A Calcium and Strontium Analysis of Shark Teeth to Constrain Past Ocean Chemistry

Abstract

Fossil apatite in shark teeth is both abundant in the geologic record back to ~400Ma and likely to preserve primary isotope ratios. We thus use measurements of Ca and Srcontent in fossil shark teeth as a tool to estimate both the Ca isotopic content \((\delta^{44/40}\textrm{Ca}_{sw})\) and ratio of Sr to Ca, \(\frac{Sr}{Ca}\), of ancient seawater. Ca from an array of modern teeth (N= 30) was separated from other trace elements on a DIONEX ICS 5000 ionchromatography (IC) system, and isotope abundances were measured on an MC-ICP-MS.These results were used to construct an empirical model of Ca sources, which suggeststhat both seawater \((10\%)\) and diet \((90\%)\) contribute to the Ca budget in shark teeth with atrophic level effect of $-0.23$ parts per thousand, per trophic level. \(\delta^{44/40}\textrm{Ca}_{tooth}\) measurements ranging from the Jurassic to the present (N = 74) show that \(\delta^{44/40}\textrm{Ca}_{sw}\) values have changed through time, while measurements of \(\frac{Sr}{Ca}\) in bioapatite imply that \((\frac{Sr}{Ca})_{sw}\) and seawater Srconcentration, \([Sr^{2+}]_{sw}\), have both decreased throughout the Cenozoic. The evolution ofoceanic Ca and Sr has important implications for our understanding of the processes that control sources (river input) and sinks (sedimentation, diagenesis, and weathering fluxes)of these elements on a global scale. An analysis of shark tooth \(\delta^{44/40}\textrm{Ca}_{tooth}\) indicates preservation of samples and supports the use of bioapatite as a reliable proxy for reconstructions of seawater chemistry.