Shark Tooth Enameloid-Bound Organic Matter Carbon Isotopic Composition as a (Paleo)ecological Indicator

Abstract

The long-standing fossil record of sharks has driven interest in reconstructing the diets and ecology of both modern and ancient species. Stable carbon isotope analyses of inorganic carbonate (δ¹³CCO₃) and tooth dentin collagen (δ13Ccollagen) are commonly used to investigate dietary information and environmental conditions. However, preservation of collagen does not exceed tens of thousands of years and δ¹³CCO₃ may be confounded by environmental signals. Enameloid is resistant to diagenetic processes and protects enameloid-bound organic matter (EBOM) over million-year timescales. This study ground-truths the carbon isotope signature of EBOM (δ13CEBOM) in shark teeth as a novel proxy for reconstructing diet. These measurements are made possible with a modified elemental analyzer–isotope ratio mass spectrometry (EA-IRMS) system (“nanoEA”) that can analyze small quantities of enameloid (2-4 mg containing ~100 nmol carbon). In this thesis, I demonstrate a strong correlation between δ13CEBOM and δ13Ccollagen in modern shark teeth, supporting its potential as a dietary proxy. I additionally measure δ13C across various carbon pools within the tooth (enameloid carbonate, dentin collagen, dentin-bound organic matter, and bulk dentin) to better understand the organic matter protected in the mineral structure and the observed offset of ~7‰ between δ13CEBOM and δ13Ccollagen. Even with a δ13C offset, the strong correlation between δ13CEBOM and δ13Ccollagen highlights the promise of δ13CEBOM for reconstructing shark diets and trophic dynamics in Earth history, which can provide new insight into shark ecology and their resilience throughout past climate and marine environmental changes.