Browsing by Subject "speleothem"

Now showing 1 - 4 of 4
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    Characterization and modeling of materials responsible for planetary crustal magnetism
    (2016-08) Strauss, Becky
    Earth and Mercury are the only terrestrial planets in our solar system with present-day magnetic dipole fields generated by internal dynamo systems. In contrast, Mars and the Moon show evidence of past dipole fields in the form of crustal magnetic anomalies; to hold measurable magnetizations, crustal materials must have been exposed to an applied field. While the physical principles of magnetic recording are consistent between terrestrial planets, the particular conditions at each planet control the mechanisms by which crustal materials may be magnetized and limit the types of minerals that can retain magnetic remanence. As the suite of magnetic materials used for studies of remanence expands, the need for new methods follows. The integration of rock magnetic techniques with microscopy and chemical analyses enables the reconstruction of increasingly comprehensive narratives of remanence acquisition and alteration, even in materials that are challenging to study using traditional methods. This thesis demonstrates the utility of a materials approach to rock magnetism by applying techniques designed for terrestrial use in a planetary context. The first of two case studies focuses on calcite cave deposits as a means to demonstrate how novel techniques can be used to unlock previously inaccessible archives of magnetic information. Tandem magnetic and microscopic analyses improve our understanding of the rock magnetic properties of weakly magnetic stalagmites and their potential for paleomagnetic research, as well as illuminating the pathways of remanence acquisition in cave systems. The second case study addresses the magnetic anomalies recently detected by the MESSENGER orbiter at Mercury. These anomalies are consistent with remanence acquired in a dipole field. However, in the absence of physical samples, the types of magnetic minerals that could be holding remanence in Mercury’s hot, highly reducing surface environment have not yet been determined. Orbital data is combined with fundamental rock magnetic principles to constrain the magnetic mineralogy of Mercury and to propose mechanisms of magnetization and remagnetization in the lithosphere.
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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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    New insights into Great Basin hydroclimate: past and present
    (2016-05) Cross, Mellissa
    From thermodynamic principles, Held and Soden (2006) predicted that in a globally warming world, dry places become drier, and wet places wetter; or by contrapositive, dry places become wetter in a globally cooling world. This hypothesis holds profound implications for our increasingly warmer Earth and vital water resources. The magnitude of this drying and whether it holds true on regional scales, however, is not clear. To explore this further, there are two options: we can model the future, or reconstruct the past. This thesis is an exploration of the past and present regional hydroclimate of the Great Basin. The Great Basin region is a large internally drained area in the western United States that has experienced significant hydroclimate changes. Large interbasin freshwater lakes dominated the landscape during glacial periods, and much time and effort has been expended understanding the timing and causes of lake level fluctuations. Until recently, there was a relatively poor understanding of Great Basin hydroclimate in times beyond the reach of radiocarbon dating. The Devils Hole record added another layer of uncertainty by indicating a Termination II ( T-II ) at least 10,000 years earlier than insolation rise. After nearly 30 years, a new Devils Hole record offers a promising reconciliation with orbital theory, indicating that certain geochemical and hydrological processes may have caused the older apparent ages of the original record. However, many aspects of Great Basin hydroclimate change are uncertain. As such, I explore two new records of Great Basin hydroclimate from Lehman Caves, Nevada, speleothems in this work. Speleothems are cave formations, which can be dated very precisely using the uranium-thorium radio-isotope system and have a number of chemical and isotopic parameters that can be interpreted to represent response to various facets of climate. For each record, both temperature and potentially a seasonal precipitation contribution change response, using δ 18 O values, and a hydroclimate response, using δ 13 C values, Mg/Ca, and Sr/Ca ratios (in Chapter 4), or transition metals and rare earth elements (in Chapter 5) are explored. Overall, Held and Soden (2006)’s prediction holds true. However, on shorter time scales climate response is more complicated, and millennial-scale events in the North Atlantic can influence moisture delivery to the Great Basin such that hydrologic changes do not always occur in lockstep with temperature variations. I further examine the potential for a hydrologic signal in my δ 18 O record by combining modern trajectory analysis with precipitation isotope data.
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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.