PALEOELEVATION OF THE NORTH AMERICAN CORDILLERA FROM THE LATE CRETACEOUS TO LATEST EOCENE: AN INTEGRATED CLIMATE MODEL-STABLE ISOTOPE APPROACH

Because stable isotope ratios of precipitation and temperatures decrease in a regular manner as elevation increases (i.e. ~ uniform lapse-rates are observed), proxies for these variables have the potential to provide information regarding paleoelevation and paleotopography. Application of lapse-rates is most successful when proxy data from a particular time-slice can be compared among a number of localities that follow the presumed trajectory of rising air masses in the past. However, in areas such as the Cordillera of western North America, where air mass trajectories are variable, where post-range front topography is complex, or where multiple air masses with different sources and rainout histories are present, simple lapse-rate interpretations of isotopic and temperature data can be more ambiguous.

In an effort to address these problems, we use a global climate model to simulate atmospheric transport, precipitation, temperature, and isotope ratios of precipitation over North America during the early and late Paleogene for a prescribed geography and topography. This approach allows widespread proxy data to be considered in the context of an integrated climatic-atmospheric-topographic framework. The model is then run several times, changing only the topography of western North America (e.g. high-Sevier orogenic belt, low Sevier-orogenic belt, low-Laramide orogenic belt, high-Laramide orogenic belt, etc.).

Results of these simulations are somewhat counterintuitive, suggesting the primary control on isotope ratios of precipitation in the Laramide region is the elevation of the Sevier hinterland, and that ratios decrease as the elevation of the hinterland decreases. Proxy data are broadly consistent with model results, although limits in the spatial and temporal resolution of proxy data make it difficult to discriminate among alternative model results.