Distributions of dissolved and acid-dissolvable bioactive trace metals in the North Pacific Ocean

Dissolved and acid-dissolvable Fe, Co, Ni, Cu, Zn, Cd, and Pb were measured in the North Pacific Ocean (40-17 degreesN, 151 degreesE-1600 degreesW) in June-July 2000. Immediately after the collection, a portion of seawater for dissolved species was filtered with a 0.2-mum filter and acidified to pH 2.2 with HCl. A portion of seawater for acid dissolvable species was acidified to pH 2.2 with HCl without filtration and stored for 3 years at an ambient temperature. In our laboratory, the dissolved and acid dissolvable trace metals were concentrated using a chelating column technique and determined by ICP-MS. Profiles of dissolved and acid dissolvable Fe show a large intra-basin variation. The concentrations are 0.3-1.4 nM for dissolved Fe (D Fe) and 0.7-4.7 nM for acid dissolvable Fe (AD Fe) in deep water. The concentrations are high at stations in the northwestern area and southwest of the Hawaiian Islands. AD Fe increases below 4000 in suggesting resuspension of sedimentary material. For Co, Ni, Cu, Zn, and Cd, the concentrations of dissolved and acid dissolved species mostly agree within an experimental error. Deep water distributions of Ni, Cu, Zn, and Cd are fairly uniform in the basin. AD Pb is significantly higher than D Pb especially in surface water. Comparison to our previous data on the dissolved trace metals in the subarctic North Pacific in July-September 1997 (Fujishima et al., 2001) indicates non-uniform distributions of the bioactive trace metals in the surface mixed layer in the North Pacific. In the subtropical region, chlorophyll biomass is very low owing to depletion of major nutrients, resulting in surface maxima of D Fe and D Zn. In the subarctic region, D Fe and D Zn are depleted in the surface mixed layer. In contrast, D Co and D Ni are high in the northwestern subarctic area and low in the subtropical gyre.