Impact of modern cattle feeding practices on milk fatty acid stable carbon isotope compositions emphasise the need for caution in selecting reference animal tissues and products for archaeological investigations

Degraded animal fats, characterized by the presence of palmitic (C_16:0) and stearic (C_18:0) fatty acids and related glycerolipids are the most common class of preserved lipids in organic residues trapped in the porous clay matrix of archaeological ceramic vessels. The ubiquitous presence of fatty acids in animal fats and plant oils precludes identification of fat types by the solely molecular composition of residues. Hence, animal fats are identified by determining their fatty acyl lipid distributions and stable carbon (δ¹³C) values allowing distinctions to be drawn between non-ruminant and ruminant, and dairy and adipose fats. The Δ¹³C proxy (= δ¹³C_18:0 - δ¹³C_16:0) originally proposed in the 1990s by Evershed and co-workers was based on modern reference fats sampled from animals raised in Britain on C₃ plant diets. Further analyses on adipose and dairy fats from ruminants grazing in a wide range of isoscapes have shown that the Δ¹³C proxy can be applied in mixed C₃/C₄ environments, such as in Africa. Here we show, however, through the investigation of milk fats, how the Δ¹³C proxy can be perturbed when animals are reared on modern diets, specifically maize silage. It is thus shown that extreme care has to be taken when choosing modern reference fats for archaeological studies, and especially that insecurely-sourced animal fats should be precluded from such databases.
Keywords: Modern reference materials, animal fats, lipid residue analyses, dairy fats, silage