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Climate, corn, and culture: Modeling paleoenvironmental change and Native American Zea mays agriculture in Utah at the MCA-LIA transition


In chapter 1, Fremont Ancestral Puebloan (AP) maize (Zea mays) cumulative growing degree days (cGDD) were reconstructed for the years 850-1449 CE by statistically downscaling temperature dailies from a state-of-the-art, fully coupled climate model (CESM LME). In addition, radiocarbon dates from geolocated Fremont artifacts (n=982) used were to map time series of spatially discrete summed probability distributions (SPD). Fremont occupation of the Colorado Plateau was shown to coincide with warming conditions during the early Medieval Climate Anomaly (MCA), ~850-1050 CE, and abandonment with shorter and increasingly variable growing seasons after ~1050 CE. The disappearance of Fremont-identified material culture from Utah was shown to coincide with increased hydrologic variability on the Colorado Plateau with the transition to the Little Ice Age (LIA, ~1350-1850 CE). In chapter 2, the downscaled climate model was used to drive a process-based crop model (EPIC) to reconstruct maize yields under 7 different crop management strategies for irrigated and non-irrigated cases. Individual sites were clustered within three separate subregions for analysis (i.e., the Great Salt Lake shoreline, the highland Uinta Basin, and an archetypal Fremont canyon occupation at Range Creek). All occupations analyzed were caught within the same agro-ecological vise of increasing exogenous climatic and endogenous (anthropogenic) soil nutrient degradation, with varying degrees of intensity. Among the subregions, Range Creek was the most constrained by exogenous changes in climate (cooling and increasing variability at the MCA-LIA transition), and the Great Salt Lake shoreline the least; but the Great Salt Lake sites were the most sensitive to endogenous soil degradation. The methods employed in chapters 1 and 2 are broadly applicable to reconstruct GDD and crop yields, especially in other low-technology subsistence dryland agricultural contexts. In chapter 3, subfossil chironomid (Insecta: Diptera: Chironomidae) assemblages from sediments were analyzed to infer mean July air temperatures for 3 subalpine lakes in Utah. Results showed warm and cool periods over the Markagunt Plateau (i.e., the location of Alpine Pond) in southwestern Utah and over the Wasatch Plateau (i.e., Blue Lake) in southern central Utah were consistent with those expected from other proxy records during the MCA and LIA. (The record for Denise Lake, in the Uinta Mountains of northeastern Utah, was discontinuous.) Further, warming over the past century was unprecedented over the past 1570 years.

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