Eventually, the modern ocean evolved about 1.2 billion years ago in which O2 of photosynthetic origin permeated almost all of the ocean's depth, not to mention the atmosphere, laying the foundation for the rich and complex biological systems we know today. Under these conditions, iron has become a very rare trace element in the oceans, having concentrations in most regions of the order of 1 nM or less, except in coastal and estuarine regions under the influence of terrestrial runoff.
in surface waters, where Fe is needed for phytoplank-ton growth, the lowest Fe concentrations are found in those oceanic regions most remote from land. More specifically, away from the direct runoff of Fe in rivers, the main external source of Fe entering oceanic surface waters in the modern Earth system is soil-derived dust transported over great distances from the arid areas of the Earth's surface (Table 2). Of particular importance are the Sahara and Sahel desert regions which deliver Fe to the equatorial and North Atlantic Ocean, and the Asian deserts which are a major source for the western North Pacific Ocean. The Southern Ocean contains no major dust sources other than the desert regions of Australia and Patagonia well to the north. Not surprisingly, this region turns out to be particularly depleted in iron.
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