ABSTRACT

Mountains produce local changes in climate through their control of vegetation and precipitation, but they may also have significant effects on hemispheric climate by setting up long-period waves in the atmosphere and preventing the simple zonal circulation that would sharply limit latitudinal heat transport through the atmosphere. As such, accurate estimates of paleotopography are an important boundary condition in global climate models of past warm periods. Detailed reconstruction of mountain belts, particularly their average elevation and aerial extent, is needed to address issues of high latitude warmth during the last ‘hyperthermal’ in the early Eocene. A variety of techniques based on sedimentology, structural geology, basalt vesicularity, stable isotopes, and paleotemperature estimates from fossil plant assemblages have been devised to reconstruct the elevations of ancient mountain systems. We present a new paleoaltimeter to estimate the difference in relative elevation between intermontane basins and the high elevations of ranges near the tree line. Application of this paleoaltimeter to the Eocene Green River Formation supports recent evidence that the Laramide mountains of the western United States were as high as or higher than the modern Rocky Mountains and suggests that many recent global climate simulations have prescribed elevations that are substantially too low.

INTRODUCTION

The early Eocene was perhaps the warmest period in the past 100 million years. Floral and faunal data suggest that warm conditions extended to much higher latitudes than today both in the oceans and on the continents.