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, characterised by the presence of palmitic (C16:0) and stearic (C18:0) 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 (= δ13C18:0 - δ13C16:0) 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 excluded from such databases.