Iron distributions in the water column of the Japan Basin and Yamato Basin (Japan Sea)

In the Japan and Yamato basins (Japan Sea), dissolved Fe ([D-Fe], <0.22 mu m fraction) was characterized by surface depletion, mid-depth maxima, then a slight decrease with depth in deep water and uniform concentration in bottom waters because of biological uptake in the surface water and release by microbial decomposition of sinking organic matter in mid-depth waters. Total Fe concentrations ([T-Fe]) in the surface water of the Japan Sea were 1-4 nM, a little higher than those in the surface waters of the nutrient-deficient subtropical western North Pacific and extremely higher than the nutrient-rich subarctic western North Pacific and the nutrient-deficient subtropical central North Pacific, resulting from high atmospheric Fe input to nutrient-depleted surface water of the Japan Sea. In the Japan Basin, the [T-Fe] in bottom water were lower than those in deep water, resulting from (1) the injection of new bottom water with the lower [T-Fe] into the Japan Basin bottom water, (2) the particulate Fe removal by particle scavenging during the bottom water circulation of the Japan Basin, or (3) the injection of deep water with the higher [T-Fe] into the Japan Basin deep water. On the other hand, the [T-Fe] in deep water of the Yamato Basin and the slope regions were variable with different [T-Fe] levels among stations and depths. We found a significant relationship between [T-Fe] and water transmittance in deep water, probably resulting from the iron supply into the deep water because of the lateral transport of resuspended sediment from the slope.