AN APATITE FOR ORDOVICIAN TEMPERATURE TRENDS: USING δ18OPHOS DATA DERIVED FROM CONODONTS TO UNDERSTAND ORDOVICIAN CLIMATE AND BIODIVERSIFICATION

Events during the Ordovician Period include both the largest and longest period of sustained biodiversification—the Great Ordovician Biodiversification Event (GOBE)—and the second largest mass extinction of the Phanerozoic. Global changes in climate have been invoked as one of the primary forcing mechanisms of both events. Considerable data have been generated in the last decade to constrain better temperatures changes over the last 15 million years of the Ordovician with the goal of better understanding the end Ordovician extinction. However, data for the Early and Middle Ordovician are still sparse. A larger dataset of paleotemperature estimates from the Early to Middle Ordovician is required to determine the role of climate during the GOBE. Conodont elements and their measured δ¹⁸O_phos values are an excellent archive of changing temperatures appropriate for examining relationships between climate and the GOBE.

New δ¹⁸O_phos data derived from conodonts of the late Cambrian of Utah, late Early Ordovician of Nevada, and Middle Ordovician of the Argentine Precordillera, combined with additional δ¹⁸O_phos­ data from the Early Ordovician of Central Texas, indicate an increase in δ¹⁸O_phos values from 15.6‰ to 17.5‰ through this interval. If attributed primarily to temperature change, this increase in δ¹⁸O_phos­ corresponds to a decrease in temperature of ~9°C. Further, assuming a seawater value of −1‰_VSMOW, late Cambrian and Early Ordovician temperatures would exceed 40°C and Middle Ordovician temperatures would still be warmer than modern equatorial temperatures. This dataset includes the first late Cambrian conodont IRMS (Isotope Ratio Mass Spectrometry) δ¹⁸O_phos measurements as well as the first IRMS δ¹⁸O_phos measurements from the Middle Ordovician of the Argentine Precordillera. The Argentine Precordillera also represents a mid-latitude site whereas most data from this interval come from tropical to subtropical localities. The combined data from these four localities support widespread cooling during the GOBE but changes were not as tightly tied to diversification as studies that proposed a direct cause and effect relationship.