Pb and Pb isotope ratios in the North Pacific: observations during the KH-12-4 GEOTRACES Japan cruise

We have measured Pb and Pb isotope ratios in filtered seawater samples collected at 9 stations in the North Pacific during the KH-12-4 GEOTRACES Japan cruise of R/V Hakuho-Maru (July-Aug. 2012) and KS-13-T3 cruise of R/V Shinsei-Maru (Oct.-Nov. 2013) and report here their vertical profiles.

The concentration of Pb generally decreased with depth below subsurface layer at all the stations but distinct basin trends were observed: sharp decrease in the eastern North Pacific in contrast to moderate decrease in the western basin. The subsurface maximum of Pb in the North Pacific is a common feature but it becomes sharp toward the eastern basin through the west to east transect. The ²⁰⁶Pb/²⁰⁷Pb isotopic ratio increased with depth at all the stations but distinctively differed between basins: rapid increases in the ratio in the eastern basin in contrast to gradual increases in the western basin. Similar observations were obtained for the concentration ratio of bismuth/lead in the water column. These results suggest that Pb isotope signatures of recent anthropogenic origin propagates from surface water to deep water more rapidly in the western basin than the eastern basin, further implying higher sinking rate of marine particles in the western North Pacific and/or the more influence of sedimentary derived old lead from the continental margin in the eastern basin. The comparison of our isotope-isotope plot from the deep water column at the Izu-Ogasawara Trench with that for coral historical data displayed that Pb in the deep water of the western North Pacific was derived approximately 60 to 100 years ago.