STABLE ISOTOPE ANALYSIS OF A 12 M.Y. RECORD OF SOIL CARBONATES IN THE SOUTHERN RIO GRANDE RIFT

The purpose of this research is to document the stable isotope trends and fluctuations in soil provide a climatic record of the uplift of the Rio Grande Rift, and to provide a long-term record of climatic fluctuations. A core collected in far West Texas contains soils formed during the establishment of the Chihuahuan Desert and the gradual drying of the southwestern United States. The core offers a unique opportunity to document the climatic changes over the last 12 million years in a tectonically and ecologically important region of the world.

Analysis of the core has determined it is uniquely suitable for stable isotope analysis of soil carbonates. Soil carbonates have become an important tool for analyzing climate change. However, in most studies they suffer from three problems. First, soils typically develop over a very long time, meaning that the isotopic signature of the soil is often a composite one that does not accurately reflect climate change. Second, soils are by definition the surfaces of non-deposition. Therefore, what is seen in soils may not be representative of the conditions under which the sediment was deposited between soil formation episodes. Thirdly, there are problems with accurate dating to sediments older than the Quaternary. However, the Yucca Mesa core contains sediments with the same mineralogic and textural features for 10 of the 12 million years of deposition.

Soils have already been identified with in the core and samples of pedogenic carbonate have been extracted. The samples will be ground to powder, and a small portion dissolved to determine the percentage of in soluble minerals within the sample. The remaining sample portion will be sent to the stable-isotopes laboratory at Oak Ridge National Laboratory, Tenn. Samples will be analyzed for stable isotopes of both carbon and oxygen. The core will then be compared with the results of the isotope analyses. The isotope results will be correlated with stage development, visible changes in soil properties, and mineralogic composition of the soil.