Reconstructing paleoenvironments using triple oxygen isotope values

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.

Speaker: Jordan Wostbrock, Yale University