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

The hard parts of marine organisms reflect the elemental and isotopic composition of the seawater in which they form. We have determined the average concentrations of the following trace and minor elements in teeth from a variety of coastal and pelagic shark species: Fe, Cr, Co, Mn, Ba, Cu, Al, U, Pb, Ni, V, Zn, Mg, Ce, and Sr. The teeth analyzed were collected from the jaws of recently deceased individuals of the following 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), Carcharhinus obscurus (dusky shark),and Hexanchus griseus (bluntnose six gill shark). Trace and minor element concentrations were measured using an ICP-MS; tooth enameloid ranged from <1 ppm for U, to 1000s of ppm for Sr. For individual sharks elemental concentrations in enameloid did not vary greatly from tooth to tooth in the functional tooth row, nor was there wide variation in concentration between teeth in functional and adjacent pre-functional tooth rows. The concentrations for each particular element in enameloid were generally similar from species to species, with some notable exceptions (e.g., relatively low Sr in Carcharhinus brevipinna). We interpret the general uniformity in composition of enameloid across species to mean that the life habits of the animals we tested and their food sources were sufficiently broad to have exposed our sharks to average conditions of oceanic chemistry. The elemental concentrations of modern shark teeth reported here provide base level values that allow for the detection of diagenetic changes in fossil shark tooth chemistry. These base values were also used as a basis to quantify total diagenetic change in Carcharias sp. teeth associated with the Cretaceous/Paleogene boundary on the New Jersey Coastal Plain. Carcharias sp. teeth were analyzed using Synchrotron microbeam X-ray fluorescence (SXRF) to quantify the Fe, Pb, Sr and U concentrations. Differences in the chemistry between modern and fossil teeth can reasonably be attributed to diagenetic changes. Detecting and quantifying such variations may prove helpful in improving the reliability of geochronological dates obtained for fossil shark teeth.