Abstract
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, Ca (sub 5) (PO (sub 4) )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 (delta (super 13) C (sub CO3) ) in shark enameloid have remained enigmatic. Values of organic dietary carbon (delta (super 13) C (sub org) ) we measured from collagen in tooth dentine range from -16.0 ppm to -10.8 ppm. Surprisingly, the delta (super 13) C (sub CO3) values we measured are much higher, ranging from -6.0 ppm to 10.3 ppm. 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 (delta (super 13) C (sub CO3) , delta (super 18) O (sub CO3) , delta (super 18) O (sub PO4) ) within modern shark teeth in order to build an interpretative framework for future fossil studies. We generated delta (super 13) C (sub org) , delta (super 13) C (sub CO3) , delta (super 18) O (sub CO3) , delta (super 18) O (sub 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 delta (super 13) C (sub org) and delta (super 13) C (sub CO3) in shark teeth, but instead found that this offset correlates with delta (super 18) O (sub 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 delta (super 18) O (sub PO4) and delta (super 18) O (sub 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 delta (super 13) C (sub CO3) values and how they relate to delta (super 13) C (sub org) will allow us to estimate delta (super 13) C (sub org) values of fossils and learn more about food web dynamics of ancient ecosystems.