RECONSTRUCTING LINKS BETWEEN CENOZOIC HYDROCLIMATE, AND ECOSYSTEM PARAMETERS IN THE JOHN DAY REGION, CENTRAL OREGON

Through the long term cooling trend of the past 50 million years many terrestrial landscapes have grown more arid. An example is the western United States, where ancient, massive lakes have evaporated, and many ecosystems have transitioned from forest to grasslands and shrubs. Since rainfall in this region is largely delivered along storm trajectories, upstream (coastal) conditions strongly influence those downstream (inland). In this study we focus on John Day Fossil Beds National Monument, a near-coastal site through much of the Cenozoic with nearly-continuous sedimentation over the last 50 million years. Since the Eocene, this site has gradually shifted inland and experienced dramatic changes in aridity similar to those observed in many other places around the world. In addition, both hydroclimate and vegetation changes influenced weathering patterns in the John Day region. Combining isotope data, elemental analysis, and clay mineralogy with a vapor transport model we develop a spatially-resolved pattern of regional aridification. Specifically, our data support a transition from wet to arid conditions around the Eocene-Oligocene boundary. While this shift is likely observed on a regional scale, the eastern part of our study region may have become more arid than the west due to local topography intercepting westerly moisture. Together, these data and model results suggest that complex, localized topographic features influenced aridity and possibly vegetation evolution.