The interpretability of stable hydrogen isotopes in modern herbivore tooth enamel

Stable hydrogen isotopes (delta D values) in animal organic tissues such as collagen and keratin typically correlate with local meteoric water, but little is known about delta D in tooth enamel apatite. In this study, we analyzed comminuted tooth enamel for oxygen isotopes (delta O-18) of the CO3 component and for delta D of bulk enamel. We find positive correlations between enamel delta D and delta O-18 (R-2 = 0.70) and between enamel delta D and local precipitation delta D (R-2 = 0.53). However, the slopes of these relationships are much shallower (less variation in tooth enamel delta D) than expected from studies of other tissues. Based on mass spectrometric peak areas, H-2 contents for enamel are 2-5 times higher than expected from chemical compositions, and we interpret as much as 50-90% of measured hydrogen from tooth enamel may be adsorbed water derived from laboratory water vapor. We tested this hypothesis by equilibrating tooth enamel with very high and very low delta D values of water vapor, then exposing to laboratory air for different periods of time ranging from minutes to 8 hours. These experiments show that the apparent delta D value of enamel converges to a nearly constant delta D value in 1 to 2 hours. The large amount of adsorbed water and rapid approach to equilibrium will make it difficult to infer provenance from delta D measurements alone, or to reproduce measured tooth enamel delta D values among laboratories with different water vapor compositions. Heating at 70 degrees C for 48 hours in air does not remove adsorbed hydrogen, but does reduce delta D values by c. 10 parts per thousand compared to unheated samples. Differences in delta D values for heated vs. unheated enamel may reflect either a different, temperature-dependent partition coefficient between adsorbed water (on apatite) and water vapor, exchange of structural H at elevated temperatures, or subtle changes to crystal structure, such as loss of structural H. (C) 2019 Elsevier Ltd. All rights reserved.