[1] Asian mineral dust was sampled at Hokkaido, northern Japan, in spring 2004 and 2006. Iron solubility of the bulk aerosol, the size-segregated aerosol (0.45 < D < 11 mm), the snow containing a lot of mineral dust, and a simulated Asian dust standard (CJ-2) were measured by an iron dissolution experiment using a newly developed continuous leaching method. The iron solubility of the bulk aerosol samples was 1.2-2.2%. Within the 1.1 < D < 11 mm size range, iron solubility (0.52-8.2%) was higher in the smaller fractions of the size-segregated aerosol samples. We considered that the preferential removal of larger mineral dust particles from the air by snow resulted in the low iron solubility of the snow samples. Iron solubility of mineral dust was relatively lower in the 4.7 < D < 11 mm fraction of the size-segregated aerosol samples (0.52%), in the snow samples (0.20-0.57%), and in the CJ-2 standard (0.33%), which are dominated by large size particles (D > 4.7 mm). We suggest that an iron solubility of around 0.4% is typical for Asian mineral dust of large particles transported to Hokkaido. In the high-nutrient low-chlorophyll region of the western subarctic North Pacific near the Asian continent, where the mineral dust deposition is dominated by large particles, the iron solubility of the mineral dust entering the ocean is around 0.4%.
The particulate fluxes of carbonate carbon and organic carbon observed in various oceans have been summarized in this paper and discussed with special reference to the fate of the atmospheric carbon dioxide. The organic carbon fluxes, which act as a sink of atmospheric carbon dioxide, are generally larger than the carbonate carbon fluxes working as a source, but are comparable in the deep subtropical oceans. The areal and vertical variations of the carbonate carbon fluxes are much smaller than those of the organic carbon fluxes, indicating that organisms producing carbonate particles exist rather evenly in the world ocean and that a substantial part of carbonate produced is transported to the ocean bottom. The particulate fluxes are much larger in the coastal and hemipelagic seas. Although the productivity of shallow seas holds a key role in the fate of anthropogenic carbon, the marine biota may not be a large sink when longer time scales are considered. The carbonate carbon fluxes in the productive northern Pacific are larger in the northeast Pacific than in the northwest Pacific, reflecting a difference in the ecosystems. This suggests that the eutrophication of marine environments may not necessarily act as a sink for the atmospheric carbon dioxide.
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