ASSESSING FOSSIL BONE DEPOSITIONAL ENVIRONMENT FROM STRUCTURAL AND COMPOSITIONAL CHANGES

The structure and composition of fossil bone is a reflection of complex diagenetic processes over geologic time. No single analytical tool provides the necessary insight into bone diagenesis. By combining multiple approaches, including synchrotron-based X-ray absorption near edge structure spectroscopy (XANES), Fourier transform infrared (FTIR) spectroscopy, and electron microprobe (EMP) analyses, to investigate the atomic-level bonding within the apatite mineral, the structural and compositional transformations during diagenesis can be quantified. Notably, changes to apatite bond coordination and lattice structure may be more important for long-term preservation because we identified increased occupancies of the p-orbital at P sites within the apatite lattice accompanied by decreased occupancy of the p-orbital at the Ca sites. This may reflect flexibility of the apatite lattice. At the Ca K-edge, a shift in peak maxima, as well as peak height in fossil bone, may reflect metal substitution at the Ca(II) sites. FTIR and EMP analyses support prior work that demonstrates changes to apatite crystallinity and composition occur during fossilization. XANES spectra suggest that fossil bones reach similar structural arrangements of the lattice over time, with chemical composition playing a secondary role. Preliminary comparisons between fossils recovered from fluvial, wetland, and lacustrine deposits reveal different bone chemical compositions, but similar structures when examined with XANES. Consequently, there are significant implications for depositional environment-specific diagenetic processes leading to bone preservation, processes that may be masked if fossils reach a similar structural composition.