Distribution and Cycling in the Ocean

External sources of iron to the ocean include atmospheric dust deposition, river input, hydrothermal vents, and release from marine sediments via reductive dissolution and resuspension. The magnitudes of these sources are highly uneven in space and time, and for chemical and physical reasons, only a small fraction of the iron they deliver is retained to become bioavadable in the surface waters of the open ocean. 

Atmospheric deposition of continentally-derived particles ( aerosols , dust ) is a major external input of iron to the oceans. In some open ocean areas, including the iron-limited subarctic Pacific, dust deposition is the dominant iron source to the surface layer. The atmospheric iron source is composed largely of aluminosilicate minerals derived from arid and semi-arid mid-latitude regions in the Northern Hemisphere [73-75] the exception to this rule is downwind of heavily populated areas, where a significant fraction of aerosol iron may originate from anthropogenic sources [76-78]. 

The fraction of iron in soils and rocks at the Earth s surface varies, but generally falls between 2.9% and 4.8%, with a widely-used average of 3.5% [82,86]. 

Similar fractions have been measured in aerosol particles [103,104]. Applying this average composition to an estimated total aerosol flux of472 x 10 g year [82], the total global atmospheric flux of iron to the oceans is on the ordCT of 16—32 X lO gyear .  

Only a small fraction of the atmospheric flux of iron to the oceans ever becomes bioavailable, largely because of the low solubihty of the particulate and colloidal Fe(lll) phases that comprise the bulk of aerosol iron species. Nonetheless, bottle incubation experiments have shown that aerosol addition is an efficient stimulator of chlorophyll and biomass production in phytoplankton cultures [105,106], emphasizing the need to understand the factors and processes controlling the chemical speciation and solubihty of aerosol iron before and after deposition. 

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