Using light to describe the ancient world

The width and position of a Raman spectral band has been correlated against alteration in fossil bone.

Chemistry has become the foundation for many paleontological explorations. Very recently, chemical signals in dinosaur teeth revealed that long-necked sauropods were warm to the touch. This pinnacle example of paleo-chemistry is one of the many lofty studies that have helped understand life on ancient Earth. Biological conclusions are occasionally based on altered fossils, however, whose chemical signals give seductive, but spurious, results. Much effort has been devoted to identifying altered bones before they are analysed. So far, no single technique can confidently tell us whether the chemical signals in fossil bones are pristine, but new techniques are still being developed.

One of these new techniques uses Raman spectroscopy. Based on the scattering of light, Raman spectroscopy is a non-destructive method for analysing chemical compositions. Take fossil bone for instance – a Raman spectrum reveals the chemical composition of the bone mineral lattice. Using Raman spectroscopy alone, we can uncover the chemical differences between modern and fossil bone, and the differences between altered and unaltered fossil bone. I recently coauthored a study that applied Raman spectroscopy to fossil teeth – check out the abstract below….

 

Thomas DB, McGoverin CM, Fordyce RE, Frew RD, Gordon KC. 2011. Raman spectroscopy of fossil bioapatite — A proxy for diagenetic alteration of the oxygen isotope composition. Palaeogeography, Palaeoclimatology, Palaeoecology. doi:10.1016/j.palaeo.2011.06.016

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.

 

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