Detangling Climate and Topographic History from Cenozoic Proxy Records: Examples from Western North America and the Andes.

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 climate state.

My analysis reveals that elevation estimates derived from fossil leaf physiognomy and stable isotope compositions may be substantially biased by ignoring changes in climate states, and spatial climate patterns. Elevation biases are greatest when using fossil leaves if one assumes modern lapse rates as approximations to the past lapse rates, and are small when using stable isotope compositions due to fortuitous canceling of δ18O variations induced by changes in regional circulation, mixing conditions, and precipitation types. After accounting for climate, we estimate Western Cordillera attaining a height of 3-4 km in the early Cenozoic. In contrast, influences of changing climates on δ18O records are strong during the Neogene. These changes could explain stronger proxy δ18O enrichment surrounding the northern Sierra Nevada and central Rockies relative to enrichment within the Great Basin. Removal of this climate signal reveals elevation loss up to 1.5 – 2 km across the Great Basin, but smaller elevation loss in its surroundings, consistent with the reconstructions of early Cenozoic pre-extension topography. 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 strengthening the diabatic vertical tropospheric circulation and low cloud formation along the South American coast. Collectively, these results improve our interpretations and mechanistic understanding of Cenozoic tectonic and climate history around the eastern circum-Pacific orogenic belt.