Direct Detection of Maize in Pottery Residues Via Compound Specific Stable Carbon Isotope Analysis

Article excerpt

Introduction

Maize was domesticated in the Valley of Mexico, and became an important part of North American diets around 1000 cal AD (Doebley, J. 1990; Van der Merwe & Vogel 1978; Vogel & van der Merwe 1977). By the time of European Contact (c. 1500 AD) maize was the primary staple food in North America, comprising up to 90 per cent of the diet of some native cultures (Broida 1984; Larsen 2000; Lynott et al. 1986; van der Merwe & Vogel 1978; Wagner 1986). The rapid adoption of this productive crop has been demonstrated by stable carbon isotope analysis of human skeletal remains. Since maize is a [C.sub.4] tropical grass, its [[delta].sup.13]C value is distinctively more positive than the [C.sub.3] temperate grasses that dominate the ecosystem of North America east of the Great Plains (Buikstra & Milner 1991; Lambert et al. 1979; Larsen et al. 1992; Lynott et al. 1986; van der Merwe & Vogel 1978; Vogel & van der Merwe 1977). Such isotopic differences are passed up the food chain and may be detected in human skeletal remains. Stable carbon isotope analysis demonstrated that extensive maize adoption was contemporaneous with dramatic increases in population, social stratification, and urbanisation in North America (Buikstra et al. 1987; Fritz 1992; Kelly 1992; Rindos & Johannessen 1991; van der Merwe & Vogel 1978; Voigt 1986).

However, direct detection of maize in the pottery in which it was cooked permits study of how maize was processed, what other foodstuffs it was processed with, and which parts of the population were cooking the most maize, when and where. Such detailed examples in context should help to determine how or why this dietary shift occurred. Direct detection of maize in pottery can be accomplished by targeting compounds, e.g. lipids, which are characteristic of maize and have the potential to survive as components of organic residues absorbed into the fabric of pottery vessels.

Absorbed organic residues

Absorbed organic residues comprise complex mixtures of compounds released from foods or other organic commodities processed in vessels, which become absorbed within the walls of ceramic vessels during the cooking process. As such, they are potentially valuable sources of information on ancient diet. Once compounds are absorbed within the walls various physicochemical processes preserve the lipids for many millennia (Evershed 1993; Evershed & Charters 1995; Evershed et al. 1999; Evershed et al. 1992a; Evershed et al. 1992b; Evershed et al. 1990; Heron & Evershed 1993; Heron et al. 1989). The compounds that survive can be extracted from potsherds and identified through gas chromatography/mass spectrometry (GC/MS).

Interpretation of residue compositions

Interpretation of lipids that originate from complex mixtures of ancient foodstuffs is problematic. Correlation of fatty acid composition was the first method used to identify contents of ancient vessels (Condamin & Formenti 1978; Condamin et al. 1976). This approach may be valid in some cases, but fatty acid compositions do not provide a generally applicable method of residue interpretation for two reasons. Firstly, most ceramic vessels would have contained more than one foodstuff during their use-life; the fatty acid compositions of such mixtures can combine to mimic the composition of a single food that was not present in the residue. Secondly, unsaturated and short-chain fatty acids degrade more quickly during deposition and burial than saturated, long-chain fatty acids, thus changing fatty acid compositions over time (Dudd & Evershed 1998, Reber & Evershed 2004). Although degradation can be modelled (Malainey et al. 1999; Malainey et al. 2001), environmental variations between sites are such that fatty acid ratios can be used to determine only generalised categories, such as primarily meat or primarily plant/fish. The fatty acid composition of maize is largely limited to oleic acid ([C. …