Trace elements and their isotopes in bones and teeth: Diet, environments, diagenesis, and dating of archeological and paleontological samples

Fossil biogenic apatites display trace element (TE) compositions that can record environmental and biological signals, give insights into past water compositions, or be used for dating paleontological and archeological bones and teeth. Processes of TE incorporation into apatites of skeletal phosphatic tissues are described, ranging from those active in living organisms to those active during diagenesis. Abiotic incorporation mechanisms have been modeled theoretically and experimentally and include crystallographic controls on TE partitioning coefficients, inorganic surface adsorption and adsorption mediated through chelation and diffusion–adsorption processes, each leading to specific fractionation patterns. Predictions from models and experiments have been tested against TE concentration and isotopic composition data on remains of contemporary wild or raised animals and on fossils of various ages and environments. In living organisms, TEs incorporated in apatite are separated in two categories, non-essential elements whose concentration is compared to that of an essential element with similar behavior (e.g. Sr/Ca) to reconstruct trophic chains, and essential elements whose isotopic ratios are used to trace metabolic activity, diet, etc. In fossils, elements are incorporated during diagenesis, such as rare earth elements (REEs), and trace diagenetic processes ranging from very early quantitative adsorption likely mediated by chelators, protracted diagenesis through inorganic adsorption and fractionation through diffusion–adsorption, to low-grade metamorphism associated with intense redistribution under crystal-chemical control. These different steps are also identified as steps of radionuclide incorporation. Only samples where the earliest steps can be deciphered are useful for determining stratigraphic and archeological ages. Other samples will date late diagenetic events that have obliterated the paleoenvironmental signals, but may be of geological significance, e.g. tectonic events.