Lead (Pb) is a naturally occurring toxic and particle reactive element that is commonly found at trace levels in global oceanic surface waters. However, its geochemical cycling—including downward fluxes and transport rates—remains largely unknown. This study employs the ²³⁴Th–²³⁸U disequilibrium method to examine the downward Pb fluxes and scavenging residence times within the upper 200 m of the northwest Pacific Ocean. Using a one-dimensional steady-state model coupled with size-fractionated Pb/²³⁴Th ratios, we estimated the particulate Pb fluxes to range from 0 to 65.1 ± 12.5 nmol m^-2 d^-1, with a mean of 5.7± 1.2 nmol m^-2 d^-1. Spatially, Pb export was higher in the North Equatorial Current (NEC) region (mean: 1.2 ± 0.3 nmol m⁻² d⁻¹) than in the North Pacific Subtropical Gyre (NPSG, 0.5 ± 0.4nmol m⁻² d⁻¹), with the maximum fluxes (65.1 ± 12.5nmol m⁻² d⁻¹) observed at Station K1 in the Kuroshio Current. This spatial distribution was broadly consistent with the pattern of nutrient diapycnal flux in this region. However, the upper-ocean Pb inventories exhibited a different spatial pattern, with higher values observed in the northwestern part of the study region—a distribution that is consistent with both atmospheric deposition and proximity to continental sources. The residence time, calculated as the ratio of Pb inventory to flux, ranged from 0 to 118 ± 522 years, with a mean of 22 ± 45 years. Stations distal from the coastline exhibited relatively short residence times (on the order of 10 years), whereas the near-coastal station showed much longer residence times (on the order of 100 years), likely attributable to the elevated Pb inventories at those locations.
These findings improve our understanding of the sources, sinks, and transport pathways of Pb in the marine environment, and offer valuable insight into the broader biogeochemical cycling of trace metals in the open ocea.
 

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