Comparison between the ratio of dissolved iron (dFe) concentration to apparent oxygen utilization (AOU) with the Fe/C ratio of phytoplankton along US GEOTRACES GP15 transect in the Tropical and North Pacific, showsthat the dFe/AOU ratio in the subsurface oceans is substantially lower than the Fe/C ratio of phytoplankton.  By assuming that dFe is supplied to the subsurface ocean in inverse proportion to the AOU equivalent to the Fe/C ratio of phytoplankton, we calculate vertical profiles of apparent residence time of dFe (τ) along the GP15 transect. The results show that τ increases linearly with increasing depth in the upper 1000-1500 m of the water column, and remains rather constant below this depth. The important finding of this results is that dFeresidence time in the deep ocean below 1500 m depth falls in the range of 200-400 years that is much longer than the <100 years residence time as thought previously,suggesting a potential for the increase of dFe supply by low latitude dust flux in the Atlantic to alleviate Fe limitation in the Southern Ocean through the upwelling of the North Atlantic Deep Water in this region during glacial period.  

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