In order to investigate the behavior of manganese and iron, which may be an index of the movement and circulation of hydrothermal water, seawater and particulate samples were obtained from H3 and H2 in the Myojin knoll caldera, H1 in the Izu-Ogasawara Trench and H4 in the Shikoku basin in April, 2004, and the concentrations of manganese, iron and phosphorus were measured. As a result, the concentrations of dissolved manganese and iron in the maximum layer deeper than 1,000m at the Myojin Knoll Caldera were very high, i.e., 70~100nmol L
–1 and 2.6~8.0nmol L
–1, respectively. The concentrations of dissolved manganese and iron were higher in H2 than in H3. Its concentration ratio (Fe/Mn) was also higher in H2. It was deduced that there was a difference between the hydrothermal influences in H2 and H3. The concentrations of particulate Mn and Fe were also higher at a level deeper than 1,000m at the Myojin Knoll caldera. The proportion of Fe in oxide fraction to the particulate fraction is higher for H2 than for H3. The data showed that ferrous iron supplied through the hydrothermal actions was changed from hydroxide to oxide, and removed.
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