Boundary Exchange and Benthic Fluxes Drive Trace Element Cycling in North Pacific Marginal Seas

Trace elements are essential micronutrients for marine primary production, playing key roles in a variety of metabolic processes. This study investigated the biogeochemical cycling and benthic fluxes of Mn, Fe, Co, Ni, Cu, Zn, and Cd in the two contrasting marginal seas of the northwestern Pacific, East/Japan Sea and Yellow Sea. Rare earth element fractionations ([Nd/Er]_PAAS and Ce/Ce* ratios) were used to trace scavenging and water mass inputs.

In the East/Japan Sea, trace element distributions were grouped into three categories. Mn, Fe, and Co were influenced by atmospheric deposition in surface waters and benthic input in the bottom layer, with fluxes of 742, 96, and 0.8 μmol m^-2 yr^-1, respectively. Ni and Cu showed a depletion from surface waters and a limited influence from benthic inputs. The distributions of Zn and Cd were more strongly regulated by biological activity. On top of that, an unusual decoupling between the concentrations of Zn and SiO4^4- was discovered in this study. Zn correlated positively with SiO4^4- in the upper 500 m but negatively at greater depths, likely owing to shelf inputs. In the Yellow Sea, all trace elements exhibited a vertically conserved distribution owing to rapid water mixing.

These results contribute to the current biogeochemical understanding of the region by providing higher-resolution cross-transect investigations and report the decoupling of Zn–SiO4^4- in the East/Japan Sea for the first time.