Intra-tooth variations in δ18O (PO4) of mammalian tooth enamel as a record of seasonal variations in continental climate variables

Regular intra-tooth variations in the delta(18)O value of mammalian tooth enamel phosphate (delta(18)O(p)) have been considered a potential measure of seasonal changes in continental climate variables since they were first observed. In order to investigate this possibility in more detail, analyses were made of teeth from a number of mammalian herbivores (sheep, cattle, elk, and pigs) that lived over a wide range of geographic locations, ecological settings, and climatic conditions (Iowa, Florida, Wyoming, Iceland,. England, Croatia, and the Philippines). The lack of intra-tooth delta(18)O(p) variations in teeth of cattle that were given tap water to drink provides strong evidence that the underlying cause of observed intra-tooth variations is primarily a change in the isotopic composition of ingested water. In concert with this interpretation, the range of intra-tooth delta(18)O(p) values and their absolute values from each locality mirror observed differences in the range and absolute delta(18)O values of local precipitation (delta(18)O(pt)) and in climate variables. Thus intra-tooth delta(18)O(p) values can indeed be considered a qualitative measure of seasonal climate change in continental settings. Quantitative use of intra-tooth delta(18)O(p) values as a climate proxy is possible, but is hindered by lack of detailed information on aspects of mammalian physiology, behavior, and perhaps local hydrology that may also play a role in influencing delta(18)O(p). This problem is exemplified by the different range in delta(18)O(p) values measured for sheep and cattle from the same locality around York, UK (3.4 vs. 2.6 parts per thousand, respectively). The observed difference most likely reflects a difference in the relative amount of leaf water ingested by the two species. Future studies of well-constrained samples are required to test physiological models and to develop empirical relations that accurately relate delta(18)O(p) to delta(18)O(pt). In addition to their use as indicators of seasonality, intra-tooth variations in delta(18)O(p) values provide valuable information for longer-term climate change and paleobiological investigations. Copyright (C) 1998 Elsevier Science Ltd.