UC Merced

Shark teeth are abundant in the fossil record and serve as ancient data buoys, recording physiological information, ecological interactions, and paleo-oceanographic conditions. A tool often used in paleobiological studies to access this recorded information is stable isotope analysis. Fossil shark teeth are well suited for stable isotope analysis because their enameloid is primarily fluorapatite, Ca5(PO4)3F, which is resistant to diagenetic alteration due to its high chemical stability. Although often used in paleoecological studies of mammals, carbonate carbon isotope compositions (δ13CCO3) in shark enameloid have remained enigmatic. Here, we investigate multiple stable isotope systems (δ13Corg, δ13CCO3, δ18OCO3, δ18OPO4) within modern shark teeth to determine relationships between the different systems and build an interpretative framework for future fossil studies. Interestingly, there is no correlation between δ18OPO4 and δ18OCO3 values in modern shark teeth, which contrasts with mammalian studies to date and suggests this metric is not an appropriate test for diagenetic alteration in fossil shark teeth. Organic carbon isotope composition (δ13Corg) measured from collagen in tooth dentine ranges from -16.0‰ to -10.8‰. Surprisingly, the δ13CCO3 values we measured are much higher, ranging from -6.0‰ to 10.3‰, and there is no direct relationship between δ13Corg and δ13CCO3 values in shark teeth. Instead, we found the fractionation (ε) between δ13Corg and δ13CCO3 values to correspond with δ18OCO3 values but not δ18OPO4 values. It is possible that the source of carbon in shark enameloid is partitioned between dietary carbon and dissolved inorganic carbon (DIC), similar to fish otoliths. We applied the fractionation factor from modern teeth to carbonate isotope composition of fossil shark teeth to predict organic carbon isotope values. The ability to estimate δ13Corg values in fossils will provide better insight into carbon cycling and food web dynamics of ancient marine ecosystems.