A refined sampling strategy for intra-tooth stable isotope analysis of mammalian enamel A. Zazzo a,⇑ , R. Bendrey b , D. Vella c , A.P. Moloney d , F.J. Monahan e , O. Schmidt e,f a CNRS - Muse ´um national d’Histoire naturelle, UMR 7209 ‘‘Arche ´ozoologie, Arche ´obotanique: Socie ´te ´s, Pratiques et Environnements’’ USM 303 - De ´partement Ecologie et Gestion de la Biodiversite ´ CP 56, 55, rue Buffon, F-75231 Paris cedex 05, France b Department of Archaeology, University of Reading, Whiteknights Box 226, Reading RG6 6AB, UK c OCCAM, Mathematical Institute, University of Oxford, 24-29 St Giles’, Oxford OX1 3LB, UK d Teagasc, Animal and Grassland Research and Innovation Centre, Grange, Dunsany, Co. Meath, Ireland e UCD School of Agriculture and Food Science, University College Dublin, Belfield, Dublin 4, Ireland f UCD Earth Sciences Institute, University College Dublin, Belfield, Dublin 4, Ireland Received 15 September 2011; accepted in revised form 12 January 2012; available online 28 January 2012 Abstract Serial sampling and stable isotope analysis performed along the growth axis of vertebrate tooth enamel records differences attributed to seasonal variation in diet, climate or animal movement. Because several months are required to obtain mature enamel in large mammals, modifications in the isotopic composition of environmental parameters are not instantaneously recorded, and stable isotope analysis of tooth enamel returns a time-averaged signal attenuated in its amplitude relative to the input signal. For convenience, stable isotope profiles are usually determined on the side of the tooth where enamel is thick- est. Here we investigate the possibility of improving the time resolution by targeting the side of the tooth where enamel is thinnest. Observation of developing third molars (M3) in sheep shows that the tooth growth rate is not constant but decreases exponentially, while the angle between the first layer of enamel deposited and the enamel–dentine junction increases as a tooth approaches its maximal length. We also noted differences in thickness and geometry of enamel growth between the mesial side (i.e., the side facing the M2) and the buccal side (i.e., the side facing the cheek) of the M3. Carbon and oxygen isotope vari- ations were measured along the M3 teeth from eight sheep raised under controlled conditions. Intra-tooth variability was sys- tematically larger along the mesial side and the difference in amplitude between the two sides was proportional to the time of exposure to the input signal. Although attenuated, the mesial side records variations in the environmental signal more faith- fully than the buccal side. This approach can be adapted to other mammals whose teeth show lateral variation in enamel thickness and could potentially be used as an internal check for diagenesis. Ó 2012 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Over the last 15 years, serial sampling and stable isotope analysis of tooth enamel has emerged as a powerful tool for exploring various aspects of the life history of living and fossil animals. Koch et al. (1989) were the first to show that animal dental tissues (in this case, proboscidean tusk den- tine) could record intra-tooth isotopic variations. Fricke and O’Neil (1996) adapted the sampling procedure to tooth enamel, and most studies have focused on enamel because of the high resistance of this tissue to diagenetic alteration (Wang and Cerling, 1994; Zazzo et al., 2004a). Mammalian tooth enamel forms incrementally from the top to the base of the tooth crown, and these differences in oxygen isotopes were attributed to seasonal variation in climate recorded sequentially during tooth mineralization. Today, intra- 0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2012.01.012 ⇑ Corresponding author. E-mail address: zazzo@mnhn.fr (A. Zazzo). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 84 (2012) 1–13