Establishing the potential of Ca isotopes as proxy for consumption of dairy products

Abstract

A procedure has been developed which allows precise determination of Ca isotope ratios in natural and organic samples such as bones, milk and other biological materials. In this study the procedure is used to determine Ca isotope ratios in modern dietary systems and to establish the potential of Ca isotopes as a paleodiet tracer by analysis of bones. Multi-sampling across a 5 cm portion of a red deer jawbone shows invariant Ca isotope ratios and suggests negligible isotopic effect during bone remodelling. The difference between Ca isotopes in red deer diet and bones from one location was 0.65‰, in agreement with a previous study of diet/bone offsets. Similar values for modern deer-bone δ 44/42Ca from four geographically diverse populations demonstrate that geological/environmental conditions do not cause large variability and suggest that diet is the major cause for variations in bone δ 44/42Ca. δ 44/42Ca of herbivore milk is found to be ≈0.5 to 0.6 higher than the corresponding diet. Modern human milk has a δ 44/42Ca of −1.15 ( n = 4) and is isotopically the lightest material reported in this study. This suggests that, for these samples, a significant portion of Ca intake was from dairy sources, and that human milk has Ca which is, again, ≈0.6‰ isotopically lighter than dietary Ca intake. Finally, Ca isotope ratios are presented from a variety of samples formed during fermentation processes (e.g., curds, whey, etc.) which indicate that these processes do not fractionate Ca isotopes significantly. Together, the data in this paper indicate that, because milk is an important dietary source of Ca with a distinctive signature, Ca isotope ratios should provide a tracer for past dairy consumption. A simplified model is outlined to demonstrate the ability to quantify dairy consumption by the analysis of Ca isotopes in bones.