http://hdl.handle.net/2027.42/77949 2013-06-19T01:52:52Z http://hdl.handle.net/2027.42/98095 Variable Hf-Sr-Nd Radiogenic Isotopic Compositions in a Saharan Dust Storm over the Atlantic: Implications for Dust Flux to Oceans, Ice Sheets and the Terrestrial Biosphere Aarons, Sarah M. Isotopic characterization of aerosol mineral particles (atmospheric dust) of varying sizes is essential in classifying source areas, and for determining the source of dust deposited over oceans and icesheets. However, the e_ect of atmospheric transport on radiogenic isotopic compositions is not well constrained, making provenance interpretation di_cult. In order to investigate the isotopic variability of 176Hf/177Hf, 87Sr/86Sr and 143Nd/144Nd we analyzed eight airborne dust samples in two size fractions collected along a cross-Atlantic transect through a dust storm originating in the Sahara in late 2002. Past measurements of 176Hf/177Hf, 87Sr/86Sr and 143Nd/144Nd of dust have focused primarily on coarse sized particles (<30 _m), whereas far field deposition is primarily finer particles (<2 _m). Strontium or neodymium isotopic sorting based on distance is not evident in our dataset; however, the combined isotopic ratios of the dust collected suggests a Saharan origin. Hafnium isotopic compositions show an east to west trend towards more radiogenic compositions across the Atlantic, suggesting grain and mineral sorting during dust transport along the _4000 km transect. Transport models with variable dust particle diameter and wind speed demonstrate that the Hf isotopic compositions can be explained by the loss of the high-density mineral zircon during transport of dust from the source area. Modeling of this “zircon e_ect” in the Hf isotopic composition of marine, terrestrial and glacial dust deposits can reveal additional information concerning dust transport and sources in the geologic past. Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology or Earth and Environmental Sciences, Department of Earth and Environmental Sciences 2013-01-16T00:00:00Z http://hdl.handle.net/2027.42/98094 Regional Paleovegetation Records of the Eocene-Oligocene Transition of Montana Dennis, Meredith The climate change that occurred over the Eocene-Oligocene transition (EOT; ~33.7 Ma) is the most significant and dramatic of the Cenozoic, reflecting a change from “hothouse” to “icehouse” conditions. Paleoclimate and paleovegetation studies over the last 50 years have put together a broad picture of vegetation change in southwestern Montana. Although general predictions concerning biome response to climate change are useful, a refined understanding is necessary to forecast the effects of future anthropocentric climate change. This study investigates five sites in southwestern Montana (North Hough Draw, Little Spring Gulch, Big Stonerpipe, Little Pipestone, and Matador Ranch) using geochemical analyses of fossilized soils (paleosols) in combination with plant biosilica (phytoliths) to infer the regional response of vegetation during the EOT. In contrast with the marine record of the EOT, whole rock geochemical analyses indicate little change in mean annual temperature and precipitation. Phytoliths from the five sites are indicative of a forested or woodland landscape with a small component of (likely understory) grasses (~22.1%). The most significant vegetation change occurs in regard to the percentage of open-habitat grasses at each site, with a clear increase in open habitat grasses around 34.5 Ma, followed by a return to a more heavily forested landscape over the EOT. Stable carbon isotope data from organic carbon inclusions in paleosols change in tandem with the abundance of open-habitat grasses, and indicate that open-habitat grasses became more abundant as drier conditions prevailed. These results affirm findings of lower resolution studies of southwestern Montana, suggest a long-term, gradual cooling and drying trend over the EOT, and highlight the importance of local geography as a predictor of response to climate change. Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology or Earth and Environmental Sciences, Department of Earth and Environmental Sciences 2012-05-29T00:00:00Z http://hdl.handle.net/2027.42/98093 Representational Bias in Phytoliths from Modern Soils of Central North America: Implications for Paleovegetation Reconstructions Hyland, Ethan G. Understanding localized patterns and community compositions of vegetation in an environment is critical to the reconstruction of climatic and ecological conditions across all spatiotemporal scales. One of the most accurate and useful ways to characterize vegetation, and therefore to describe the climatic and ecological conditions of a location, is through the plant fossil record. Phytoliths (plant silica microfossils) are often preserved in the absence of other paleobotanical data and are becoming more widely used for deep-time vegetation reconstructions. Significant work has been done to standardize the analytical methodology of phytolith extraction, statistical analysis, and interpretation, but more detailed investigations are needed to understand how well a given phytolith assemblage represents the aboveground plant biomass for a given ecosystem. We present results from paired soil phytolith assemblages and local vegetation assemblages across the central United States, from temperate forest, grassland, and rangeland/scrubland ecosystems. Phytolith assemblages from soil A-horizons were compared to percent cover of species and plant biomass estimates obtained via field observations and aerial estimates of tree cover to analyze differences in the relative abundance of forest or woody vegetation versus grasses. Soil phytolith assemblages from all sites average a 32% bias toward the grass morphotypes as compared to actual aboveground biomass observations, and comparisons to percent cover yielded broadly comparable bias figures. Percent bias estimates do not show significant correlations to most environmental factors (temperature, precipitation, local elevation), however, an extremely strong correlation (p< 0.001) was observed with soil order type. As a result, we suggest further research into the development of correction factors between phytolith sample assemblages and their inferred past counterpart ecosystems based on estimates derived from modern analyses of each major soil order type. Such corrections are essential to the continued use of phytoliths as a proxy for past vegetation and ecological reconstructions throughout the Phanerozoic record. Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology or Earth and Environmental Sciences, Department of Earth and Environmental Sciences 2012-05-29T00:00:00Z http://hdl.handle.net/2027.42/98092 Dehumidification over Tropical Continents Reduces Climate Sensitivity and Inhibits Snowball Earth Initiation Fiorella, Richard P. The enigmatic Neoproterozoic geological record suggests the potential for a fully glaciated “Snowball Earth.” Low-latitude continental position has been invoked as a potential Snowball Earth trigger by raising surface albedo and reducing atmospheric CO2 concentrations through increased silicate weathering. Herein, climate response to reduction of total solar irradiance (TSI) is tested using four different land configurations (aquaplanet, modern, Neoproterozoic, and low latitude supercontinent) with uniform topography in the NCAR Community Atmosphere Model (CAM, version 3.1) general circulation model with a mixed-layer ocean. Despite a lower surface albedo at 100% TSI, the threshold for global glaciation decreases from 92% TSI in the aquaplanet configuration to 85% TSI with a low-latitude supercontinent. The difference in thresholds is principally due to the sensitivity of total specific humidity and therefore greenhouse forcing to reductions in TSI. Dehumidification of the troposphere over large tropical continents decreases greenhouse forcing and also increases direct heating by decreasing cloud cover. Continental heating intensifies the Walker circulation and the transport of dry air over the ocean, enhancing surface evaporation and marine tropospheric humidification, maintaining a high specific humidity and greenhouse effect over the ocean. Topography also provides an important control on Snowball Earth initiation. Modern topography in the modern continental arrangement lowers the initiation threshold by up to 2% TSI relative to a modern continental arrangement without topography. In the absence of potential silicate weathering feedbacks, large tropical landmasses raise the barrier to initiation of Snowball events. More generally, these simulations demonstrate the substantial influence of geography on climate sensitivity. Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology or Earth and Environmental Sciences, Department of Earth and Environmental Sciences 2013-01-16T00:00:00Z