Characterizing the binders in rock paintings by THM-GC–MS: La Casa de Las Golondrinas, Guatemala, a cautionary tale for radiocarbon dating

Abstract

Chemical characterization of the organic material in rock paintings is an important new direction in directly determining the age of such artifacts. Using plasma-chemical oxidation and accelerator mass spectrometry, radiocarbon dates (6250–5550 cal b.c. and 1500–900 cal b.c.) were obtained for two red-pigmented pictographs from La Casa de Las Golondrinas, the largest recorded rock art site in the Guatemalan Highlands. While the dates fell within or near the 6000-year span of human activity at the site, plasma-chemical oxidation yielded significant amounts of carbon from the unpainted tuff samples that were collected for comparison with the paint. The presence of organic material not related to the paint will render a radiocarbon date irrelevant at best. Qualitative analyses using thermally assisted hydrolysis/methylation (THM)-GC–MS were undertaken to clarify the nature of the organic matter in these paint samples from Guatemala and to determine if any binder could be identified in the paint samples. Results of the analyses show only small differences in composition between the paint samples and unpainted substrate. The older date is not related to a binder, but more likely to humic acids derived from soil organic matter that were not removed by the standard chemical pretreatments; that date, then, should not be considered anthropologically relevant to placing the painting activity in time. The results demonstrate the importance of collecting both appropriate substrate and paint samples for any attempt to date rock art. The THM-GC–MS method we describe has demonstrated excellent potential as a rapid screening method for the comparison of substrate and paint samples to determine which ones have the best chance of yielding a reliable direct radiocarbon date.