Iron oxide minerals in dust of the Red Dawn event in eastern Australia, September 2009

Richard L. Reynolds, Stephen R. Cattle, Bruce M. Moskowitz, Harland L. Goldstein, Kimberly Yauk, Cody B. Flagg, Thelma S. Berquó, Raymond F. Kokaly, Suzette Morman, George N. Breit

Research output: Contribution to journalArticlepeer-review

19 Scopus citations

Abstract

Iron oxide minerals typically compose only a few weight percent of bulk atmospheric dust but are important for potential roles in forcing climate, affecting cloud properties, influencing rates of snow and ice melt, and fertilizing marine phytoplankton. Dust samples collected from locations across eastern Australia (Lake Cowal, Orange, Hornsby, and Sydney) following the spectacular "Red Dawn" dust storm on 23 September 2009 enabled study of the dust iron oxide assemblage using a combination of magnetic measurements, Mössbauer spectroscopy, reflectance spectroscopy, and scanning electron microscopy. Red Dawn was the worst dust storm to have hit the city of Sydney in more than 60years, and it also deposited dust into the Tasman Sea and onto snow cover in New Zealand. Magnetization measurements from 20 to 400K reveal that hematite, goethite, and trace amounts of magnetite are present in all samples. Magnetite concentrations (as much as 0.29wt%) were much higher in eastern, urban sites than in western, agricultural sites in central New South Wales (0.01wt%), strongly suggesting addition of magnetite from local urban sources. Variable temperature Mössbauer spectroscopy (300 and 4.2K) indicates that goethite and hematite compose approximately 25-45% of the Fe-bearing phases in samples from the inland sites of Orange and Lake Cowal. Hematite was observed at both temperatures but goethite only at 4.2K, thereby revealing the presence of nanogoethite (less than about 20nm). Similarly, hematite particulate matter is very small (some of it d<100nm) on the basis of magnetic results and Mössbauer spectra. The degree to which ferric oxide in these samples might absorb solar radiation is estimated by comparing reflectance values with a magnetic parameter (hard isothermal remanent magnetization, HIRM) for ferric oxide abundance. Average visible reflectance and HIRM are correlated as a group (r2=0.24), indicating that Red Dawn ferric oxides have capacity to absorb solar radiation. Much of this ferric oxide occurs as nanohematite and nanogoethite particles on surfaces of other particulate matter, thereby providing high surface area to enhance absorption of solar radiation. Leaching of the sample from Orange in simulated human-lung fluid revealed low bioaccessibility for most metals.

Original languageEnglish (US)
Pages (from-to)1-13
Number of pages13
JournalAeolian Research
Volume15
DOIs
StatePublished - 2014

Bibliographical note

Funding Information:
We are grateful to Christoph Geiss, Slobodan Nickovic, and an anonymous reviewer for very helpful reviews. We thank Andrew Rawson for collecting the dust sample from Orange and, for laboratory assistance, Cody Flagg and Jiang Xiao (reflectance analysis), Heather Lowers (SEM), and Peat Solheid (Mössbauer spectroscopy). We are also grateful to Roger Clark and Todd Hoefen for use of the USGS Spectroscopy Laboratory, Denver. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The Institute for Rock Magnetism is supported by grants from the Instruments and Facilities Program of the National Science Foundation Division of Earth Science. This is IRM Contribution 1306. Funding was provided by the U.S. Geological Survey Climate and Land Use Program .

Keywords

  • Dust magnetic properties
  • Dust radiative properties
  • Goethite
  • Hematite
  • Iron oxide minerals
  • Nanoparticles

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