Geochronologic Implications of Strontium Isotopic Signatures in the Teeth of Upper Cretaceous Sharks

The ⁸⁷Sr/⁸⁶Sr isotopic composition of the enameloid tips of teeth of Scapanorhynchus texanus, a common and widespread upper Cretaceous shark, differs from that found in the dentine inner portions of the same teeth. Using a marine ⁸⁷Sr/⁸⁶Sr isotopic evolution curve, we converted the ⁸⁷Sr/⁸⁶Sr isotopic composition of different sections of the teeth to numerical ages. Portions of fossil teeth dominated by enameloid (but that include some dentine) yield numerical ages (74.2 ± 0.8 Ma, 2 standard error [s.e.]) that fall within the upper Cretaceous, and are consistent with the known geologic age range of these fossils. As the proportion of dentine present in samples of the teeth increases, the ⁸⁷Sr/⁸⁶Sr-derived numerical ages trend from biostratigraphically reasonable upper Cretaceous ages to those that are far younger and biostratigraphically impossible (about 32 Ma, i.e., Oligocene). This deviation in isotopic composition (and numerical age) is likely to be the result of post-burial diagenetic alteration of dentine, which is generally more susceptible to such alteration than enameloid. To avoid the contaminating effects of dentine, we sampled only the enameloid portion of twelve S. texanus teeth by scratching their surfaces. The enameloid scratch samples yield numerical ages (78.8 ± 0.4 Ma, 2 s.e.) older than those obtained for any of the sectioned samples. The numerical age for the enameloid scratch samples is consistent with the known early Campanian-early Maastrichtian occurrence of S. texanus in North America and with the radiometric age of the deposit from which these samples were recovered. This suggests that sampling only the surface enameloid of shark teeth is useful in reducing spurious isotopic results arising from post-burial alteration of teeth.