EARLY ORDOVICIAN CLIMATE FLUCTUATIONS INFERRED FROM CONODONT OXYGEN ISOTOPES

Climate likely played an important role in the evolution of the Ordovician biosphere. Global cooling has been invoked as a primary driver of the Great Ordovician Biodiversification Event (GOBE) and the end Ordovician mass extinction event. However, climate trends during the Ordovician are only broadly constrained, and short-term patterns of climate change (not to mention their correlation to paleontological events) are poorly documented. The Early Ordovician is marked by reoccurring intervals of extinction and taxonomic recovery. To study the dynamics of climate change and relatively rapid taxonomic turnover in the Early Ordovician we measured species-specific and mixed-species conodont oxygen isotopic ratios from the Threadgill Creek Member of the Tanyard Formation in the Llano Uplift of central Texas.

Oxygen isotopic results from the Lange Ranch section record an ~2‰ negative excursion in the Skullrockian (early Tremadocian) Stage. Values decrease through the Cordylodus angulatus conodont Zone, reaching the lowest values measured at ~15.5‰ in the lower Rossodus manitouensis conodont Zone. Values return to an average of ~17‰ in the upper R. manitouensis Zone. The decrease in δ¹⁸O values occurs just prior to a positive carbon isotope excursion and extinction event at the base of the Stairsian Stage in North America. These results are consistent with a brief (<3 m.y.) warming pulse superimposed on already very warm Early Ordovician conditions. Our results, assuming ambient seawater δ¹⁸O values of -1‰_VSMOW, would suggest local sea surface temperatures in excess of 40°C (104° F). On-going work will focus on extending higher resolution studies through the Early Ordovician, testing regional gradients, and examining possible links between short-term climate change inferred from δ¹⁸O data and both turnover rates and fluctuations in marine redox conditions.