Constraining the tempo of Ordovician cooling

The Ordovician Period (485-444 Ma) is of particular interest to geologists and paleontologists because it contains a major interval of biodiversification followed by a mass extinction that occurred at a time of global glaciation. To constrain what factors are responsible for the drastic change in climate that resulted in the end-Ordovician glaciation, we must first understand when this cooling began and whether it was continuous or punctuated. It is well-established that the beginning of the Ordovician was characterized by a warm climate and the end of the period by a major glaciation, yet the trajectory of cooling through the period remains unclear. By creating biostratigraphically-constrained age models, we compile δ ¹⁸ O _phosphate data developed from Ordovician conodont elements to create a more comprehensive oxygen isotope curve. In this study, we paleogeographically reconstruct the position of the sediments, allowing for a new understanding of seawater temperature difference as a function of latitude at the given time. Additional conodont elements from Oklahoma provide data to increase the robustness of this δ¹⁸O_phosphate composite curve. The tempo of seawater δ¹⁸O_phosphate change becomes more apparent. In contrast to a prevailing interpretation of cooling, stasis, and further cooling, these compiled data reveal a progressive cooling trend through much of the Ordovician. Further constraining the timing and tempo of cooling is essential for evaluating hypotheses for the initiation of the glaciation.