Karen L. Steelman

Nondestructive radiocarbon dating: Naturally mummified infant bundle from SW Texas

Plasma oxidation was used to obtain radiocarbon dates on six different materials from a naturally mummified baby bundle from the Lower Pecos River region of southwest Texas. This bundle was selected because it was thought to represent a single event and would illustrate the accuracy and precision of the plasma oxidation method. Five of the materials were clearly components of the original bundle with 13 dates combined to yield a weighted average of 2135 ±11 B.P. Six dates from a wooden stick of Desert Ash averaged 939 ± 14 B.P, indicating that this artifact was not part of the original burial. Plasma oxidation is shown to be a virtually nondestructive alternative to combustion. Because only sub-milligram amounts of material are removed from an artifact over its exposed surface, no visible change in fragile materials has been observed, even under magnification. The method is best applied when natural organic contamination is unlikely and serious consideration of this issue is needed in all cases. If organic contamination is present, it will have to be removed before plasma oxidation to obtain accurate radiocarbon dates.

AMS Dates from Four Late Prehistoric Period Rock Art Sites in West Central Montana

Abstract In 2002, eight pigment samples were collected from three rock art sites in the Big Belt Mountains of west central Montana. Samples from Hellgate Gulch (24BW9), Avalanche Mouth (24BW19), and the Gates of the Mountains (24LC27) were dated using plasma-chemical extraction and accelerator mass spectrometry. The dates were statistically indistinguishable with ages of 11701 ± 45, 1225 ± 50, and 1280 1 ± 50 B.P. When calibrated, these ages range from 650 to 990 cal A.D. This corresponds to the early Late Prehistoric period on the Northwestern Plains. An oxalate accretion sample overlying a painted area at another site, Big Log Gulch (24LCI707), provided a minimum age of 1440 ± 45 B.P. for the rock art present at this site. The dated images at the four sites fit within the Foothills Abstract and Eastern Columbia Plateau rock art traditions.

Strategies for 14 C Dating the Oxtotitlán Cave Paintings, Guerrero, Mexico

ABSTRACT Oxtotitlán Cave paintings have been considered among the earliest in Mesoamerica on stylistic grounds, but confirmation of this hypothesis through absolute dating has not been attempted until now. We describe the application of advanced radiocarbon strategies developed for situations such as caves with high carbon backgrounds. Using a low-temperature plasma oxidation system, we dated both the ancient paint and the biogenic rock coatings that cover the paint layers at Oxtotitlán. Our research has significantly expanded the time frame for the production of polychrome rock paintings encompassing the Early Formative and Late Formative/Early Classic periods, statistically spanning a long era from before ca. 1500 cal B.C. to cal A.D. 600. Los murales de la Cueva de Oxtotitlán, de acuerdo con criterios estilísticos, han sido considerados entre los más tempranos de Mesoamérica. Sin embargo, hasta la fecha esta hipótesis no había sido corroborada mediante fechamiento absoluto. En este trabajo se describe la aplicación de técnicas de radiocarbono avanzadas, las cuales han sido desarrolladas para lugares como cuevas con un elevado fondo de carbón. Fechamos tanto la pintura antigua como los recubrimientos biogénicos que cubren las capas de pintura utilizando un sistema de oxidación de plasma a temperatura baja. Nuestras investigaciones han ampliado de manera significativa el intervalo temporal de la pintura mural policroma en Mesoamérica, abarcando los periodos del Formativo Temprano al Formativo Tardío/Clásico Temprano, desde antes de aproximadamente 1500 cal a.C. hasta 600 cal d.C.