Browsing by Subject "paleoclimate"

Now showing 1 - 3 of 3
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    High-resolution speleothem record of climate variability during the late Pleistocene from Spring Valley Caverns, Minnesota
    (2018-02) Nissen, Julia
    Understanding how the Earth's oceanic and atmospheric systems responded to abrupt climatic forcings in the past is crucial in determining potential effects of anthropogenic climate change. This is of particular importance in the mid-continental United States, an agricultural hub that produces much of the world's corn and soybeans. High resolution paleorecords in the region remain sparse, restricting the predictability of global climate models and limiting our knowledge of atmospheric teleconnections across North America. This study analyses the growth and stable isotopic concentration of late Pleistocene speleothem samples from Spring Valley Caverns, located in SE Minnesota. Timing of growth for six samples was determined using 230Th dating and confocal microscopy, recording ages between 114 - 29 ky BP. This places all sample growth within the last glacial period. Sample growth largely correlates with warm conditions in the North Atlantic, indicating elevated temperature and moisture availability. Growth during MIS4 may be due to enhanced anti-cyclonic activity over the Laurentide Ice Sheet, resulting in anomalous southerly moisture transport. High resolution stable isotope analysis was completed for two samples, showing both long term trends and short term variability. Samples show a steady decrease in d13C values from 104 - 29 ky BP, suggesting a transition from C4 to C3 dominated ecosystems consistent with global cooling. However, d18O values climb steadily throughout this period. This is indicative of drier conditions and an increase in Gulf of Mexico sourced moisture in the region. Short term variability from 64 - 44 ky BP shows impressive correlation to global paleorecords. North Atlantic cold events are shown as an increase in d18O values, and North Atlantic warm events as a decrease d18O values. As d18O and d13C generally covary during this period, elevated d18O values suggest regional aridity. The transport of Gulf of Mexico summer moisture into the region appears significantly susceptible to changes in North Atlantic climate, most likely through large scale atmospheric systems such as the North Atlantic Oscillation and Pacific-North America teleconnection. The results of these records agree with a previous study of Spring Valley Caverns from the Holocene (Dasgupta, 2008). This signifies that large scale dynamics present during the last glacial period continue to affect regional climate, and thus these results may prove useful not only in improving our understanding of abrupt climate events during the late Pleistocene but in predicting impacts on the region from anthropogenic warming.
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    Magnetic minerals in soils and paleosols as recorders of paleoclimate
    (2017-06) Maxbauer, Daniel
    It is a fundamental challenge for geologists to create quantitative estimates of rainfall and temperature in past climates. Yet, records of past climates are integral for understanding the complexities of earth system dynamics. The research presented in this dissertation begins to establish a framework for reconstructing paleoclimates using the magnetic properties of fossilized soils. Magnetic minerals are ubiquitous in soils, and their composition, grain size, and concentration is often directly related to the ambient climatic conditions that were present during soil formation. Using rock magnetic methods, it is possible to sensitively characterize the magnetic mineral assemblages in natural materials - including soils and paleosols. The fundamentals of rock magnetism and many of the common methods used in rock magnetic applications are presented in chapter 2 and chapter 3, respectively. Chapter 4 reviews the physical, chemical, and biological factors that affect magnetic mineral assemblages in soils, the magnetic methods we use to characterize them, and the known relationships between magnetic minerals in soils and climate. A critical component to developing replicable tools for reconstructing paleoclimate is developing analytical and statistical tools that are accessible to the greater community. Chapter 5 introduces a new model, MAX UnMix, that was developed as an open-source, online tool for rock magnetic data processing that is designed to be user-friendly and accessible. Two case studies, on both fossil (Chapter 7) and modern (Chapter 6) soils, are presented and discuss many issues related to applying magnetic paleoprecipitation proxies in deep time. Chapter 7 discusses difficulties in disentangling the effects of pedogenesis, diagenesis, and recent surficial weathering in Paleocene-Eocene (56-55 Ma) paleosols. Chapter 6 explores the relative influence of soil forming factors (vegetation vs. climate) on controlling the pedogenic formation of magnetic minerals in soils developing across the forest-to-prairie ecotone in NW Minnesota. The body of research presented in this dissertation provides many challenges to future workers, while at the same time highlighting that rock magnetism should be a useful tool for researchers interested in deep time paleoclimates moving forward.
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    A tunnel speleothem based stable-isotope record of Atlantic Multi-Decadal Oscillation forcing of precipitation in the Midlands, United Kingdom
    (2019-08) Shull, Carolyn
    Cave speleothems are an established source of preserved data used in paleo-environmental reconstruction, as climate and land use information can be recorded in the carbon and oxygen isotopes. Speleothems in the tunnels of the canals in West Midlands, UK were investigated as another potential record as they appear to experience a rapid growth rate, a requirement to detect short-term climate events and low-amplitude climate signals. Formation in artificial structures restrict the potential record to the past 150 years, while other speleothem-based proxies span millennial time scales. Upon analysis, speleothem oxygen isotopes reflect the Atlantic Multi-Decadal Oscillation (AMO), and carbon and oxygen isotopes are correlated to land use changes. The results indicate speleothems from canal tunnels in central England potentially provide a record of land use changes and precipitation source water related to the AMO. Tunnel speleothem isotope data have the potential to serve as valuable datasets in climate teleconnection and modeling studies.