ENAMEL MATURATION RESETS APPOSITIONAL ISOTOPE COMPOSITIONS
Enamel formation occurs in two stages with different geometries and CO3/PO4 ratios. The first stage (apposition) precipitates enamel with a high CO3/PO4 ratio at a low angle to the dentine-enamel junction. The second stage (maturation) precipitates enamel with a lower CO3/PO4 ratio at a high angle. Because CO3 and PO4 are deposited in different ratios during the two stages the resulting bulk isotopic composition for each component should be weighted differently. For example, if apposition and maturation both contribute isotope compositions (with different CO3/PO4 ratios), and if isotope compositions vary seasonally, δ18OC isotope patterns should be shifted earlier seasonally relative to δ18OP.
We collected paired δ18OC and δ18OP zoning patterns from several ungulate teeth (Bison, Capra, Cervus, Equus; 86 subsamples across 5 teeth) to test whether CO3 isotopes are phase shifted relative to paired analyses of PO4 isotopes. These teeth display moderate to strong zoning in both δ18OC and δ18OP values. Cross correlations between δ18OC and δ18OP values show no detectable lag between the two components (R2 = 0.80 to 0.95). Similar comparisons with published data show no resolvable lag. The lack of phase shifts in strongly zoned teeth suggests that enamel mineral re-equilibrates during maturation, removing the contribution of apposition from the final isotope composition. Because enamel records the isotope composition of maturation, not apposition, sampling procedures should preferentially mimic the geometry of maturation, not apposition. This also suggests that the preservation of apposition-stage incremental features (i.e. striae of Retzius) in mature enamel reflect relic pseudomorphic textures.