Influence of chemical weathering and aging of iron oxides on the potential iron solubility of Saharan dust during simulated atmospheric processing

Abstract

The flux of bioavailable Fe from mineral dust to the surface ocean is controlled not only by the processes in the atmosphere but also by the nature and source of the dust. In this study, we investigated how the nature of Fe minerals in the dust affects its potential Fe solubility (Fepsol) employing traditional and modern geochemical, mineralogical, and microscopic techniques. The chemical and mineralogical compositions, particularly Fe mineralogy, in soil samples as dust precursors collected from North African dust source regions were determined. The Fepsol was measured after 3 days of contact with sulfuric acid at pH 2 to simulate acid processes in the atmosphere. Fepsol of the soil dust samples were compared with calculated predictions of Fepsol based on the amount of individual Fe‐bearing minerals present in the samples and Fe solubilities of corresponding standard minerals. The calculated Fepsol deviated significantly from the measured Fepsol of the soil dust samples. We attributed this to the variability in properties of Fe minerals (e.g., size of Fe oxides and heterogeneity of chemical compositions of clay minerals) in soil dusts in comparison to the standard minerals. There were, however, clear relationships between the degree of chemical weathering of North African soils and Fepsol. The Parker index and ratio of ascorbate plus dithionite Fe to total Fe ((FeA+FeD)/FeT) are positively and negatively correlated with Fepsol, respectively. In addition, the ratio of FeA/(FeA+FeD), which decreases with aging of the Fe oxides, was found to be positively correlated with Fepsol in the soil dusts. Overall, our results indicate that there is a significant regional variability in the chemical and Fe mineralogical compositions of dusts across North African sources, as a result of the differences in chemical weathering and aging of Fe oxides. Furthermore, the indices for these weathering processes can provide an estimate of the fraction of Fe which can be solubilized if acid processed in the atmosphere.

Iron oxide minerals in dust-source sediments from the Bodélé Depression, Chad: Implications for radiative properties and Fe bioavailability of dust plumes from the Sahara

“…Our results extend earlier research on the occurrences of iron-bearing minerals in Bodélé surface sediments and elsewhere in North Africa that aimed to establish linkages between dust mineralogy and potential environmental effects of dust (Lafon et al, , 2006Alfaro et al, 2004;Moreno et al, 2006;Mounkaila, 2006;Journet et al, 2008Journet et al, , 2104Chudnovsky et al, 2009;Lázaro et al, 2011;Maher, 2011;Shi et al, 2011;Moosmüller et al, 2012;Adetunji, 2014;Formenti et al 2014 a, b). These studies employed many methods for dust and soil mineralogy, including magnetic analyses, X-ray absorption analysis, X-ray diffraction, microscopy, elemental and isotopic chemistry combined with leaching experiments, as well spectroscopic techniques in the laboratory and applied to satellite retrievals.…”

The impact of Saharan dust and black carbon on albedo and long-term mass balance of an Alpine glacier

“…Since they have different light absorption spectra (e.g., Lafon et al, 2006) their relative proportion has to be known for calculating the radiative properties of dust. According to Shi et al (2011) the mass ratio of the mineral haematite to the minerals haematite plus goethite for Saharan dust is 0.42 on average. Based on the assumption that about 45-64 % of the total Fe is encompassed in lightabsorbing oxides , the mass of goethite and haematite is calculated following Kaspari et al (2014) and used as proxy for the absorption of mineral dust.…”

Response of the Eastern Mediterranean Microbial Ecosystem to Dust and Dust Affected by Acid Processing in the Atmosphere

“…The proxy dust was resuspended using a custom-made resuspension chamber and the PM10 was separated for subsequent use. Previous studies have used similar procedures for experimental work with Saharan dust (Lafon et al, 2006;Guieu et al, 2010;Shi et al, 2011). Within the text we use the term PM10 dust or simply dust for this size fractionated proxy.…”