IN THE FOOTPRINTS OF KIRK BRYAN AND CLAUDE C. ALBRITTON, JR.: A REINVESTIGATION OF SOILS AS EVIDENCE OF LATE-QUATERNARY CLIMATE CHANGE IN THE BIG BEND REGION OF SOUTHWESTERN TEXAS

During the 1930s and 40s, Kirk Bryan and Claude Albritton defined the stratigraphy of late-Quaternary alluvial fills in the Chihuahuan Desert of the Big Bend region, southwest Texas. An outcome of that work was a benchmark paper, “Soil phenomena as evidence of climatic changes,” demonstrating the value of paleosols as indicators of climatic change that helped set the stage for the use of soil science in Quaternary studies focused on climate.

Here, we present the results of our soil-stratigraphic investigations at localities studied by Bryan and Albritton, plus several new sites. Macromorphology, micromorphology, and physical and chemical properties of buried soils were analyzed. Also, δ¹³C analysis of soil organic matter was used to reconstruct late-Quaternary bioclimatic change that drove landscape evolution.

Four lithologically distinct late-Quaternary alluvial fills occur in the study area: the Neville, Calamity, Terlingua, and Kokernot members. Aggradation of the Neville occurred ca. 14-7 ka. Paleosols in the Neville have truncated, well-developed Bk horizons with rubified matrix colors, stage II carbonate morphology (CM), and 6-15% CaCO₃ equivalent (CCE). Aggradation of the Calamity began soon after ca. 7 ka and continued until ca. 5 ka. Buried soils sometimes occur within the Calamity but are weakly developed and not widely traceable. A soil with a truncated, moderately expressed Bk-BCk profile, brown matrix colors, stage I CM, and CCE values of 5-8% is developed at the top of the Calamity. The Terlingua slowly aggraded ca. 3.1-0.5 ka and has a cumulic A horizon that spans the entire thickness of the unit; there is stage 0-I CM and CCE values are <6%. The Kokernot aggraded ca. 0.5-0.1 ka and has surface soils with thin A horizons grading to stratified alluvium.

δ¹³C values indicate a C₃-dominated plant community at the onset of the Younger Dryas, but a shift towards a C₄-dominated community occurred by ca. 12.5 ka and continued through the mid-Holocene along with warming and increased aridity. The soils and δ¹³C values indicate that the northern Chihuahuan Desert was a C₄ grassland ca. 3.1-0.5 ka with greater effective annual precipitation compared to ca. 7-5 ka. After ca. 0.5 ka, overgrazing transformed the grassland into a shrubby desert. Our findings generally support Bryan and Albritton’s field interpretations.