HR: 08:30h
AN: GP51A-03    [Abstracts]
TI: Dusty Gusts and Rusty Dust: The Role of Iron in the Contrast in Physical Properties of Surface Soil between Sahara and Sahel
AU: * Williams, E
EM: earlew@ll.mit.edu
AF: Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, United States
AU: Balsam, W
EM: wbalsam@cybermesa.com
AF: Department of Earth and Environmental Sciences, University of Texas, Arlington, TX 76019, United States
AU: Ellwood, B
EM: ellwood@lsu.edu
AF: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, United States
AU: Hicks, E
EM: ehicks@univ-ag.fr
AF: Universite des Antilles et de la Guyane, Guadeloupe, Pointe-a-Pitre, 12345, France
AU: Oldfield, F
EM: oldfield.f@gmail.com
AF: Department of Geography, University of Liverpool, Liverpool, 12345, United Kingdom
AU: Prospero, J
EM: jprospero@rsmas.miami.edu
AF: Rosenstiel School of Marine and Atmospheric Sciences, 4600 Rickenbacker Causeway, Miami, FL 33149, United States
AU: Arimoto, R
EM: rarimoto@nmsu.edu
AF: Carlsbad Environmental Monitoring, Research Center, Carlsbad, NM 88220, United States
AU: Huang, S
EM: suilou@ll.mit.edu
AF: Lincoln Laboratory, Massachusettts Institute of Technology, Lexington, MA 02173, United States
AU: Thorncroft, C
EM: chris@atmos.albany.edu
AF: Atmospheric Sciences, University at Albany, SUNY, Albany, NY 12222, United States
AB: In the treatment of dust lofting and its advection out of Africa to the Americas, many studies neglect important distinctions between the Sahara Desert and the Sahel on its southern fringe, as source regions for the dust. The Saharan sources are primarily dry processes sustained by synoptic-scale winds and occasional cold fronts from Europe. The Sahelian sources involve moist convection and ensuing dry dusty gust fronts (aka, hoboobs), and have recently been investigated with a variety of methods (Doppler radar, visibility, thermodynamic) during the African Monsoon and Multi-Disciplinary Analysis. Evidence for a latitudinal contrast in optical properties in West Africa comes from satellite measurements of surface albedo, showing typical factor-of-two differences between albedo maxima (~0.5) in the Sahara to the regions in the Sahel (0.2-0.35) where vegetation begins to influence albedo strongly. Subjective visual observations indicate that the Sahara is ‘white' and the Sahel is ‘red'. In the present more quantitative study, soil samples were collected at ~50 km intervals between Bamako, Mali in the Sahel and Timbuktu, Mali in the Sahara Desert, to explore the role of iron compounds in influencing the physical properties of the soil: measurements included optical reflectance (the laboratory equivalent of albedo) and magnetic susceptibility (MS). Diffuse reflectance spectroscopy in the red, green and blue regions of the spectrum (250-800 nm in 1nm increments) were undertaken on these samples. Percent red is generally a good indicator of (red) hematite and increases from the Saharan samples to the Sahelian samples. The overall increase in reflectance is a factor-of-three from Sahel to Sahara. The MS measurements by three groups all show consistent results and a strong increase from Sahara to Sahel following the gradient in rainfall as in other studies elsewhere. The maximum spread in values is nearly two decades (from 1 x 10-8 m3/kg to 100 x 10-8 m3/kg). MS is known to be strongly influenced by ferrimagnetic minerals (magnetite and maghemite), which appear to be substantially more abundant in the surface material of the Sahel than the Sahara, but the hematite concentrations in the Sahel may be 2-3 orders of magnitude larger. The use of MS as a tracer of the origin of dust from Africa will be critically evaluated.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0322 Constituent sources and sinks
DE: 1519 Magnetic mineralogy and petrology
DE: 1865 Soils (0486)
DE: 5109 Magnetic and electrical properties (0925)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting