The Bering Sea is characterized by high primary productivity despite being limited by iron scarcity in its surface waters at the basin area. The intermediate waters contain high concentrations of trace metals and serve as a source of the subarctic Pacific intermediate water. Manganese is known to be a good tracer for coastal sediment materials in the open ocean because of its abundant supply from the coastal areas. On the other hand, lead is primarily delivered to the ocean surface via atmospheric deposition from anthropogenic sources. Both Pb and Mn are particle-reactive trace metals that have relatively short residence times in oxygenated seawater. In this study, we provide detailed distributions of dissolved manganese (dMn) and lead (dPb) in the western Bering Sea and investigate the biogeochemical processes influencing their distributions.
   Seawater samples were collected during cruise Mu18 onboard R/V Professor Multanovskiy from August to October 2018 in the western Bering Sea. Sample collection was carried out using acid-clean Teflon-coated 12 L X-Niskin bottles, which were mounted onto a modified clean conductivity-temperature-depth carousel. Immediately after sampling, the seawater samples were passed through a 0.2 micrometer pore-size capsule filter. The concentrations of dMn and dPb in the samples were analyzed using high resolution inductively coupled plasma mass spectrometry following a pre-concentration step with a chelating resin column in the land-based laboratory.
   Our results indicated that the coastal region of the western Bering Sea is a major source of dMn, with maximum dMn concentrations reaching up to 6.6 nM at a nearshore station. However, this region is not a significant source of dPb. Intense scavenging processes reduced dPb concentrations in both surface and intermediate waters. Surface dPb concentrations ranged from 10 to 15 pM were observed, substantially lower than typical open ocean concentrations. The vertical distribution profile of dPb shows a pronounced maximum in the intermediate water at around sigma-theta 27.2. This depth is deeper compared to the dPb maxima observed in the subarctic North Pacific and Gulf of Alaska at sigma-theta 27.0. In deeper waters, dMn was supplied from sedimentary sources under reducing conditions, and dMn concentrations exhibited a strong correlation with dissolved iron concentrations. Meanwhile, dPb showed a negative relationship with dissolved oxygen, implying that some dPb is released during the degradation of biogenic particles, likely through reversible scavenging. In surface waters, elevated concentrations of dMn suggested substantial inputs from coastal sources, potentially alleviating Fe limitation and supporting phytoplankton growth. In contrast, relatively low dPb concentrations indicated efficient boundary scavenging, particularly at the nearshore regions, possibly due to a higher particle flux.

trace metals,boundary scavenging,intermediate waters