Iron oxide minerals in dust: New views of their types, sizes, amounts, and effects on the Earth System

Richard Reynolds 1, Bruce Moskowitz1, Harland Goldstein2, Stephen Cattle3, Charlie Bristow4, Thelma Berquó5, Ray Kokaly2
1University of Minnesota, Minneapolis, Minnesota, USA, 2U.S. Geological Survey, Denver, Colorado, USA, 3The University of Sydney, Sydney, New South Wales, Australia, 4University of London, London, UK, 5Concordia College, Moorhead, Minnesota, USA

Iron oxide minerals typically compose only a few weight percent of bulk atmospheric dust but are important for roles in forcing climate, affecting cloud properties, accelerating rates of snow and ice melt, fertilizing marine phytoplankton, and impacting human health by generating reactive oxygen species and other pathways. Moreover, the partition between hematite and goethite is important to know to improve models for the radiative effects of ferric oxide minerals in dust. The combination of magnetic property measurements, Mössbauer spectroscopy, reflectance spectroscopy, chemical analysis, and scanning electron microscopy at 9-nm resolution reveals the occurrences, sizes, and abundances of iron oxide minerals in samples from the “Red Dawn” dust storm (23 Sept. 2009, Australia), the Bodélé Depression, and numerous dust and dust-source sediments in the interior American West. In each case, discrete nano-phase and microcrystalline iron oxides, hematite and (or) goethite were identified on and within clay coatings on PM2.5 and PM10 particles. “Red Dawn” dust samples collected from locations across eastern Australia (Lake Cowal, Orange, Hornsby, and Sydney) each contained hematite, goethite, and magnetite. Magnetite concentrations (as much as 0.29 wt %) were much higher in eastern, urban sites than in sites in central New South Wales (0.01 wt %), strongly suggesting addition of magnetite from local urban sources. Mössbauer spectroscopy (at 300 K and 4.2 K) indicated that goethite and hematite compose approximately 25-45 % of the Fe-bearing phases in samples from the inland sites. In samples from the Bodélé Depression, dominant goethite and subordinate hematite together composed about 2% of yellow-reddish dust-source sediments. Magnetite was ubiquitous in small amounts (0.002-0.57 wt. %). The average iron apportionment was estimated at 32% in ferric oxide minerals, 1.4 % in magnetite, and 66% in ferric silicates. In all sample groups, (high) abundance of ferric oxide correlated with (low) reflectance, indicating that these minerals have capacity to absorb solar radiation. Moreover, the high surface-to-volume ratios of ferric oxide nanoparticles may facilitate atmospheric chemical and physical processing, as well as affect iron solubility and bioavailability to marine and terrestrial ecosystems and in human lungs.