HR: 1330h
AN: B33B-03    [Abstracts]
TI: Advances in Dating of Desert Varnish by Portable X-Ray Fluorescence Spectroscopy
AU: * Pingitore, N E
EM: nick@geo.utep.edu
AF: The University of Texas at El Paso, Department of Geological Sciences, El Paso, TX 79968-0555 United States
AU: Lytle, F W
EM: fwlytle@lcturbonet.com
AF: The EXAFS Company, HC 74 Box 236 Eagle Valley, Pioche, NV 89043-9521 United States
AB: In prior studies we have estimated ages of desert varnish (DV) from the mass per area of Mn and Fe on flat rock surfaces, compared to a measured average accumulation rate across the US Southwest. To extend the technique to non-horizontal surfaces, we compared the apparent ages of horizontal and vertical surfaces of petroglyphs of the same type in the same area, i.e., glyphs that most likely are coeval. The source of the material in desert varnish is from the fall of dust, but other important factors in the formation of the varnish include moisture and exposure to sunlight, which may regulate the bacterial activity responsible for deposition of Mn and Fe. Fall of dust on a non-horizontal surface is proportional to the projected horizontal area, i.e., to cos A, the angle of the surface from the horizontal. For two stylistically coeval glyph panels that we measured, one at 30° to the horizontal and one at 60°, the projected areas scale as 1.0 : 0.866 : 0.5, where 1.0 is a horizontal surface. These ratios suggest that the "true" age of the glyphs could be calculated from a measurement on a sloping surface if the only operative factor was the accumulation of dust. For the 30 and 60 degreee panel pair, 0.866/0.5 = 1.73, which is DV thickness ratio predicted by orientation. But the ratio of the measured ages in years, i.e., the measured actual thicknesses, is 9250/8400 = 1.10. Since these ratios differ, it is apparent that, as expected, factors other than the accumulation of dust on the surface of the rock also are important. Nonetheless, the ratios are at least reasonably close. Taking many measurements of this type may prove more efficient than trying to model the effects of orientation on sunlight, moisture retention, dust accumulation, bacterial activity, etc. We anticipate that building an empirical data base of this type will eventually allow better estimation of the age of glyphs on any sloping rock. Research supported by US Bureau of Reclamation Grant Agreement No. 02-FG-30-0028.
DE: 0315 Biosphere/atmosphere interactions
DE: 1615 Biogeochemical processes (4805)
DE: 8494 Instruments and techniques
SC: Biogeosciences [B]
MN: 2005 Joint Assembly