The increase of d¹⁸O values in the rock record (where older samples have lower d¹⁸O values than modern) has been attributed to changing ocean temperature, an increase in the d¹⁸O value of the ocean water, or diagenesis obscuring paleoenvironmental information. Unlike conventional d¹⁸O analysis where the formation water’s isotopic value is assumed, paired d¹⁷O-d¹⁸O measurements allow for the water’s isotopic composition to be calculated because there is only one unique solution for equilibrium fractionation using triple oxygen isotope values. This is particularly useful when measuring samples from the rock record, where the water is no longer present. These measurements are also useful for a definitive identification of diagenesis – samples that fall off the unique triple oxygen isotope equilibrium curve must have undergone some degree of diagenesis.  A fluid-rock mixing model can be used to ‘see through’ alteration in samples such that the initial temperature of formation in which the carbonate formed can be calculated. In this talk, I will provide an overview of carbonate-water triple oxygen isotope fractionation. We will then look at three case studies: Late Cretaceous ammonites from the Western Interior Seaway, Devonian fossil brachiopods from the Appalachian Basin, and Hirnantian to Silurian fossil brachiopods from Anticosti Island.

Jordan Wostbrock is an assistant professor in Earth and Planetary Sciences at Yale University. She received her B.Sc in Environmental Sciences from Westfield State University, and her M.Sc and Ph.D from the University of New Mexico. Her research focuses on method development for the analysis of triple oxygen isotope values of low temperature minerals and the application to paleoclimate and paleoenvironmental reconstructions. In her spare time, you can find her outside either rock climbing, mountain biking, or hiking with her two dogs.