Using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (La-Icp-Ms) to Source Shell in Shell-Tempered Pottery: A Pilot Study from North Mississippi

Article excerpt

Ceramic provenance studies usually rely on characterizing the elemental composition of clays. Complicating factors include dealing with temper and amassing background data from potential source areas. Because freshwater mussels incorporate elements into their shells in proportion to the chemical makeup of waterways, hypothetically the drainage of origin could be determined for shell-tempered pottery by chemically analyzing the temper particles. A chemical analysis of whole-shell samples is presented which indicates that separation by drainage is possible. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) provides a rapid, precise, and essentially nondestructive way to analyze shell-temper particles without having to remove them from the sherd. A pilot study employing LA-ICP-MS on shell-tempered ceramics from Lyon's Bluff (22OK520), a late prehistoric/Protohistoric period site in northeast Mississippi, suggests that while some stylistically exotic artifacts appear to be imports, others appear to have been made locally. These results, although preliminary in terms of sourcing ceramics from the site, are useful in introducing the method and for discussing procedural considerations arising from the study.

Introduction

Provenance studies based on the chemical or mineralogical characterization of archaeological materials have a long history in archaeology. Methods commonly employed include X-ray diffraction and X-ray fluorescence (EDXRF), particle-induced X-ray emission (PIXE), and neutron activation analysis (NAA). Each method has some technical limitations or complicating factors, such as accounting for the diluting effects of temper (Neff 1995; Larson et al. 2005:100). Another major problem stems from the nature of the material being tested. Depending on the geological context, clay sources may be homogenized or thoroughly mixed over large areas, making precise sourcing difficult (e.g., Neff, in press; Steponaitis et al. 1996). Although these limitations may be addressed by, for example, focusing on surface slips or pigments rather than the clay matrix (e.g., Neff 2003; Sail et al. 2005; Speakman 2005; Vaughn et al. 2005), the development of new methods and techniques for sourcing pottery and other ceramic artifacts is desirable. In this paper, we present the results of a pilot study in which we first analyze whole-shell samples from a number of sites via microwave digestion and Inductively Coupled Plasma-Mass Spectrometry to demonstrate the potential for sourcing freshwater mussel shell. We then employ Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) in chemically analyzing the sheU in sheU-tempered ceramics from the Lyon's Bluff site (22OK520), a Mississippian/Protohistoric period mound and viUage complex in Oktibbeha County, Mississippi. Because shell-tempered ceramics are a widespread archaeological phenomenon, the LA-ICP-MS method described here should have broad applications. The method is essentially nondestructive and rapid and materials for establishing background sourcing data are widely available. Procedural considerations arising from our work also are discussed.

Chemical Uptake in Aquatic Mollusks

Shell chemical analysis is routinely employed in paleoenvironmental and pollution-monitoring work, based upon the fact that mollusks intake chemicals in general proportion to what is in the environment (Brown et al. 2005; Fuller 1974; Lee and Wilson 1969). Mussels are filter feeders and gill breathers, passing water across the gills via cilial movement and retaining suspended phytoplankton and other materials in a cilial mesh and/or mucus (McMahon 1991). Because freshwater is a dilute medium, mussels are in a constant state of ion flux, replacing lost salts and other ions via "active transport over the gills and other external epithelial surfaces" (McMahon 1991:334). The relationship between water chemistry and shell secretion is not especially well studied (Burky 1983:296-297), but it is understood that dissolved ions of calcium and other elements are taken up both from the water and from digested food; the calcium carbonate shell is secreted in discrete layers by the underlying mantle tissue, with ions of other elements being incorporated (e. …