Raman spectroscopy of fossil bioapatite — A proxy for diagenetic alteration of the oxygen isotope composition

Fossil bioapatite may yield biogeochemical signals of paleoenvironments captured by living organisms. Bioapatite may be diagenetically altered, however, with ions added or removed post-mortem; such change is typically assessed using destructive and demanding techniques. Here, Raman spectroscopy is used as a rapid and non-destructive way to identify significant diagenetic alteration of fossil bioapatite. We found spectral parameters of phosphate symmetric stretching (ν1-PO43−) to be very sensitive to variations in apatite chemistry, particularly with respect to common diagenetic components (CO32−, F−, Sr2+). The Raman spectral parameters were subsequently applied to a set of modern (biogenic) and geologic (magmatic) apatite samples as potential endmembers for diagenetic alteration. Raman spectra were also collected from enamel and dentin (respectively resistant vs. alteration-prone) of fossil teeth. Phosphate-oxygen isotopic values from the same enamel–dentin samples were used as an index of alteration and provided definition of Raman spectral parameters as relates to diagenetic alteration. Diagenetically altered samples were characterised by spectra with ν1-PO43− widths (at half maximum height) less than 13.0 cm− 1, and ν1-PO43− band positions greater than 964.7 cm− 1. Raman spectroscopy is shown to have potential as a tool for pre-screening fossil apatite samples before further analyses.