Congratulations to Ran Feng who successfully defended her dissertation on August 28, 2015.

Advisor: Chris Poulsen

Abstract: Cenozoic topographic histories of the North American Cordillera and Andes are essential to understand the tectonic and climate evolution over these regions. However, because proxy records document both elevation and climate signals, detangling these signals is difficult and often associated with large uncertainties. To overcome this difficulty, this dissertation describes an interdisciplinary approach involving the application of global climate models (GCMs) to search for coherent solutions of paleoelevations and climates that match Cenozoic proxy records. This approach is used to investigate both early Cenozoic uplift and Neogene extension history of western North American Cordillera; and the late Cenozoic co-evolution of the Andean uplift and tropical Pacific climate.

Our analysis reveals that elevation estimates derived from fossil leaves and stable isotope compositions may be substantially biased due to ignoring changes in climate states, and patterns associated with the uplift. Elevation biases are greatest when using fossil leafs because of the assumptions that modern lapse rates are applicable to past climates.  Elevation biases are reduced when using stable isotope compositions due to fortuitous canceling of δ18O variations induced by changes in regional atmospheric circulation, mixing conditions and precipitation types. After accounting for climate, we estimate that the Western Cordillera attained a height of 3-4 km in the early Cenozoic. In addition, our work helps explain the conflicting distribution of Neogene proxy δ18O changes across the western U.S. The Great Basin region, despite attaining its topographic peak during the early Cenozoic, shows smaller δ18O enrichment (indicating less elevation loss), relative to the surrounding northern Sierra Nevada and central Rockies. δ18O enrichment over these two regions, however, is mostly attributable to Neogene large-scale circulation changes. Removal of this climate signal reveals localized elevation loss of 1.5 – 2 km across the Great Basin and minimal loss of its surroundings, consistent with the early Cenozoic topographic reconstructions. Finally, we demonstrate that the Andean uplift played an integral role in creating the present-day east (cold) –west (warm) SST contrast across the tropical Pacific, through the impacts of the uplift on strengthening the diabatic vertical atmospheric circulation and on low cloud formation along the South American coast. Collectively, these results improve our interpretations and mechanistic understandings of Cenozoic tectonic and climate history around the eastern circum-Pacific orogenic belt.