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In order to investigate if high elevations existed in the Rocky Mountains before the late Miocene, we examined oxygen isotope ratios of 63 Tertiary smectite samples as a proxy for the isotopic composition of precipitation. Of these samples, 51 were also analyzed for hydrogen isotope ratios. These smectites were formed as a result of the weathering of volcanic air-fall deposits that blanketed much of western North America during the Tertiary. Smectite-bearing ashfall samples were collected from Eocene, Oligocene, and Miocene deposits along a transect that extends from the western Great Plains to Yellowstone National Park at modern elevations from ∼900 to ∼2800 m. In general, oxygen and hydrogen δ values of smectite lie along a line parallel to the meteoric water line, which suggests that the isotopic composition of these ash-derived smectites records the meteoric water composition during its formation. There is little evidence for postdepositional exchange with basinal brine fluids, evaporative effects, or diagenesis of these smectites. The δ18O values of Oligocene and Miocene samples increase ∼6‰ linearly from sample sites located at the crest of the Rocky Mountains to sites in western Nebraska and South Dakota. These results mimic the distribution and values of calculated oxygen isotope ratios of theoretical modern smectite over this same geographic traverse of decreasing elevation. This result suggests modern atmospheric circulation patterns and that the resulting distribution of δ18Oprecipitation has persisted since the Oligocene. The δ18O values of Eocene samples increase ∼8‰ between the Yellowstone region and central Wyoming, a result that does not correlate with modern δ18Oprecipitation trends. Our Eocene results may be explained by climate conditions extant at that time, but tectonic modification in the region between 50 Ma and 37 Ma cannot be excluded as the cause of our results. Because the modern climate system requires interaction with and modification by high-elevation areas, our results suggest that the Rocky Mountains have been at high elevation since at least 50 Ma.

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