VERTEBRATE FOSSIL GEOCHEMISTRY AS AN ARCHIVE OF BIOLOGY AND CLIMATE

When bones and teeth form, their mineral constituent, apatite, adopts an isotopic and trace element chemistry reflective of the animal’s composition. Fossils often preserve original biogeochemistry despite diagenesis, providing a rich archive of information regarding the animal and its environment. Dietary studies commonly employ C-isotopes because plant ¹³C depends on photosynthetic pathways and aridity. First used to explore the rise of C4 grasses and dietary niche partitioning, C-isotopes from tooth enamel are now also used to infer mean annual precipitation in continental settings. Stable isotopes of O reflect water sources and animal physiology, and ¹⁸O enrichment reflects warmer/drier climates, climatic shifts to water sources, or physiological drought tolerance. For example, tooth enamel isotopes document drought tolerance for some (but not all) "camels," systematic rise of mountain ranges in western North America, and abrupt changes to moisture sources during the Pleistocene in the desert Southwest. Trace elements and their isotopes are more susceptible to diagenesis, especially in bone, but in favorable cases enamel preserves biogenic compositions. Isotopes of Sr are used to infer place of origin (at the time of tissue formation) and geographic movement either from progressively precipitated and zoned teeth or from different teeth and bones that formed at different times in an animal’s lifetime. Numerous archeological examples explore human movements and interactions, for example documenting locals vs. "foreigners" in primitive cultures and the origin and movements of the ~5200 year-old iceman recovered from an Alpine glacier. Trace element ratios (Sr/Ca and Ba/Ca) are sometimes used to infer position within food chains, e.g. degree of carnivory in our ancestors and ancestral cousins. These ratios are most susceptible to diagenesis, however, and recent data suggest that dust and soil ingestion can potentially bias interpretations. Future work looks towards novel isotopic systems, e.g. Ca, Mg, Zn isotopes and clumped isotopes. Additional research is needed both in calibrating these systems and identifying diagenetic overprint potential, but possibly these isotopes can be used to identify trophic level (herbivory vs. carnivory), sex, or body temperatures of extinct organisms.