Magnesium stable isotope ecology using mammal tooth enamel

Geochemical inferences on ancient diet using bone and enamel apatite rely mainly on carbon isotope ratios (delta C-13) and to a lesser extent on strontium/calcium (Sr/Ca) and barium/calcium (Ba/Ca) elemental ratios. Recent developments in nontraditional stable isotopes provide an unprecedented opportunity to use additional paleodietary proxies to disentangle complex diets such as omnivory. Of particular relevance for paleodietary reconstruction are metals present in large quantity in bone and enamel apatite, providing that biologically mediated fractionation processes are constrained. Calcium isotope ratios (delta Ca-44) meet these criteria but exhibit complex ecological patterning. Stable magnesium isotope ratios (delta Mg-26) also meet these criteria but a comprehensive understanding of its variability awaits new isotopic data. Here, 11 extant mammal species of known ecology from a single locality in equatorial Africa were sampled for tooth enamel and, together with vegetation and feces, analyzed for delta Mg-26, delta C-13, Sr/Ca, and Ba/Ca ratios. The results demonstrate that delta Mg-26 incorporated in tooth enamel becomes heavier from strict herbivores to omnivores/faunivores. Using data from experimentally raised sheep, we suggest that this Mg-26 enrichment up the trophic chain is due to a Mg-26 enrichment in muscle relative to bone. Notably, it is possible to distinguish omnivores from herbivores, using delta Mg-26 coupled to Ba/Ca ratios. The potential effects of metabolic and dietary changes on the enamel delta Mg-26 composition remain to be explored but, in the future, multiproxy approaches would permit a substantial refinement of dietary behaviors or enable accurate trophic reconstruction despite specimen-limited sampling, as is often the case for fossil assemblages.