Atmospheric Biogeochemistry - Ecological Systems - page 16

Human perturbations to atmospheric iron deposition to oceans

Human
emissions
CO 2 , SO 2 , NO x ,
Fe

Human
perturbations

Human land
use

Desert Dust

Climate

Soluble iron
deposition to
oceans

CO 2
concentrations
(other GHG,
aerosols)

Natural cycle of how
desert dust iron may
modulate ocean co 2
uptake

Ocean
productivity

Fig. 2.2 Skematic representing feedbacks between natural ocean carbon cycle, carbon dioxide
concentrations, and iron inputs also shows humans could be perturbing the iron deposition
(Figure from [ 65 ])

estimated to be less than 5% of the total iron in aerosols [ 64 , 65 ]. Crustal material is
on average about 3.5% iron [ 66 ], with some minerals having substantially higher or
lower concentrations [ 67 , 68 ]. However, estimates and observations suggest that
desert dust aerosols vary in their iron content by only a factor of 2, suggesting that
the high heterogeneity in the soils is mixed in the atmosphere [ 65 , 69 ]. There are
also small sources of atmospheric iron from volcanoes [ 70 , 71 ], cosmic dust [ 72 ],
and combustion [ 64 ].
Because atmospherically deposited dust only resides in the mixed layer of the
ocean for a short time, many researchers consider the soluble fraction of the iron the
most relevant for ocean iota [ 73 ]. However, which fraction of the iron is really
bioavailable is not well understood, but is likely to be a small fraction of the total
iron in aerosols (1-80%) (see reviews in [ 65 , 73 ]). Because the soluble fraction of iron
in soils is much smaller than what is observed in the atmosphere, it is thought that
atmospheric processing of iron is important [ 73 , 74 ]. It is likely that the acidity and
insolation play a role in processing of iron [ 73 - 75 ]. Some iron-containing minerals
are more easily solubilized than others and iron in combustions is significantly more
soluble than iron in mineral aerosols; these factors complicate our understanding of
iron solubility [ 68 , 76 - 78 ]( Fig. 2.2 from [ 65 ]).
Some regions of the ocean are iron deficient and additions of iron can result in
phytoplankton blooms [ 79 , 80 ]; however,

there is limited evidence showing