Stable Isotope Analysis of a 12 M.Y. Record of Soil Carbonates in the Southern Rio Grande Rift

A core collected in Trans-Pecos, Texas has been dated by paleomagnetic techniques and records almost continuous deposition between 12.5 million years ago and 350,000 years ago. The core was deposited in the floodplain of small arid stream over most of this time and the invariant depositional environment and continuous record make this core uniquely suitable for stable isotope analysis of soil carbonates. We have collected samples from over 90 soils, extending back as far as 10 million years. These soils record the uplift associated with formation of the Rio Grande rift, which occurred between 15 million years ago and the present, as well as the establishment of the Chihuahuan Desert and the gradual drying of the southwestern United States. The core also documents climatic changes over the last 12 million years in a tectonically and ecologically important region of the world. Because the depositional setting of the core was similar over such a long time and the setting is a closed basin, factors such as changing sediment sources and other depositional factors can be discounted.