Evidence of an extensive spread of hydrothermal dissolved iron in the Indian Ocean

Ocean scale transect observations covering the entire water depth enable a comprehensive picture of the chemistry including the circulation and biogeochemical cycling of elements in seawater. The large-scale investigation of dissolved iron (Fe) took place through the Japanese-GEOTRACES study and here we report a basin-scale full-depth section profile of dissolved Fe in the Indian Ocean, from the Arabian Sea to the Southern Ocean. The data clearly shows the hydrothermal Fe distributed over 3000 km distance in the deep layer centered at a depth of approximately 3000 m, around the Central Indian Ridge segment, and a large part of the dissolved Fe from the hydrothermal sources was in the real soluble fraction rather than the colloidal fraction. In the intermediate water in the north Arabian Sea, another dissolved Fe rich water mass existed where Fe was enriched by remineralization processes from settling particles and/or adjacent reducing sediments, and preserved in the suboxic water. The basin-scale section profile indicates that there are several sources supplying dissolved Fe to deep waters, such as the hydrothermal sources and terrestrial Fe input with a persistent condition in the oxygen minimum zone (OMZ), between the northern-subtropical section, though few Fe sources were apparent in the Southern Ocean. Combining our size-fractionated Fe data with numerical modeling study suggests that the Fe physical–chemical form in seawater differs between the sources and is a key factor for controlling residence time and explaining the large scale distributed hydrothermal Fe.