INVESTIGATING ENIGMATIC CARBONATE CARBON ISOTOPIC VALUES IN SHARK TEETH: EVIDENCE FOR TEMPERATURE DEPENDENT PARTITIONING

Shark teeth are extremely abundant in the fossil record and can be used as ancient data buoys, recording physiological information, ecological interactions, and paleo-oceanographic conditions of prehistoric shark taxa. 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)F, which has a high chemical stability and is resistant to diagenetic alteration. Despite being regularly used in paleoecological studies of other organisms, carbonate carbon isotopic values (δ¹³C_CO3) in shark enameloid have remained enigmatic. Values of organic dietary carbon (δ¹³C_org) we measured from collagen in tooth dentine range from -16.0‰ to -10.8‰. Surprisingly, the δ¹³C_CO3 values we measured are much higher, ranging from -6.0‰ to 10.3‰. It is possible that, like otoliths, the source of carbon in shark enameloid is partitioned between dietary carbon and dissolved inorganic carbon (DIC). Here, we investigate the relationships of stable isotope systems (δ¹³C_CO3, δ¹⁸O_CO3, δ¹⁸O_PO4) within modern shark teeth in order to build an interpretative framework for future fossil studies. We generated δ¹³C_org, δ¹³C_CO3, δ¹⁸O_CO3, δ¹⁸O_PO4 values for modern teeth (N=137) of wild caught, captive study, and aquarium reared species. We then used these relationships to interpret the ecology and environment of Miocene–Pliocene fossil sharks. We found no relationship between δ¹³C_org and δ¹³C_CO3 in shark teeth, but instead found that this offset correlates with δ¹⁸O_CO3 values. This anomalous trend suggests that carbonate carbon isotope composition is temperature dependent and prone to partitioning or fractionation. Interestingly, there is no correlation between δ¹⁸O_PO4 and δ¹⁸O_CO3 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. Gaining a better understanding of δ¹³C_CO3 values and how they relate to δ¹³C_org will allow us to estimate δ¹³C_org values of fossils and learn more about food web dynamics of ancient ecosystems.