Enigmatic carbonate isotope values in shark teeth: Evidence for environmental and dietary controls

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 (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 studies of both modern and fossil sharks. There is a weaker than expected correlation between δ18OPO4 and δ18OCO3 values in modern shark teeth (r2 = 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 (δ13Corg) measured from modern dental collagen ranges from −16.0‰ to −10.8‰. The enameloid δ13CCO3 values we measured are much higher than collagen, 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. 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 δ13Corg 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.