ORIGIN OF THE WHEWELLITE-RICH ROCK CRUST IN THE LOWER PECOS REGION OFSOUTHWEST TEXAS AND ITS SIGNIFICANCE TO PALEOCLIMATE RECONSTRUCTIONS

A calcium oxalate (whewellite)-rich crust occurs on exposed limestone surfaces in dry rock and open air shelters in the Lower Pecos region o f southwest Texas, The crust, which also contains gypsum and clay, for med over silica-rich limestone during the Holocene. SEM and optical ph otomicrographs reveal similarities between whewellite microstructures and the lichen Aspicilia calcarea. This desert lichen is known to prod uce calcium oxalate, and has been found in several sites in the region , The ubiquity of the whewellite-rich crust in the Lower Pecos shelter s suggests that the lichen flourished in the past, Since A. calcarea i s a desert species, the virulence of the organism likely peaked during xeric climate episodes then waned during mesic periods. Thus, radioca rbon ages of whewellite would correspond to dry climate periods experi enced in the region, while periods with few or no C-14 data would indi cate wet climate episodes, A preliminary paleoclimate reconstruction b ased on fourteen AMS C-14 dates indicates the Lower Pecos experienced dry to wet climate fluctuations during the late Holocene, This reconst ruction generally agrees with other models established for Texas. (C) 1996 University of Washington.