1Department of Anthropology, The Graduate Center, City University of New York, 2The New York Consortium in Evolutionary Primatology, NYCEP, 3Department of Biology, University of Utah, 4Department of Geology and Geophysics, University of Utah, 5Department of Biomaterials and Biomimetics, New York University College of Dentistry, 6WiscSIMS, Department of Geoscience, University of Wisconsin-Madison
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Stable carbon and oxygen isotopes in vertebrate tooth enamel are commonly used to understand the diets and environments of modern and fossil animals. Isotope variation within the lifetime of individual animals can be recovered by microsampling enamel along the direction of growth. Intra-tooth oxygen isotope profiles can be used to reconstruct seasonal changes in environmental parameters and document paleoseasonality in the human fossil record. However, conventional sampling of the enamel surface provides highly time-averaged records. The innermost enamel layer, about 10 μm in width, rapidly mineralizes after formation and may record less attenuated signals. We used secondary ion mass spectrometry (SIMS) to sample with approximately 4 μm spots the innermost enamel of a woodrat incisor from an animal included in a controlled water-switch experiment. SIMS oxygen isotope values are compared with values obtained from the enamel surface and breath CO2,a proxy for body water and the primary input signal. The range in isotope values from the enamel surface (12.1‰) was reduced compared to breath (16.0‰). The innermost enamel profile captured the full shift (15.9‰), and exhibited more variability than the conventional profile. Because rodent incisors mineralize rapidly, the amplitude of the surface enamel profile was already similar (75%) to the primary signal. Signal resolution increased to 99% when sampling the innermost enamel layer. However, an increase of 24% in signal amplitude would be negligible in teeth recording natural environmental variation. This approach will be more beneficial in taxa with more slowly mineralizing and thus more highly time-averaged enamel.
This research was funded by the New York Consortium of Evolutionary Primatology NSF DGE 0333415, NSF BCS 0621542, and Sigma Xi G20110315157181.