Stratigraphic Variations In Geochemical Signatures and Crystallographic Parameters of Fossil Bioapatite and Associated Sediment Fractions

The primary constituent of modern and fossilized bone is the mineral, bioapatite (~Ca ₅(PO ₄) ₃(F, OH, Cl)). Geochemical conditions control the fossilization of bioapatite and diagenetic processes in sediments. Chemical species present in fossilized bone and certain sediment fractions represent the original depositional environment.

There are four different types of crystallographic positions in the bioapatite structure where elemental exchanges can occur, and where a variety of cations and anions can be accommodated. During diagenesis, these substitutions into bioapatite affect the chemical composition of fossilized bone. These variations in the chemical composition of bioapatite control changes in unit cell dimensions (UCD). Since these substitutions are dependent on the species that are available during diagenesis, the depositional environment determines the fossil bioapatite structure. Geochemical signatures from fossil bioapatite and associated sediment fractions also represent the depositional environment and can be used with UCD for interpretation of paleoenvironments and stratigraphic correlation.

A representative sampling of bioapatite and associated sediments from the marine deposits of the Pierre Shale in Central South Dakota were used for this research. The results obtained in the geochemical analysis of the bioapatite samples indicate the chemical composition directly controls the variations in the UCD. Through comparison of the geochemical signatures and UCD of the samples, described lithographic units can be subdivided into sub-units based on the depositional environment. The geochemical variations seen in fossil bioapatite within lithostratigraphic units also correspond to variations in the geochemical signature of certain sediment fractions.

Geochemical signatures of bioapatite and sediments, and UCD of fossil bioapatite are consistent within individual lithologic units or sub-units and vary between these units. These variations in UCD, geochemical signatures of fossil bioapatite, and certain sediment fractions are unique for a particular lithostratigraphic subdivision and can be used to discriminate between lithostratigraphic units or sub-units for stratigraphic correlation.