Iron in the Japan Sea and its implications for the physical processes in deep water

Labile dissolved Fe (DFe, <0.22 μm fraction) and total dissolvable Fe (TDFe, unfiltered) were examined throughout the water column of the northeastern (Japan Basin) and mid-eastern (Yamato Basin) basin regions in the Japan Sea. We observed extremely high vertically integrated TDFe inventories and low nutrient concentrations at the surface water in both basins, probably resulting from high atmospheric Fe input to nutrient-depleted surface water. DFe in both basins was characterized by mid-depth maxima and, below that, a slight decrease with depth in deep water and uniform concentration in bottom water. In the Japan Basin, surprisingly, TDFe concentrations in bottom water were lower than those in deep water, resulting from the injection of new bottom water. On the contrary, TDFe concentrations in bottom water of the Yamato Basin increased gradually with depth and were higher than those in deep water probably due to the supply of iron during the horizontal transport of the bottom water in the Japan Basin towards the Yamato Basin. The TDFe with a high percentage of labile particulate Fe (TDFe minus DFe) may have important implications for understanding the physical processes of bottom water in the Japan Sea.