Herbivore tooth oxygen isotope compositions: Effects of diet and physiology

Abstract

The applicability of rapid and precise laser probe analysis of tooth enamel for δ 18O has been verified, and the method has been applied to different modern herbivores in East Africa. Sampling and pretreatment procedures involve initial bleaching and grinding of enamel to <75 μm, and elimination of adsorbed water and organic compounds with BrF 5. Typical analytical reproducibilities for 0.5–2 mg samples are ±0.08‰ (± 1σ). Chemical and spectroscopic characterization of pretreated but unanalyzed samples show no alteration compared to fresh enamel. Solid reaction products are nearly pure CaF 2 with little evidence for residual O 2. Because laser probe fluorination extracts oxygen from all sites in the apatite structure (phosphate, structural carbonate, and hydroxyl), only unaltered tooth enamel (>95% apatite) can be analyzed reliably. Different East African herbivores exhibit previously unsuspected compositional differences. Average enamel δ 18O values (V-SMOW) are approximately: 25‰ (goat), 27‰ (oryx), 28‰ (dikdik and zebra), 29‰ (topi), 30‰ (gerenuk), and 32‰ (gazelle). These compositions differ from generalized theoretical models, but are broadly consistent with expected isotope effects associated with differences in how much each animal (a) drinks, (b) eats C3 vs. C4 plants, and (c) pants vs. sweats. Consideration of diet, water turnover, and animal physiology will allow the most accurate interpretation of ancient teeth and targeting of environmentally-sensitive animals in paleoclimate studies.