Compound-Specific Stable Carbon Isotopic Detection of Pig Product Processing in British Late Neolithic Pottery
Mukherjee, Anna J., Berstan, Robert, Copley, Mark S., Gibson, Alex M., Evershed, Richard P., Antiquity
By extracting lipids from potsherds and determining the [[delta][sup.13.C]] of the most abundant fatty acids, degraded fats from ruminant animals, such as cattle, and non-ruminant animals, such as pigs, can be distinguished. The authors use this phenomenon to investigate Late Neolithic pig exploitation and find that the pig 'signature' was more frequently found among residues from Grooved Ware than other prehistoric pottery types.
Keywords: Neolithic, Britain, archaeological chemistry, fatty acids, Grooved Ware, pigs, residues, ruminants, TAGs
It is now well established that unglazed pottery vessels possess the capacity to absorb substantial concentrations of fats, oils and waxes (i.e. lipids) during the processing of animal and plant products (Beck et al. 1989; Evershed et al. 1991; 1992; 1994; 2002; Charters et al. 1993; 1995; 1997; Dudd & Evershed 1998; Mottram et al. 1999; Evershed 2000; Copley et al. 2001a; 2001b; 2003; 2005a; 2005b; 2005c; 2005d; 2005e; Hansel et al. 2008; Reber et al. 2004). Significantly, degraded animal fats are the most commonly observed components, raising a number of possibilities for using absorbed residues in pottery as an alternative source of information relating to the exploitation of both domestic and wild species (Evershed et al. 1999; Copley et al. 2003; 2005d; Mukherjee et al. 2005). However, while degraded animal fats are readily detected, the animal species from which they derive is more difficult to determine. The chromatograms displayed in Figure 1 show how degraded ruminant adipose, dairy and porcine adipose fats resemble each other chemically.
Triacylglycerols (TAGs) are the major constituents of flesh animal fats and their distributions can be diagnostic to species level. Studies of modern animal fats have shown that bovine adipose fats contain saturated TAGs of total acyl carbon numbers between [C.sub.42] and [C.sub.54], similarly, ovine adipose fats contain TAGs ranging from [C.sub.44] (trace) to [C.sub.54]. Artificially degraded dairy fats have a characteristically wide TAG distribution ([C.sub.40]-[C.sub.54]), whereas for porcine fats it is 'narrow' ([C.sub.44]-[C.sub.54] but with very low abundances of [C.sub.44], [C.sub.46] and [C.sub.54]; Dudd 1999). Recognisable distributions have been found to persist in animal fats extracted from archaeological potsherds (Figure 2; Dudd & Evershed 1998; Kimpe et al. 2001; 2002), although there are differences in the abundances of individual TAGs between modern and ancient animal fats (Figure 2). This is probably due to alteration of the more chemically labile and water soluble/volatile constituents taking place during cooking and degradation (Dudd 1999). Burial conditions at the vast majority of sites are not conducive to TAG preservation and over archaeological timescales they become hydrolysed to form di- and monoacylglycerols (DAGs and MAGs) and free fatty acids, with the latter usually becoming the major constituents of lipid residues. Hence, alternative means of animal fat identification have to be sought.
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A more reliable method for distinguishing degraded animal fats is compound-specific stable carbon isotope analysis to determine stable carbon isotope ratios [[delta][sup.13.C]] values; Evershed et al. 1994; 1997; 2002; 2003; Mottram et al. 1999) of individual fatty acids. We have shown that while the original TAGs are hydrolysed over time the [[delta][sup.13.C] values of their constituent free fatty acids are not affected significantly by degradation (Evershed et al. 1999). The scatter plot presented in Figure 3 shows [[delta][sup.13.C]] values of [C.sub.16:0] and [C.sub.18:0] fatty acids of modern reference animal fats (bovine, ovine and porcine adipose and bovine milk) from animals raised on isotopically comparable diets to those that would have been available to domesticated animals in antiquity (Dudd & Evershed 1998; Copley et al. …