Paleoclimatic Reconstruction Based on Molluscan (Gastropoda, Pelecypoda) Environmental Indicators-Late Quaternary of Northwestern Texas

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Paleoecologic interpretation of fossil molluscan faunas provides a basis for indirect reconstruction of paleoclimates. Terrestrial and aquatic mollusks are abundant in late Quaternary sedimentary deposits of the western Rolling Plains of Texas. These taxa compose a succession of distinct faunal assemblages. Most mollusks represented in late Pleistocene to middle Holocene assemblages of the region are absent from the modern fauna. However, none of the extirpated species are extinct; their distribution has merely been reduced such that at present, these taxa are found northeast or in montane areas west of the Southern High Plains and Rolling Plains. Because they are living species, their environmental requirements, habitat, and climate are relatively well known. Environments that sustain these mollusks today are presumed to have existed in northwestern Texas in the past. Ecological conditions throughout the Pleistocene and Holocene Epochs can be inferred by tracing the range of environmentally sensitive taxa through radiocarbon-dated stratigraphic sequences. Allowance is made for local variations owing to facies changes and temporary modifications of habitat. Conditions in this region during the late Pleistocene favored diverse molluscan faunas. By comparison, the living fauna is depauperate and virtually restricted to species with broad environmental tolerances; climatic and ecological change was gradual, affecting different species at different times. Regional extirpation of a number of species with comparable ecologic requirements indicates a profound change in environment probably related to climate. Data from three well-constrained stratigraphic sections permit refinement of existing paleoclimatic reconstructions. Climatic variations during the Holocene have taken two related but somewhat independent paths. A summer warming trend that began in the latest Pleistocene greatly increased temperatures by about 8,000 years before present. A similar trend toward desiccation was accelerated between 8,000 and 6,000 years before present until essentially modern conditions were attained 3,000 years before present. Other, comparatively minor fluctuations of the regional paleoclimate are evident as well.


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