Olive oil or lard?: Distinguishing plant oils from animal fats in the archeological record of the eastern Mediterranean using gas chromatography/combustion/isotope ratio mass spectrometry

Distinguishing animal fats from plant oils in archaeological residues is not straightforward. Characteristic plant sterols, such as beta-sitosterol, are often missing in archaeological samples and specific biomarkers do not exist for most plant fats. Identification is usually based on a range of characteristics such as fatty acid ratios, all of which indicate that a plant oil may be present, none of which uniquely distinguish plant oils from other fats. Degradation and dissolution during burial alter fatty acid ratios and remove short-chain fatty acids, resulting in degraded plant oils with similar fatty acid profiles to other degraded fats. Compound-specific stable isotope analysis of delta C-13(18:0) and delta C-13(16:0), carried out by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS), has provided a means of distinguishing fish oils, dairy fats, ruminant and non-ruminant adipose fats, but plant oils are rarely included in these analyses. For modern plant oils where C-18:1 is abundant, delta C-13(18:1) and delta C-13(16:0) are usually measured. These results cannot be compared with archaeological data or data from other modern reference fats where delta C-13(18:0) and delta C-13(16:0) are measured, as C-18:0 and C-18:1 are formed by different processes resulting in different isotopic values. Eight samples of six modern plant oils were saponified, releasing sufficient C-18:0 to measure the isotopic values, which were plotted against delta C-13(16:0). The isotopic values for these oils, with one exception, formed a tight cluster between ruminant and non-ruminant animal fats. This result complicates the interpretation of mixed fatty residues in geographical areas where both animal fats and plant oils were in use.