| dc.description.abstract | This dissertation takes a multiple-proxy approach to a paleoenvironmental reconstruction of the Holocene--Pleistocene Transition represented by the Brady Soil in the central Great Plains. To better understand this dynamic time of climate change, and to overcome the limitations of previous analyses using stable carbon isotope data, this study employs phytoliths to provide specific information on paleovegetation communities and quantitative shifts of plant taxa. Climatic indices based on both short- and large-cell phytolith frequencies add needed data on shifts of relative temperature (C3:C4 Grassland Ratio), water stress on plants (Bulliform Index), and soil moisture (from a proposed Soil Moisture Index). Analysis of short cell phytoliths reveals quantitative plant taxa shifts from Pooideae (C3) dominant grasses, with relatively large numbers of arboreal dicot spheres and a few Cyperaceae (sedge) present in a savannah or open woodland setting in the Bølling-Allerød climatic period (~14.6 ka to 12.9 ka), to a mixed, open grassland of Chloridoideae (C4) and Pooideae (C3) in the early Holocene. Stipa-type Pooideae, a cool-season grass preferring drier soil conditions, marks the onset of the Younger Dryas (~12.9 ka to 11.7 ka), which was previously not revealed in δ13C analysis. Brady Soil bioturbation was examined to provide a new proxy for paleoenvironmental conditions in addition to determining the effect of bioturbation particular to this soil on sediment transportation through the profile. Through macro- and micromorphology, particle size, soil color, and phytolith signatures, it was determined that invertebrate activity (primarily by cicada nymphs (Cicadidae) or burrower bugs (Cydnidae) of the Naktodemasis bowni ichnotaxon) was highly localized (diffusive mixing). Similar modern burrowers tend to be correlated with woody vegetation necessary throughout their lifecycle. Increased activity of cicada within the Bkb horizon corroborates phytolith reconstruction, indicating savannah to open woodland conditions during this time of Brady Soil pedogenesis. Non-local (advective) mixing is also prevalent due to vertebrate activity. Burrow and chamber construction, size parameters, and scratch traces indicate activity by prairie dog (Cynomys sp.) or ground squirrel (Spermophilus sp.). | |
