TRACE ELEMENT CHEMISTRY OF MODERN SHARK TEETH AND IMPLICATIONS FOR SHARK TOOTH GEOCHRONOMETRY

We have determined the average trace element concentration in the dentine and enameloid of teeth from a variety of coastal, inshore and pelagic shark species: Galeocerdo cuvier (tiger shark), Carcharhinus limbatus (black tip),Carcharias taurus (sand tiger), Carcharhinus leucas (bull shark), Prionace glauca (blue shark), Isurus oxyrinchus (mako shark), Carcharhinus brevipinna (spinner shark), Sphyrna zygaena (smooth hammerhead) and Hexanchus griseus (bluntnose six gill shark). Teeth were collected from the jaws of recently deceased individuals of these species, and the trace element concentration of aliquots prepared from samples of the interior and root dentine and cusp enameloid were analyzed using the inductively coupled plasma mass spectrometer housed in the Environmental Sciences Analytical Center at Brooklyn College. The average concentration ranges for each shark species measured so far are: REE and U, <1 ppm; Ba, Ni, Mn, Vn, 1 to 10 ppm; Al, Zn, Cu, 10 to 100 ppm; and Sr, 1000 to 3000 ppm. Dentine, which is much more porous than enameloid, shows significantly wider fluctuations in within-species trace element concentrations than does enameloid. We have not yet been able to detect noticeable trace element differences among species in tooth enameloid. This result supports the view that trace element uptake and deposition in tooth enameloid reflects the average trace element concentration of ocean water. Sharks do not appear to be preferentially fractionating trace elements metabolically and concentrating them in their teeth. We interpret this to mean that the life habits of the animals we tested, and the food sources they utilized, are sufficiently broad to have exposed our sharks to average conditions in oceanic chemistry. This result needs further corroboration but suggests that the trace element composition of well preserved fossil shark teeth may be indicative of the trace element distribution in the ancient oceans which they inhabited. It also suggests a possible approach to developing a mechanism for evaluating the degree of diagenetic alteration in the trace element composition of fossil shark tooth enameloid, and possibly make fossil shark teeth reliable geochronometers for post-Silurian marine sediments.