CLIMATIC CHANGE DURING THE LAST 3.5 M.Y. INFERRED FROM 18O AND 13C ISOTOPES FROM TRANS-PECOS, TEXAS, USA: COMPARISON WITH OTHER REGIONAL STUDIES

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 streams 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.

The stable isotope values from the core have been compared with known global climatic events and other regional stable isotope studies to determine if there is any correlation. Our d¹⁸O values show much heavier values than data from studies by Mack et al. (1994) and Wang et al., (1993). The following trends are seen in our d¹⁸O values: a decrease from ~3.4-2.5 Ma; an increase from ~2.5-2.1 Ma; a decrease from ~2.1-1.4 Ma; an increase from ~1.4-0.8 Ma; fluctuating values from ~0.8-0.35 Ma; all superimposed on a long-term decrease in d¹⁸O. Our observations are in contrast to trends observed in other studies. Specifically, while we see either a slight increase in d¹⁸O from ~2.4-2.1, Wang et al. (1993) and Mack et al. (1994) both show a decrease. From ~2.1-1.6 Ma, our data indicates a decrease in d¹⁸O, whereas Wang et al. (1993) and Mack et al. (1994) show an increase in d¹⁸O.

The d¹³C values from our study show a gradual increase from ~3.4-1.8 Ma, followed by a long-term decrease to ~0.35 Ma. Overall, the trends that we find in our data is in basic agreement with both Mack et al. (1994) and Wang et al. (1993), with a few specific exceptions. From ~1.9-1.8 Ma, ~1.5-1.3 Ma, and ~1.1-0.9 Ma, we show a decrease in d¹³C values, while the other studies show an increase; from ~0.9-0.8 Ma, our data indicates an increase in d¹³C values, where the other studies indicate a decrease.