Abstract
[Research Article]Bedrock features represent various economic, social, and symbolic aspects of past societies, but have historically received little study, particularly in North America. Fortunately, new techniques for analyzing spatial configurations, use-wear, and organic residues are beginning to unlock more of the interpretive potential of these features. Though preliminary in nature, the present study contributes to this trend by documenting an application of lipid analysis to bedrock features in a dry rockshelter. Results of this initial application indicate that bedrock features in dry rockshelters may provide especially favorable conditions for the preservation and interpretation of ancient organic residues. Abundant lipids, comparable to concentrations present in some pottery sherds, were extracted from a bedrock grinding surface at Gila Cliff Dwellings National Monument and analyzed using gas chromatography-mass spectrometry. Though the lipids were highly oxidized, degradation products indicative of former unsaturated fatty acids were retained. Comparisons to experimentally aged residues, and absence of a known biomarker for maize, indicate that the bulk of the lipids preserved in the milling surface probably derive from processing an oily nut or seed resource, and not from processing maize. Substantially lower amounts of lipids were recovered from a small, blackened cupule. It is hypothesized that some portion of the lipids in the blackened cupule was deposited from condensed smoke of cooking and heating fires in the caves. Potential for the preservation of organic residues in similar sheltered bedrock contexts is discussed, and a practical method for sampling bedrock features in the field is described.
Highlights
For several decades, lipid analysis via gas chromatography-mass spectrometry (GC-MS) has helped identify sources of organic residues absorbed in the fabric of ancient pottery (Condamin et al 1976; Evershed et al 1992; Evershed 2008b)
The core sample taken from the grinding surface on Rock Modification Panel (RMP)-33 had 278μg g-1 of fatty acids, approximately 85 times the fatty acid concentration detected in the corresponding control sample
It is worth noting that this appreciable concentration of fatty acids is in the range of those recovered from pottery sherds that have been used to directly date the use of some pottery vessels (Stott et al 2001; Stott et al 2003; Berstan et al 2008)
Summary
Lipid analysis via gas chromatography-mass spectrometry (GC-MS) has helped identify sources of organic residues absorbed in the fabric of ancient pottery (Condamin et al 1976; Evershed et al 1992; Evershed 2008b). I discuss a recent study of lipids present in bedrock features at Gila Cliff Dwellings National Monument. These may be the first samples of bedrock features from dry caves to be analyzed for lipid content, as similar applications were not encountered during a literature search. Sampling for this initial application was limited, and was intended to provide preliminary information to see if a larger study might be merited. Results presented here suggest that sheltered bedrock milling features in arid settings could provide especially favorable contexts for the analysis of lipids or other organic residues
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