BIOGENIC APATITE COMPOSITION AND THE EFFECTS OF DIAGENESIS

Biogenic apatite, the mineral in living vertebrates contain relatively high concentrations of carbonate, sodium, and other ions. During fossilization the bioapatite composition changes from a meta-stable carbonate hydroxylapatite to a thermodynamically stable Fluorapatite incorporating fluoride, Rare Earth Elements (REE) and other trace elements during diagenesis. The total REE concentrations (S REE) in modern bones and teeth are generally less than 20 ppm. S REE in fossil bones, however, may be >1,000 to 10,000 ppm. This SREE increase occurs during early diagenetic recrystallization and therefore REE signatures in fossils do not reflect the diet or phylogenetic position of the organism. Recent studies have suggested that once REE are incorporated into bone, they are retained,thus providing a stable record reflecting the early diagenetic environment. Apatite becomes a “flight recorder” for the environment of early diagenesis and can be used to infer the condition of the paleoenvironment. Substitution by REE and other trace elements into the cation sites and specifically into the seven fold Ca (II) site will change the unit cell dimensions. In a comparison of lattice parameters between modern and fossil apatite there is a change in the a-axis from modern values of ~9.4 Å, primarily due to substitution by similar size cations for Ca²⁺ while the c-axis remained consistently stable at ~6.88 Å. Since substitutions are dependent on what species are available during diagenesis, the early diagenetic waters will ultimately determine the fossil apatite structure. Fossil vertebrates were collected with tight stratigraphic control from five members and three sub-members of the Pierre Shale of central South Dakota. REE signatures were found to be distinct for the lithostratigraphic subdivisions and also variations can be seen in differences in the a-axis dimensions according to changes in lithology. In this study unit cell parameters and REE signatures of fossils Bioapatite are consistent within individual sedimentary facies or paleoenvironmental units but are significantly different between lithostratigraphic units and therefore can be used to discriminate between lithostratigraphic units and for the interpretation of paleoenvironments.