Lead concentrations and isotopic compositions in the Western Philippine Sea

Abstract

Dissolved lead (Pb) concentrations and isotope ratios in seawater of the Western Philippine Sea (WPS) were determined to investigate sources of Pb in the region. Surface seawater concentrations at seven sampling stations ranged from 36.2 to 68.1pmolkg−1. Isotopic composition of surface water, with 206Pb/207Pb ranging from 1.162 to 1.170, 208Pb/207Pb ranging from 2.445 to 2.451, and 206Pb/204Pb ranging from 18.14 to 18.27, reflects Asian anthropogenic aerosols input to the WPS. Shallow water Pb concentrations within the Kuroshio Current domain are about 15pmol kg−1 lower than at other sites and, together with a distinct isotopic signature (206Pb/207Pb=1.167–1.170, 208Pb/207Pb=2.447–2.451 and 206Pb/204Pb=18.22–18.27), reflect water originating from the Equatorial Pacific that is relatively less impacted by contamination from anthropogenic inputs. An isotopically distinct sub-surface Pb maximum at about 100 to 250m, representing water originating from the Western North Pacific where anthropogenic Pb deposition is high, was seen at all seven sites. Lead concentrations in deep water in the stations further from shore are typically lower than in the surface layer and ranged from 11.2 to 51.6pmolkg−1. Lead isotopic signatures in deep water at these sites (206Pb/207Pb=1.162–1.184; 208Pb/207Pb=2.448–2.471; 206Pb/204Pb=18.13–18.51) are offset from pre-anthropogenic values and suggest that anthropogenic sources have penetrated the deep water column. Elevated concentrations and isotopic signatures observed in the deep water at stations closer to shore, where sediment transport is prevalent, indicate that sedimentary input is a major source of dissolved Pb at these sites. Differences in Pb concentrations and isotopic signatures between samples collected from the Pacific Deep Water (PDW) water-mass during this study and those collected a decade ago suggest that Pb inputs even in deep water change on decadal scales.