EXTREME HEAT IN THE EARLY ORDOVICIAN

Abstract

Global cooling and the establishment of a moderate climate in the Middle Ordovician has been invoked as the primary driver of the tenfold increase in marine biodiversity that characterized the Great Ordovician Biodiversification Event (GOBE). Arguments suggesting that climate change played a significant role in biodiversification purport that the Early Ordovician was dominated by warm (possibly even extremely warm) temperatures. In this scenario, biodiversification occurred only after sea surface temperatures approached the range observed in modern tropical settings. Temperature constraints for the Early Ordovician, however, are limited in number and documentation of short-term climatic trends is lacking. This study aims to begin to address these shortcomings by presenting a new high-resolution phosphate oxygen isotope record of species-specific and mixed conodont assemblages from the Lange Ranch section of central Texas. We document consistently low δ18O values indicating that the Early Ordovician was characterized by extreme warmth. In addition, variations observed in δ18O values through the section are consistent with a short-lived (< 4 m.y.) warming event and/or oceanographic shift leading to a decrease in the δ18O value of local seawater. All new data generated for this study are consistent with very warm temperatures during the Early Ordovician, an important starting condition in arguments that climate played an important role in Middle Ordovician biodiversification, but variation found also demonstrates the need for high resolution studies to constrain conditions on time scales relevant to evolutionary diversification.