Abstract
Shark teeth are abundant in the fossil record and integrate physiological information, ecological interactions, and paleo-oceanographic conditions in their chemistry. Fossil shark teeth are well suited for stable isotope analysis because their enameloid is resistant to diagenetic alteration due to its high chemical stability. Although often used in paleoecological studies of mammals, carbonate carbon isotope compositions (delta C-13(CO3)) in shark enameloid have remained enigmatic. Here, we investigate multiple stable isotope systems (delta C-13(org), delta C-13(CO3), delta O-18(CO3), delta O-18(PO4)) within modern shark teeth to determine relationships between the different systems and build an interpretative framework for future studies of both modern and fossil sharks. There is a weaker than expected correlation between delta O-18(PO4) and delta O-18(CO3) values in modern shark teeth (r(2) = 0.44), 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 (delta C-13(org)) measured from modern dental collagen ranges from 16.0 parts per thousand to 10.8 parts per thousand. The enameloid delta C-13(CO3) values we measured are much higher than collagen, ranging from 6.0 parts per thousand to 10.3 parts per thousand, and there is no direct relationship between delta C-13(org) and delta C-13(CO3) values in shark teeth. Instead, we found the fractionation (epsilon) between delta C-13(org) and delta C-13(CO3) values to correspond with delta O-18(CO3) values but not delta O-18(PO4) values. This could be due to the carbon source in shark enameloid being partitioned between dietary carbon and dissolved inorganic carbon (DIC) or physiological differences in the tooth formation process changing the fractionation of carbonate isotopes. We applied the fractionation factor from modern teeth to carbonate isotope compositions of fossil shark teeth to predict delta C-13(org) values. Although the carbon sources to shark enameloid carbonate needs further investigation, our results suggest that fossil shark teeth could provide insights into carbon cycling of ancient marine ecosystems.
