New Mexico Geological Society Annual Spring Meeting — Abstracts

An understanding of paleoclimate and its effect on past ecosystems is important in " understanding and predicting future responses of ecosystems to climatic shifts. The goal of this research was to 1) determine the latest Pleistocene and Holocene vegetational history and paleoclimate of the Hueco Basin in the northern Chihuahuan Desert, 2) determine how other factors within the ecosystem, primarily sedimentation and erosion rates respond to changes in vegetation types and densities, and 3) how the above changes may have affected past human populations, and/or the preservation and integrity of the archaeological record.

The method used to determine past vegetation types was analyses of stable isotopes in pedogenic carbonate. This method is a particularly useful tool in environments where packrat middens and fossil pollen may not be preserved. In addition, buried calcic soils within fault troughs in the Hueco Basin are ideal because these soils have formed in noncalcareous parent materials and in an aggrading environment, which protected older pedogenic carbonates from contamination by younger fluids. Approximately 140 profiles were excavated and described throughout the Hueco Basin to determine the eolian stratigraphy. Five trenches within two fault complexes were sampled at 10 cm intervals for carbon and oxygen isotopes. In addition, three lateral transects were sampled at 10, 20 and 30 cm apart and analyzed to determine the lateral variability of the isotopic signatures.

δ¹³C analyses indicate an abrupt shift from C₄ grasses to C₃ desert scrub during the Altithermal, at approximately 8 ka. This change is also found in the alluvial fans adjacent to the basin indicating that this event was significant enough to affect both the higher and lower elevations throughout southern New Mexico and west Texas. In addition, this vegetational shift resulted in a basin-wide deflational event, which eroded Pleistocene paleosols in many areas within the central basin, created a paleolag similar to the one on the surface today, and eventually resulted in the deposition of the Organ unit. In contrast, δ¹⁸O values remain comparatively constant during the 8 ka shift in δ¹³C values, suggesting that a decrease in precipitation was the major driving factor contributing to this vegetational shift, rather than temperature change. An increase in atmospheric CO₂ may have also contributed.

The largest enrichment of δ¹⁸O values begins at approximately 4 ka, and suggests an increase in mean annual temperature and/or a shift from winter precipitation to summer monsoons. This increase in δ¹⁸O values is coupled with increasing δ¹³C values from approximately 4 to 2 ka. This indicates an increase in C₄ grasses and is reasonable since summer monsoonal precipitation favors their development.

Isotopic signatures from soils younger than 2 ka are rare. However, δ¹³C values from the only profile containing carbonate of this age indicate an increase in aridity and C₃ desert scrub between 2120 and 1550 yr. B.P.. This corresponds to the Fairbanks Drought and the Formative period of cultural evolution. A corresponding shift in 8180 values is not present. The stable isotopic data indicate major vegetational shifts during the latest Pleistocene and Holocene and correspond to periods of eolian erosion and deposition within the Hueco Basin. These processes may have affected both the cultural practices of indigenous people in this area as well as the preservation and stratigraphic integrity of their artifacts.