When an archaeological sample is radiocarbon dated, it typically undergoes three separate steps: (1) chemical pretreatment to remove contamination or isolation of sample-specific chemical compounds, (2) conversion to a measurable form, and (3) measurement of 14C to determine the age. This current study demonstrates that step 2, conversion to a measurable form, is a virtually nondestructive process using plasma oxidation. Combustion has been the traditional method for converting a sample to carbon dioxide, which is then reduced to graphite for accelerator
mass spectrometry (AMS). Because chemical pretreatment and combustion are destructive, small samples are removed from artifacts for radiocarbon dating.
As an alternative to combustion, plasma oxidation is a virtually nondestructive method for converting solid organic carbon to carbon dioxide. An oxygen plasma, electrically excited oxygen gas, gently removes organic carbon from the surface of a whole artifact at low temperatures (-500C). Since
only 100 pg of carbon is needed for AMS measurement, the effect upon an artifact is negligible. Plasma oxidation has the potential to resolve one of the major problems facing archaeologists working with rare, unique, or sacred objects. The need to place these artifacts in a secure chronological context is often offset by a reluctance to destroy even the small part that must be combusted for current dating methods. The preservation of normally perishable organic material in the dry rock shelters of southwest Texas provided an opportunity to test the method on a suite of materials that were thought to be from a single event-the mortuary furnishings of an infant grave. Using plasma oxidation, along with traditional combustion, we have now determined 19 radiocarbon dates on six materials
from an infant burial bundle found in a dry shelter in the Lower Pecos River region of southwest Texas. Two clusters of AMS dates demonstrate that the process clearly differentiated between artifacts of different ages and that the results were replicable.
Plasma Oxidation
Plasma-chemical extraction was originally used by our chemistry laboratory at Texas A&M University to radiocarbon date ancient rock paintings (e.g., see Rowe 2001 and references therein; Russ et al. 1990). The primary advantage of this process is that the low temperatures of the plasma gas (50-150’C depending upon our operating conditions) are
below the decomposition temperatures of carbon-containing minerals. This allows the plasma to selectively remove only organic carbon from a sample and leave accompanying carbonates and oxalates intact (Chaffee et al. 1994; Russ et al. 1992).
More recently, we employed plasma oxidation on perishable organic materials to obtain nondestructive radiocarbon dates (Steelman and Rowe 2002, 2003). Low-temperature (-50-150oC) oxygen plasmas, electrically excited ionized gas, were used to extract sub-milligram amounts of organic carbon from surfaces of archaeological artifacts for
radiocarbon dating. It is possible to collect multiple aliquots of carbon dioxide from a single material for replicate accelerator mass spectrometry (AMS) and carbon stable isotope measurements without any observable change in an artifact’s appearance. In contrast, when combustion is used to collect carbon for dating, milligram-size or often
larger samples must be physically removed from an artifact and destroyed.
In some cases, it is undesirable to remove even a small portion of an artifact for destructive analysis. Nondestructive plasma oxidation should be employed when: (1) the amount of material needed for combustion is a large fraction of artifact size, (2) the information content of sample structure should be preserved, (3) an object is rare or has
intrinsic value, and (4) an artifact is a sensitive item that might not otherwise be dated. Plasma oxidation may also be preferable for any type of sample containing significant amounts of carbon-containing minerals. Two previous studies found that in unusual circumstances, acid treatment is insufficient to remove offending calcium oxalate minerals (Armitage et al. 2001; Hedges et al. 1998). The characteristics of plasma extraction have great potential for the study of artifacts associated with burials, where the least intrusive techniques are especially critical. Museum artifacts such as basketry, textiles, cordage/twine, and paleobotanical samples would also benefit from this application of the plasma oxidation process.
Naturally Mummified Infant Bundle All samples used in this study were taken from a bundled infant burial that was unsystematically removed from a dry cave on the Pecos River in the 1950s (Figure 1). After 40 years of storage in a
closed compartment, the bundle was brought to the attention of professional archeologists in 1989 by a local resident who described the original context of the burial. He provided a number of details about the find and pointed to the location of the cave on a USGS topographic map. To him, this was the old Marcos place, but it had been registered in the state trinomial system as Hinds Cave, 41VV456.
Texas A&M University carried out excavations at Hinds Cave for years, contributing several master’s theses and doctoral dissertations on environmental data gleaned from fauna, flora, and coprolites (Dering 1979; Lord 1984; Stock 1983; Williams-Dean 1978; Woltz 1998). An NSF report
on Hinds Cave was submitted by Shafer and Bryant (1977). Others have also studied various aspects of the archaeology of the cave (Andrews and Adovasio 1980; Bement and Turpin 1987). No burials were reported from the Texas A&M excavations, but many of the details provided by the donor of the infant burial bundle are consistent with the lay.
out of the Hinds Cave site. The exact provenience of the baby burial is not critical to the current project since it is indisputable that the bundle came from a dry cave on the Pecos River and therefore belongs within the cultural matrix devised from over seven decades of archeological research (e.g., Shafer 1986; Turpin 1991). Cultural Context of Bundled Burial The cultural chronology of the Lower Pecos region generally follows the quadripartite sequence used throughout Texas, but hundreds of radiocarbon dates generated by excavations permit the subdivision of these long periods into more precise subperiods (Turpin 1991).
Although very few burials have been dated, it can be assumed that the practice of bundle burial in dry rock shelter deposits is predominantly a Late Archaic trait and more specifically one attributable to the Flanders-Blue Hills subperiods (2300-1300 B.P.). These periods follow immediately after a mesic interlude (Cibola Subperiod) in which the Great Plain’s habitat expanded into the Lower Pecos region for a few hundred years. The resumption of a general trend toward aridity that had been in effect since the end of the Pleistocene presaged the return to a desert-adapted economic strategy and settlement pattern, possibly by an influx of people from northern Mexico. Rock shelter occupation regained favor arid the dead were often buried in the refuse left by the living. The dry climate and sheltered environment contributed to the excellent preservation of perishables and occasionally led to natural mummification, especially of infants such as the one selected for this study. Bundled Infant Burials Despite the irregularities in their collection, the baby burials of the Lower Pecos region share certain similarities that are indicative of a structured mortuary pattern, at least insofar as evidenced by interments in dry rock shelters.
With few exceptions, the infant corpse is wrapped in a skin shroud, such as a rabbit fur robe, coyote skin, antelope hide, fawnskin, or unidentified membrane, or in a finely woven mat (Banks and Rutenberg 1982; Butler 1948; Martin 1933; Pearce and Jackson 1933; Turpin 1988:Table 3). This bundle is then placed in some form of nest or cradle. Examples are pillows of grass held in place by wooden stakes, a pit lined with twigs, a finely woven basket or net, a crude cradle made of crossed sticks, or their broken cradleboards. Next, up to three mats are placed over or wrapped around the bundle. The sequence invariably begins with the most finely woven mat on the interior and the most coarse on the exterior. The interior mats are often small and intact, suggesting they were specially made for the baby, but the exterior coarser checkerweaves can be folded into segments of larger pieces. The entire burial is then usually covered with a metate, limestone slab, or in one instance, a large stick that held the bundle down. Occasionally, exotic offerings are included in the bundle; examples are woven fiber artifacts decorated with red designs, an olivella shell necklace with coyote teeth pendants, mussel shell plaques, and human hair cords or ropes.
