FEASIBILITY OF MEASURING STRUCTURAL STABLE H ISOTOPES IN TOOTH ENAMEL BIOAPATITE VIA IN-VACUO, STEP HEATING DEHYDRATION-DEHYDROXYLATION EXPERIMENTS

Stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotopes in bioapatite are frequently used for paleoclimate and paleoenvironmental reconstructions. Despite the prevalence of research on stable hydrogen (δD) isotopes from animal organic tissues such as collagen and keratin, δD in bioapatite has only been minimally investigated. This reflects several factors which may skew the resulting δD values: (1) a sometimes high concentration of hydrogen-bearing organic inclusions, (2) strongly surface-bound waters which occur in geologically young bioapatite, (3) achieving controlled and separated breakdown of the hydrogen-bearing moieties of lattice-bound water versus (4) structural hydroxyl groups which co-exist in bioapatites. This study aims to understand the thermal properties of the hydroxyl-group (OH) in apatite during in-vacuo extraction experiments and thereby evaluate if there is a methodology which enables the use of bioapatite δD as a paleoenvironmental proxy.

We present preliminary results of step heating dehydration-dehydroxylation experiments of both biologic and geologic apatite samples which were conducted via high-vacuum extraction lines. There is a correlation between thermal and chemical stability, and dehydration-dehydroxylation helps to better understand temperature effects on the crystal structure, and the breakdown of its hydrogen-bearing components. Highly exchangeable hydrogen-bearing materials require less thermal energy to be released from the bioapatite crystal (factors 1, 2 above). The crystal-bound hydrogen (factors 3, 4 above) released at higher temperatures have lower levels of contamination from factors 1 & 2. We evaluate whether non-crystalline components can be removed at lower temperatures, and if the higher-temperature extraction experiments provide a reasonably close approximation of the original, environmental record of bioapatite δD values.