the 22nd Pacific−Asian Marginal Seas meeting (PAMS)

Lead (Pb) is a particle reactive trace metal in seawater and the biogeochemical cycle of Pb in the ocean is mainly controlled by oceanic processes including interaction with marine particles. Pb has four stable isotopes with mass number of 204, 206, 207 and 208, and the isotope ratios (e.g., ²⁰⁶Pb/²⁰⁷Pb ratio) are useful as a fingerprint of anthropogenic/natural input to the ocean and in studying the biogeochemical cycle of Pb. Pb and Pb isotope ratios in the Atlantic, Indian and Pacific Oceans have been studied by international communities (Noble et al., 2015; Lee et al., 2015; Shuo et al., 2025). In a previous study on Pacific meridional section distributions of Pb, the importance of particle veils is clarified where high particle flux/abundance drives an efficient transport of anthropogenic Pb to the deep water (Lanning et al., 2023). At a sea region around equatorial Southeast Asia as an ocean-land boundary, particulate-dissolved exchange is demonstrated to be an important mechanism controlling the concentration and isotopic composition of dissolved Pb in the ocean (Chen et al., 2023). The subarctic North Pacific is an important region for vertical transport by particles as well as the influence of boundary exchange. Here we present the distributions of dissolved Pb isotope ratios in the subarctic North Pacific including the eastern boundary as well as open ocean where the number of published data are relatively small up to today. We mainly report new data from 8 stations in the eastern basin during the GEOTRACES Japan KH-17-3 cruise together with our KH-12-4 cruise data from this region for comparison.
Filtered and acidified seawater samples were analyzed by our chemical separation methods using chelating resin columns, and Pb and Pb isotope ratios in the purified solutions were measured by ICPMS of ELEMENT XR and Neptune, respectively.
Vertical profiles of Pb concentrations show a high concentration in the upper water with subsurface maximum followed by decrease with depth, which is a typical one in the North Pacific. Vertical profiles of Pb isotope ratios at a crossover station (47˚N, 152˚W) observed by US GEOTRACES (2018) and Japan GEOTRACES (2017) show a good agreement. Vertical profiles of ²⁰⁶Pb/²⁰⁷Pb ratios show low values of ~1.16 in the upper water reflecting a large influence of anthropogenic input while higher values of ~1.18 near the sea floor with a probable natural source contribution. Plots of ²⁰⁶Pb/²⁰⁷Pb vs. ²⁰⁸Pb/²⁰⁶Pb ratios indicate linear relationships. In particular, the eastern basin near the Juan de Fuca Ridge provides distinctive trend showing that Pb in bottom waters partially originates in hydrothermal system of the ridge.

lead isotopes subarctic North Pacific