Assessing diagenetic stability of enamel bioapatite oxygen isotope compositions by alteration in isotopically enriched tracer solution

Oxygen isotope compositions in enamel bioapatite are considered as robust proxy archives for ingested drinking water, body temperature and physiology of fossil vertebrates. However, only few alteration experiments have tested this assumption.

Here we present the results of alteration experiments of dental tissues. Three mm-sized dental cubes, comprising both enamel and dentine, were cut from a modern African elephant molar and placed into acidic aqueous solution highly enriched in 18O (3.43 mols/mol%) and were reacted in sealed Teflon vials at temperatures of 30 to 90 °C for up to two months. The δ18OSIMS distribution across the reacted dental cubes were measured in-situ to determine the degree of bioapatite alteration via profiles with step distance as short as 20 μm.

In the 30 °C experiments only a few permil enrichment in δ18OSIMS occurred at the outer and inner enamel rim. In contrast, for 90 °C experiments enamel was strongly enriched (≈ 100 to 600 ‰) throughout the complete enamel transect, increasing with experimental duration. The 18O-enriched tracer solution penetrated the enamel and exchanged significant amounts of oxygen with all three different oxygen-bearing moieties PO4, CO3 and OH. Thus, even mm-thick mammalian enamel is susceptible to pervasive oxygen isotope alteration, including the PO4-group, which is generally considered to be robust against abiogenic low temperature oxygen isotope exchange. This has implications for the reliability of enamel δ18O values as palaeoclimatic and palaeoecologic proxy.