Lead concentration and isotopic compositions in the central Pacific basin: GEOTRACES PMT (GP15)

Edward A Boyle, Massachusetts Institute of Technology, Earth Atmospheric and Planetary Sciences, Cambridge, MA, United States and Shuo Jiang, East China Normal University, State Key Laboratory for Estuarine and Coastal Marine Research, Shanghai, China
One of the key GEOTRACES trace elements, oceanic lead (Pb) is highly influenced by human activities. During a cruise (GEOTRACES GP15) in 2018 along 152°W, seawater samples were collected for Pb concentration and isotope ratio profiles. Here, data from station 8 (47°N), station 18 (22°N), station 25 (7.5°N), and station 35 (10.5°S) will be presented. The Pb concentration of the two northern stations increased from 18-19 pmol/kg in the mixed layer to a maximum at 200-600m and then decreased to the bottom. At 200-600m, the Pb concentration at station 18 (45-67 pmol/kg) was significantly higher than that at station 8 (41-46 pmol/kg), but at station 18 , Pb concentrations at depths below 600m are lower than that at station 8. The208Pb/206Pbprofiles showed a similar pattern as the concentration, where the 208Pb/206Pb at station 18 (2.103-2.108) at 200-600m was significantly higher than that at station 8 (2.100-2.104), but was ~0.050 systematically lower from 600m to the bottom. Below 4500m, the Pb concentration of the two stations reached were both uniformly 4-5 pmol/kg, but with different isotopic values. In the diagram of 206Pb/207Pb vs 208Pb/206Pb, both stations fit on a linear regression with the same slope, implying the similar Pb sources with different mixing ratios which we attribute to the stations being in different circulation regimes (station 8 is in the north Pacific subpolar gyre and station 18 is in the north Pacific subtropical gyre). The 208Pb/206Pb isotope ratio at station 18 showed a strong feature at 4750 m and 5095 m which we attribute to the flow of eastward flowing Pacific Bottom Water passing through the deep passage south of Horizon Guyot as described by Edmond, Chung, & Sclater (1971, JGR 76:8089-8096). Surface seawater during this study exhibited concentrations of 18-19 pmol/kg and 208Pb/206Pb of 2.096-2.103. In contrast, previous studies near these stations reported [Pb] = 11 pmol/kg and 8/6 = 2.044 in 1979 and [Pb] = 51-62 pmol/kg and 8/6 = 2.114 in 2004 (Flegal et al., 1984; Wu et al., 2010).This temporal variation in Pb is consistent with changing Pb emissions around the Pacific, where North American-Australian Pb dominated in 1979, Chinese leaded gasoline dominated in 2004, and Chinese coal and diesel dominated after 2010. Data from southern stations 25 and 35 are in progress and will also be shown.