantic has diminished over the last three decades owing to the ban on leaded gasoline use, it has been sustained in the North Pacific through the growth of other anthropogenic sources in Asia and Russia. We propose that the enrichment factor of dM, defined as EF(dM) = (dM/dAl)seawater/(M/Al)upper crust, where (M/Al)upper crust is the molar ratio in upper crustal abundance, can be a good parameter for the sources. The median is 1.3 × 102 (n = 436) for EF(dMn), 3.2 × 102 (n = 430) for EF(dCo), and 1.2 × 103 (n = 413) for EF(dPb). The EF(dPb) found in this study is on the same order of magnitude as the EF values for aerosols found in the literature, suggesting that the deposition of aerosols is a major source for dPb. Because EF(dMn) and EF(dCo) are ten to hundred times higher than the EF for aerosols, sources other than the aerosol deposition are more significant contributors to the concentrations of Mn and Co.