Aeolian input of bioavailable iron to the ocean

Abstract

Atmospheric deposition of mineral dust supplies much of the essential nutrient iron to the ocean. Presumably only the readily soluble fraction is available for biological uptake. Previous ocean models assumed this fraction was constant. Here the variable solubility of Fe in aerosols and precipitation is parameterized with a two‐step mechanism, the development of a sulfate coating followed by the dissolution of iron (hydr)oxide on the dust aerosols. The predicted soluble Fe fraction increases with transport time from the source region and with the corresponding decrease in dust concentration. The soluble fraction is ∼1 percent near sources, but often 10–40 percent farther away producing a significant increase in soluble Fe deposition in remote ocean regions. Our results may require more rapid biological and physicochemical scavenging of Fe than used in current ocean models. We further suggest that increasing SO2 emission alone could have caused significant Fe fertilization in the modern northern hemisphere oceans.

A review of natural aerosol interactions and feedbacks within the Earth system

“…Such dust chemistry has been shown to reduce the particle lifetime (e.g., Fan et al, 2004) and hence affect dust burden, radiative forcing and deposition to the oceans. As well as enabling dust particles to act as CCN, reactive uptake of acids also increases the solubility of iron, making it more available to the phytoplankton in the ocean (Fan et al, 2006). These observations have led to the suggestion that changes in anthropogenic SO 2 emissions over East Asia (for example) may affect carbon fixation in High Nutrient Low Chlorophyll regions of the ocean via atmospheric dust deposition (Meskidhze et al, 2003) -see Sect.…”

“…This requires improvements to the modeling of emission, transport and deposition, all of which are size-dependent, as well as enhanced surface soil datasets. Changes in dust chemistry during transport may be important for iron and phosphorous availability in the oceans, although more fundamental research is needed (e.g., Fan et al, 2006).…”

Characterization of Atmospheric Iron Speciation and Acid Processing at Metropolitan Newark on the US East Coast

“…Figure 4c shows that SO4 2− was mainly accumulated in 0.32-0.56 μm, which was consistent with the size distribution of NH4 + , suggesting that the potential formation of NH4HSO4 and (NH4)2SO4 in aerosols. Fan et al (2006) [23] found that the coating of dust particles by sulfate from ambient trace gas deposition was likely the main factor for the enhanced fertilization of the modern northern hemisphere ocean. Organic acids such as low molecular weight dicarboxylic acids with low vapor pressure are soluble in aqueous solution, are consequently strongly hygroscopic [56], thus, when accumulated in sufficient quantity, have a much greater ability than the inorganic species SO4 2− and NO3 − to lower the surface tension of cloud condensation nuclei (CCN).…”

“…Photochemical reductions, particularly in more acidic cloud waters may promote dissolution of Fe [18], leading to the production of soluble Fe (II) [19][20][21]. Solubility of Fe could also be affected by the processes involving inorganic acidic species, such as sulfur-and nitrogen-containing compounds [22][23][24] and organic acidic species, such as oxalic acid [25,26]. C2-C4 dicarboxylic acids exist in the urban atmosphere [27,28], in biomass burning [29], in agricultural areas [30], over marine regions [31], and over remote regions [32].…”

Atmospheric processing of iron carried by mineral dust

“…Moxim et al (2011) specified that the iron solubilization rate is dependent on the acid chemical processing, the local sunlight and cloud processing, with the decay time ranging from 3 h (cloud processing of hygroscopic particles) to 23 days (chemical and photo processing). Fan et al (2006) used a two-step mechanism for the iron processing by sulfur, which included the acid coating of the dust phase followed by the dissolution phase. For converting the hematite iron in the dust aerosol, Meskhidze et al (2003) suggested a mechanism where iron becomes more soluble in a highly acidic, polluted environment.…”