Fig. 3.

(A) Schematic box model of Mg isotopes in the body. For a given diet δ²⁶Mg value, enamel is fractionated by +0.3‰, but the bone δ²⁶Mg value remains unchanged because the fractionation between enamel and bone equals approximately –0.3‰ (15). The feces δ²⁶Mg value is fractionated by –0.6‰ relative to diet, leaving the body isotopically unbalanced. Muscle (30% of body Mg) is a likely candidate to be the complementary ²⁶Mg-enriched reservoir. (B) According to the box model, the δ²⁶Mg value of muscle must be higher than that of bone. Measurements of δ²⁶Mg in muscle and bone in four sheep confirm this hypothesis.