USING GAMMA-RAY SPECTROMETRY TO NON-DESTRUCTIVELY DETERMINE THE AGE OF RADIOACTIVE C. MEGALODON TEETH AND OTHER URANIUM-BEARING CENOZOIC VERTEBRATE FOSSILS

Application of a new method of gamma-ray spectrometry to fossils bearing the isotopes ²³⁵U and ²³⁸U and various daughters including ²²⁴Ra, ²²⁶Ra, and ²¹⁴Pb, can potentially be used to determine the absolute age of fossils ranging from 13,000 YBP- 200MYA. Calcium ions in the hydroxyapatite crystal lattice of bones and teeth readily substitute with uranium ions. Reliable hydroxyapatite geochemical substitutions are dependent on the assumption that uranium was introduced rapidly post-mortem, enrichment occurred early in diagenesis, and bones and teeth remained closed with respect to uranium during geologic time. This study was conducted using several Pleistocene mammal remains from Texas localities. Most specimens were from sharks ranging in age from Pliocene to Eocene. A C. subauriculatus tooth and a Parotodus benedeni tooth came from Pliocene localities in North and South Carolina. Twenty C. megalodon teeth came from Pliocene and Miocene localities in Florida, Maryland, North Carolina, South Carolina, Virginia, Fiji and Cuba. An Otodus obliquus tooth and a Carcharias sp. tooth are from the Eocene of Mississippi. A C. angustidens tooth is from an Eocene locality in South Carolina. A “Carcharodon” (sic) sp. tooth is from an Eocene locality in Oregon. Two C. auriculatus teeth are from Eocene localities in South Carolina and Virginia. One Hybodus reticulatus partial spine is from the Jurassic of England. Pulse height analysis was applied to the gamma-ray spectrum produced by each specimen. The areas of peaks corresponding to daughter isotopes were added together, compared to the areas under peaks corresponding to parent isotopes, producing a peak height ratio. These ratios were plotted against the logarithmic values of age and a sigmoidal graph was produced. The sigmoidal graph shows a consistent change from 13,000 YBP to 200 MYA, serving as a function of time with relation to the decay chain of ²³⁵U and ²³⁸U, which provides reliable age dates. Most specimens were borrowed from the U. S. National Museum collections and their taxonomic identifications are those assigned by the USNM.