OXYGEN AND HYDROGEN ISOTOPE RATIOS OF CENOZOIC SMECTITE SUGGEST LONG-STANDING SIMILAR-TO-MODERN ATMOSPHERIC CIRCULATION PATTERNS IN WESTERN NORTH AMERICA

In order to investigate atmospheric circulation patterns in the Rocky Mountain region before the Late Miocene, oxygen isotope ratios of 63 Tertiary smectite samples were determined as a proxy for oxygen isotope ratios in precipitation. Additionally, hydrogen isotope analysis of 57 of these samples was conducted in order to determine if the isotopic ratios of smectite formation waters were modified by evaporative processes. We suggest these smectites were formed in equilibrium with meteoric water as a result of the weathering of volcanic airfall deposits which covered much of western North America during the Cenozoic. Oxygen isotope ratios of Eocene smectite samples from central Wyoming are approximately 2-6‰ heavier than calculated oxygen isotope values of theoretical smectite in isotopic equilibrium with modern precipitation. These relatively heavy isotopic values are consistent with average temperatures which were 10-20C warmer or a stronger summer monsoon system in North America during the Early Cenozoic. Oxygen isotope ratios of Eocene samples from western Wyoming and Oligocene and Miocene samples collected throughout the study area increase ~10‰ linearly from sample sites located at the crest of the Rocky Mountains to sites in western Nebraska and South Dakota, which is very similar to the distribution of modern oxygen isotope values. Although our hydrogen isotope ratios are more variable than our oxygen isotope results, comparison with the meteoric water line indicates that our hydrogen and oxygen isotopic ratios have not been modified by evaporation. Therefore, our results suggest modern atmospheric circulation patterns and the resulting distribution of oxygen and hydrogen ratios in precipitation have persisted since at least the early Tertiary, and these values have not been influenced significantly by evaporative processes. Because the climate system in the Rocky Mountain Region requires interaction with and modification by high elevation areas in the Rocky Mountains, these data from Tertiary smectites suggests the Rocky Mountains have been at near modern elevations since at least the Eocene. This conclusion is in agreement with many recent studies which suggest high elevations in the Rocky Mountains and a summer monsoon have existed in western North America throughout the Cenozoic.