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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.