Palaeo-dust records in sediments and ice cores show that wind-borne mineral aerosol ('dust') is strongly linked with climate state. During glacial climate stages, for example, the world was much dustier, with dust fluxes two to five times greater than in interglacial stages. However, the influence of dust on climate remains a poorly quantified and actively changing element of the Earth's climate system. Dust can influence climate directly, by the scattering and absorption of solar and terrestrial radiation, and indirectly, by modifying cloud properties. Dust transported to the oceans can also affect climate via ocean fertilization in those regions of the world's oceans where macronutrients like nitrate are abundant but primary production and nitrogen fixation are limited by iron scarcity. Dust containing iron, as fine-grained iron oxides/oxyhydroxides and/or within clay minerals, and other essential micronutrients (e.g. silica) may modulate the uptake of carbon in marine ecosystems and, in turn, the atmospheric concentration of CO2. Here, in order to critically examine past fluxes and possible climate impacts of dust in general and iron-bearing dust in particular, we consider present day sources and properties of dust, synthesise available records of dust deposition at the last glacial maximum (LGM); evaluate the evidence for changes in ocean palaeo-productivity associated with, and possibly caused by, changes in aeolian flux to the oceans at the LGM; and consider the radiative forcing effects of increased LGM dust loadings.

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We report on the simulated cloud processing of an aerosol iron sample derived from an Australian dust storm. Primary factors influencing the extent and rate of Fe mobilization were pH, duration of extraction and dust concentration. Fe was significantly mobilized below a threshold of pH ∼3.6. After initial rapid mobilization at low pH, the rate of Fe release was constant with constant pH. Between this threshold and pH ∼7.1, dissolved Fe fell to a minimum but above this pH further dissolution of iron occurs, probably due to the formation of soluble ferrates. At our lowest dust concentrations (1–20 mg L−1) the rate of Fe extraction at low pH was constant, while at higher dust concentrations the rate was inversely proportional to dust concentration. Dissolution of iron from dust is thus a complex process and these factors must be considered when modeling the input of iron to the oceans.

[1] Dust is an important vector for iron supply to the ocean, which subsequently impacts ocean productivity, atmospheric CO 2 concentrations, and hence global climate. Here, we synthesize the processes influencing the biogeochemistry of Australian dust and compare them with those from other Southern Hemisphere dust sources. Our observations range from soil and dust physical properties to abrasion and cloud chamber chemistry experiments to dust storms and their dispersion and deposition. We then present satellite observations of the impact of episodic dust deposition events on the productivity of low-iron oceanic waters north (i.e., low-nitrate, low-chlorophyll (LNLC)) and south (i.e., high-nitrate, lowchlorophyll (HNLC)) of Australia. Dust deposition from the largest dust storm in over 40 years did not result in iron-mediated algal blooms in either oceanic region. A comparison of Australia with other Southern Hemisphere source regions reveals that the relatively well sampled Australian system is a poor generic model. Furthermore, there are marked distinctions between Southern and Northern Hemisphere iron/dust biogeochemistry that must be recognized by modelers and included in future simulations. Better information is required on the relative role of the atmosphere and ocean on influencing iron biogeochemistry and how their relative influences might change in the future due to climate change.

[1] Eolian dust is a source of iron for phytoplankton in many ocean areas, and there are complex pathways of atmospheric processing from soil to ocean. Overlooked parts of the pathways are the impact of large (>10 mm) grains (including a role as proxies for the behavior of smaller grains) and the effect of multiple cycles of uplift and abrasion in the dust source region. Partitioning (readily released, acid-leachable and refractory) and dissolution rates of iron were determined for an artificial dust (produced by abrading an Australian soil), untreated soil, abraded soil (after production of the artificial dust), and a natural Australian eolian dust sample taken during a dust storm. Readily released iron is not created during abrasion, and therefore the amount of readily released iron in a dust or dust-derived soil depends on processing events since the dust or soil last experienced an abrasion event. Our study develops a method for the partitioning of iron within airborne dusts and appears to be the first to consider the effect of multiple uplift events on iron partitioning.

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