ENAMEL MATURATION RESETS APPOSITIONAL ISOTOPE COMPOSITIONS

Mammalian tooth enamel records isotopic variations reflecting seasonal variations in an animal’s body water composition (which correlates with local water compositions) and diet. Past work has interpreted oxygen isotope (δ¹⁸O) variations in the phosphate (PO₄) and carbonate (CO₃) components of enamel without considering that isotopic attenuation during enamel formation may affect each component differently.

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, δ¹⁸O_C isotope patterns should be shifted earlier seasonally relative to δ¹⁸O_P.

We collected paired δ¹⁸O_C and δ¹⁸O_P 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 δ¹⁸O_C and δ¹⁸O_P values. Cross correlations between δ¹⁸O_C and δ¹⁸O_P 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.