| Summary: | Committee members: Diefendorf, Aaron; Fischer, Mark; Pollyea, Ryan; Stansell, Nathan. Advisor: Dodd, Justin P. Includes illustrations. Includes bibliographical references. The spatial ubiquity of trees in terrestrial ecosystems worldwide facilitates the use of stable isotope research in tree-rings to address a wide-range of interdisciplinary questions related to climate, ecology, archaeology, and hydrology. This dissertation focuses on the development and application of Prosopis tamarugo tree-ring stable isotope records as a paleoclimate proxy for the Atacama Desert, Chile. Current paleoclimate proxies from this region lack the spatial and temporal resolution to quantify a causal mechanism for observed Holocene hydroclimate variability. P. tamarugo tree-ring isotopes provide a high-resolution proxy that facilitates the reconstruction of hydroclimate variability at a regional scale. In order to address this issue, annual and subannual hydrogen ([delta]²H[sub ac]), oxygen ([delta]¹⁸O[sub ac]) and, carbon ([delta]¹³C[sub ac]) isotope records of [alpha]-cellulose measured in modern P. tamarugo tree rings are used to evaluate the use of this species for climatic reconstruction. Correlation-based analysis over the past half-century (1954-2016) reveals that tree isotopes record hydroclimate conditions, including changes in vapor-pressure deficit and groundwater isotope variability/depth. Modern groundwater depth was successfully modeled from the [delta]¹⁸O[sub ac] and [delta]²H[sub ac] values of modern tree-branch a-cellulose. Based on the modern calibration dataset, tree-ring [delta]¹⁸O[sub ac] and [delta]¹³C[sub ac] values of ancient Prosopis sp. are interpreted as a record of regional hydroclimate variability over the Holocene. A discontinuous record of Prosopis sp. tree-ring [delta]¹⁸O[sub ac] and [delta]¹³C[sub ac] isotopes radiocarbon date between 9,500 and 1,200 years ago. These records provide evidence of changes in relative humidity and groundwater availability (via plant intrinsic-water use efficiency) in the Atacama Desert over the Holocene. There is a steady increase in intrinsic water-use efficiency from the early to late Holocene, indicating a regional reduction in the availability of groundwater resources. [delta]¹⁸O[sub ac]-based estimates of relative humidity (h) show increased atmospheric moisture variability beginning 4,900 years ago and continuing through to the present. A strong correlation (r = 0.96) between the average [delta]¹⁸O[sub ac] values of the Atacama tree-rings and the high Andean Sajama [delta]¹⁸O ice core record provides evidence that water availability in the Atacama Desert is linked with high altitude Andean precipitation over the last 9,500 years. The tree-ring isotope series provide a discontinuous, but annually resolved, proxy that is the highest resolution record yet developed for the Atacama Desert. This research provides evidence that Prosopis sp. trees are high-resolution archives of hydroclimate change and suggests that future exploration of arid-region tree species isotope records could increase the spatial coverage of paleoclimate records worldwide. Ph.D. (Doctor of Philosophy)
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