LATE QUATERNARY CLIMATES AND ENVIRONMENTS OF THE EDWARDS PLATEAU, TEXAS

Fossil vertebrate, pollen, and plant macrofossil data from the Edwards Plateau, Texas and throughout the southcentral United States permit reconstruction of regional changes in temperature and effective moisture. Full-glacial temperatures were significantly cooler than those of today, at least 6-degrees-C during the summer months, but by ca. 13,000 yrs B.P. summer temperatures were within 2-3-degrees-C of present values. There was more effective moisture during the full-glacial period than at any time since then. During the late-glacial, ca. 14,000-10,500 effective moisture first decreased then increased, while the early to middle Holocene was dominated by a protracted decrease in effective moisture. This long-term trend culminated in conditions that were drier than modem during the early part of the late Holocene from ca. 5000 to 2500 yr B.P. Conditions were more mesic than present from ca. 2500 and 1000 yr B.P., while the modern drought-prone climate has characterized the last 1000 years. Fossil vertebrates and the characteristics of cave fill sediments show that late Pleistocene and Holocene changes in temperature and moisture regimes were coupled with vegetation changes and a gradual degradation of upland soils. During the full- and late-glacial much of the upland landscape was covered by thick, deeply weathered reddish clay-rich soils and an open savanna vegetation with a mixed tall and short grass understory. Changes to Holocene climatic conditions promoted a diminished vegetation cover and initiated the gradual degradation of soil mantles, whereas minimum effective moisture during the earlier part of the late Holocene resulted in upland landscapes that were covered by a mixture of short grasses and scrub vegetation, and the near complete removal of the remaining soil mantle. Vegetation changes during the last 2500 years are poorly known, but the upland landscape has consisted of exposed bedrock with little soil cover. Comparison of empirical data and the results of paleoclimate models shows good correspondence for the full-glacial through middle Holocene when climate system boundary conditions that drive model simulations were substantially different from today. Climate models also provide plausible physical explanations for climatic and environmental changes identified by empirical data. Our synthetic reconstruction, based on both empirical data and climate model simulations, provides a framework for evaluation of the impacts of climatic and environmental changes on earth surface processes and landforms, and a basis for discussion of prehistoric human adaptations to different environmental conditions.