CHAPTER 2 • The Chemical and Physical Properties of Marine Aerosols: An Introduction
73
ling the primary productivity in vast ocean regions. Subsequent research has firmly
established that Fe is an important micronutrient (Archer and Johnson 2000) and that
Fe could play an important role in the Earth's carbon cycle, thereby mediating the
ocean-atmosphere exchange of CO2 (Behrenfeld et al. 1996; Coale et al. 1996a,b;
Falkowski 1997; Falkowski et al.1998; Fitzwater et al.1996; Gordon et al.1997). Furthermore, it is clear that except for coastal waters, the only significant source of Fe is that
transported by winds from the continents (Johnson et al. 1997a,b).
The work cited above largely focuses on the results of laboratory experiments or
on field experiments where Fe is added to ocean waters. Although there is considerable evidence to support the hypothesis that Fe plays an important role in the ocean
primary productivity, it is difficult to estimate the importance of dust-Fe on the global ocean. Recently, Fung et al. (2000) presented an analysis of the iron budget in the
upper ocean. They estimated the global distribution of annual iron assimilation by
phytoplankton from satellite-derived estimates of oceanic primary production and on
the measured ratios of FetC in cells. They took into account two sources of iron: that
supplied by upwelling and mixing of ocean waters and that deposited from the atmosphere. On this basis, they computed the global assimilation rate of Fe by phytoplankton in the open ocean to be 12 x 10 9 mol Fe yr- 1 . They estimated that the total deposition of Fe in dust was 96 x 10 9 mol Fe yr- 1 in the open ocean. The soluble fraction of
Fe depends on a variety of factors, especially the pH of the solution (Desboeufs et al.
1999); solubilities at pH's commonly found in precipitation and cloud water are typically in the range of 1 and 10% (Zhu et al. 1997; Spokes et al. 1994, Spokes and Jickells
1996). Thus, the estimated Fe-dust flux yields a total available-Fe deposition rate in
the range of 1 to 10 X 10 9 mol Fe yr- 1 • Thus, the estimated aeolian deposition rate of
available Fe in dust could readily supply a substantial fraction of that required by phytoplankton. In comparison, the upwelling and entrainment supply of dissolved Fe to
the upper ocean is relatively small: -0.7 x 10 9 mol Fe yr- 1 • It should be noted that there
are major uncertainties in the estimate of Fung et ai, as large as a factor of 5 to 10. The
largest limitations in the calculations are due to the dearth of information on the transport rates of dust to the oceans, the chemical form of the Fe in the dust, and the availability of dust-Fe to organisms in the water column. These are areas ripe for research.
In an earlier section, I showed that huge amounts of dust are carried from North
Africa to the Atlantic (Table 2.4). Thus, there is considerable interest in the impact of
this transport of biogeochemical processes in this region. The nutrient budgets of the
North Atlantic have been extensively studied, although major uncertainties remain
(Galloway et al. 1996). Michaels et al. (1996) compiled an N-nutrient budget for the
region. They found that the pool of N -nutrients was substantially greater than could
be accounted for by conventional oceanic sources. They concluded that though the
anthropogenic inputs of NOx and NHx were significant as discussed in an earlier section, the greatest impact was through the input of Fe associated with African dust.
Michaels et al. attribute this high rate of N-fixation to the colonial cyanobacterium,
Trichodesmium sp. Nitrogen fixation by Trichodesmium sp. requires a great deal more
Fe than that required by organisms that utilize nitrate. The ratio of N:Fe in
Trichodesmium sp. is about 1000 moles of N to 1 mole of Fe. Prospero et al. (1996) estimate the total deposition of dust to the latitude band 10° N to 40° N (excluding the
areas near the coast of Africa) at about 44 x 10 12 g yr- 1 . The concentration of Fe in African dust is close to the average crustal abundance, 3.5% by weight. Thus, the annual
73
ling the primary productivity in vast ocean regions. Subsequent research has firmly
established that Fe is an important micronutrient (Archer and Johnson 2000) and that
Fe could play an important role in the Earth's carbon cycle, thereby mediating the
ocean-atmosphere exchange of CO2 (Behrenfeld et al. 1996; Coale et al. 1996a,b;
Falkowski 1997; Falkowski et al.1998; Fitzwater et al.1996; Gordon et al.1997). Furthermore, it is clear that except for coastal waters, the only significant source of Fe is that
transported by winds from the continents (Johnson et al. 1997a,b).
The work cited above largely focuses on the results of laboratory experiments or
on field experiments where Fe is added to ocean waters. Although there is considerable evidence to support the hypothesis that Fe plays an important role in the ocean
primary productivity, it is difficult to estimate the importance of dust-Fe on the global ocean. Recently, Fung et al. (2000) presented an analysis of the iron budget in the
upper ocean. They estimated the global distribution of annual iron assimilation by
phytoplankton from satellite-derived estimates of oceanic primary production and on
the measured ratios of FetC in cells. They took into account two sources of iron: that
supplied by upwelling and mixing of ocean waters and that deposited from the atmosphere. On this basis, they computed the global assimilation rate of Fe by phytoplankton in the open ocean to be 12 x 10 9 mol Fe yr- 1 . They estimated that the total deposition of Fe in dust was 96 x 10 9 mol Fe yr- 1 in the open ocean. The soluble fraction of
Fe depends on a variety of factors, especially the pH of the solution (Desboeufs et al.
1999); solubilities at pH's commonly found in precipitation and cloud water are typically in the range of 1 and 10% (Zhu et al. 1997; Spokes et al. 1994, Spokes and Jickells
1996). Thus, the estimated Fe-dust flux yields a total available-Fe deposition rate in
the range of 1 to 10 X 10 9 mol Fe yr- 1 • Thus, the estimated aeolian deposition rate of
available Fe in dust could readily supply a substantial fraction of that required by phytoplankton. In comparison, the upwelling and entrainment supply of dissolved Fe to
the upper ocean is relatively small: -0.7 x 10 9 mol Fe yr- 1 • It should be noted that there
are major uncertainties in the estimate of Fung et ai, as large as a factor of 5 to 10. The
largest limitations in the calculations are due to the dearth of information on the transport rates of dust to the oceans, the chemical form of the Fe in the dust, and the availability of dust-Fe to organisms in the water column. These are areas ripe for research.
In an earlier section, I showed that huge amounts of dust are carried from North
Africa to the Atlantic (Table 2.4). Thus, there is considerable interest in the impact of
this transport of biogeochemical processes in this region. The nutrient budgets of the
North Atlantic have been extensively studied, although major uncertainties remain
(Galloway et al. 1996). Michaels et al. (1996) compiled an N-nutrient budget for the
region. They found that the pool of N -nutrients was substantially greater than could
be accounted for by conventional oceanic sources. They concluded that though the
anthropogenic inputs of NOx and NHx were significant as discussed in an earlier section, the greatest impact was through the input of Fe associated with African dust.
Michaels et al. attribute this high rate of N-fixation to the colonial cyanobacterium,
Trichodesmium sp. Nitrogen fixation by Trichodesmium sp. requires a great deal more
Fe than that required by organisms that utilize nitrate. The ratio of N:Fe in
Trichodesmium sp. is about 1000 moles of N to 1 mole of Fe. Prospero et al. (1996) estimate the total deposition of dust to the latitude band 10° N to 40° N (excluding the
areas near the coast of Africa) at about 44 x 10 12 g yr- 1 . The concentration of Fe in African dust is close to the average crustal abundance, 3.5% by weight. Thus, the annual