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7 7 Understanding the ancient vegetation of the region around the Allen site is central to understanding the conditions the site’s occupants lived in, both because plants provide essential resources for all human groups and because overall patterns of vegetation, and changes in those patterns, provide important clues about ancient climate. This chapter presents analyses of pollen and phytoliths from four localities within the Medicine Creek drainage as a basis for examining these two issues. As the discussion below shows, these data indicate a clear pattern of seasonal flooding at Medicine Creek during the Early Holocene. Therefore, we also consider our data in light of a series of archaeoclimatic models of overall temperature and precipitation and of stream discharge/flooding in Medicine Creek. The sites considered here include a cutbank at the north end of Harry Strunk Lake and three localities along Lime Creek (from downstream to upstream, these are the Lime Creek, Red Smoke, and Stafford sites). May (2002; see also chapter 3) discusses the stratigraphy /geomorphology and radiocarbon chronology of all four of these localities. Examination of the pollen and phytolith record is based on analysis of individual pollen and phytolith samples within stratigraphic sequences. Sample selection paralleled sample selection for radiocarbon dates and geomorphic samples to maximize comparability of these records. Samples spaced this far apart, of course, allow only a coarse-grained examination of temporal changes in the past vegetation, from which we may infer paleoenvironmental conditions. However, the close spacing of sample locales within the drainage makes it possible to assess local patterns of environmental variation that are invisible in studies that reconstruct an environment on the basis of samples taken from a single location. Two samples from the present ground surface at Lime Creek were also examined for both pollen and phytoliths. Methods Several cores were collected by drilling into the terrace above the Lime Creek site. Single combined pollen and phytolith samples were removed from one core at the same depths that radiocarbon samples were removed. Samples from the other three localities were taken from exposed cutbanks. The samples were processed simultaneously to remove any clays in the sediments. Once clay removal was complete, the samples were dried and split for subsequent pollen and phytolith processing. In this manner we are assured that the pollen and phytolith samples represent identical proveniences. Pollen/Phytolith Extraction A chemical extraction technique based on flotation is the standard preparation technique used at Paleoresearch Institute for the removal of pollen from the large volume of sand, silt, and clay with which they are mixed. This particular process was developed for extraction of pollen from soils where preservation has Chapter 6 PALEOENVIRONMENTAL INTERPRETATIONS OF THE LATE PLEISTOCENE AND EARLY HOLOCENE IN SOUTHWESTERN NEBRASKA The Pollen and Phytolith Evidence Linda Scott Cummings, Thomas E. Moutoux, and Reid A. Bryson 78 / Chapter 6 been less than ideal and pollen density is low when compared with that in peat bogs. Hydrochloric acid (10 percent) was used to remove calcium carbonates present in the soil, after which the samples were screened through 150-micron mesh. The samples were rinsed until neutral by adding water (to a vertical column height of 10 cm), letting the samples stand for two hours, and then slowly pouring off the supernatant. A small quantity of sodium hexametaphosphate was added to each sample once it reached neutrality; then the beaker was again filled with water and allowed to stand for two hours. The samples were again rinsed until neutral, filling the beakers only with water. This step was added to remove clay prior to heavy liquid separation. Once this step was complete, the samples were dried and then pulverized. Zinc bromide (density 2.1) was used for the flotation process. The samples were mixed with zinc bromide and centrifuged at 1,500 rpm for 10 minutes to separate organic from inorganic remains. The supernatant containing pollen and organic remains was decanted and diluted. This process was repeated to ensure recovery of all pollen and organic remains. After rinsing the pollenrich organic fraction obtained by this separation, all samples received a short (20-minute) treatment in hot hydrofluoric acid to remove any remaining inorganic particles. The samples were then acetolated for three minutes to remove any extraneous organic matter. After this, the samples were rinsed until neutral, then stained with basic fuschin, with a single drop of potassium hydroxide in the final rinse, in preparation for slide making. Extraction of phytoliths from these sediments also was based on heavy...

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