HIGH-RESOLUTION CONODONT OXYGEN ISOTOPE RECORD OF ORDOVICIAN CLIMATE CHANGE

The Ordovician Period was characterized by several major events, including a prolonged “super greenhouse” during the Early Ordovician, the “Great Ordovician Biodiversification Event (GOBE)” of the Middle and early Late Ordovician, and the Hirnantian ice age and mass extinction of the latest Ordovician (Webby et al., 2004, The Great Ordovician Biodiversification Event, Columbia University Press). The cause of the rapid diversification of marine invertebrates during the GOBE is not clear, however, and several scenarios have been proposed including widespread development of shallow cratonic seas, strong magmatic and tectonic activity, and climate moderation. In order to investigate relationships between climate change and marine ecosystem evolution during the Ordovician, we measured the oxygen isotopic composition of single coniform conodonts using a Cameca secondary ion mass spectrometer. Our δ¹⁸O profile shows a shift at the Early/Middle Ordovician transition that is indicative of a rapid 6 to 8⁰C cooling. This cooling event marks the termination of the Early Ordovician “super greenhouse” and may have established cooler tropical seawater temperatures that were more favorable for invertebrate animals, setting the stage for the GOBE. Additional cooling episodes occurred during the early Sandbian, early Katian, and Hirnantian, the last culminating in a short-lived (<1-Myr) end-Ordovician ice age. The much cooler conditions that prevailed at that time may have been an important factor in the end-Ordovician mass extinction. Our results differ from those of Trotter et al. (2008, “Did cooling oceans trigger Ordovician biodiversification? Evidence from conodont thermometry,” Science 321:550-554). Instead of a slow, protracted cooling through the Early and Middle Ordovician, our high-resolution record shows that cooling occurred in several discrete steps, with the largest step being at the Early/Middle Ordovician transition.