Implications for rock art dating from the Lower Pecos Canyonlands, TX: A review

The Lower Pecos Canyonlands was the first study area where the method of plasma oxidation was employed to extract organic material from prehistoric rock paintings for accelerator mass spectrometry radiocarbon dating. During the developmental phase of this method, Rowe's laboratory at Texas A&M University obtained 32 radiocarbon measurements for Lower Pecos rock art: 29 dates for 16 paintings of the Pecos River style, and 3 additional dates for paintings of other styles found within the region. We evaluate these legacy dates based on contextual, compositional, and measurement elements, concluding that these experimental results are problematic and should not be used to draw archaeological conclusions. Building on knowledge gained during the development of the technique, Rowe established field and laboratory methods to address issues impacting the reliability and precision of radiocarbon results. Steelman's laboratory at Shumla Archaeological Research & Education Center has implemented these and additional protocols, including: (1) proper documentation of sampling locations so that the provenience of the sample is known (contextual); (2) analysis of unpainted control background samples to identify the presence or lack of contaminants in the rock substrate (compositional); (3) chemical pretreatment with base to remove any potential humic acid contamination (compositional); and (4) improved laboratory procedures to ensure that laboratory contamination is avoided (measurement). Using this methodology, Steelman's laboratory has obtained eleven radiocarbon results for four rock art sites in the region: 6 dates for Pecos River style paintings; 1 date for a red zigzag painting of another style; and 4 oxalate minimum/maximum ages. Three of these AMS measurements are from a single composition and pass a χ2-test consistent with being coeval. To our knowledge, this data also presents the first minimum, direct, and maximum age for a single pictograph. This review suggests that future dating research in the region will produce a refined chronology for age comparisons between different rock art sites, painting styles, and even sub-styles – adding to our knowledge of the hunter-gatherers who lived in this painted landscape.