Temperature trends of the late Cambrian through the Middle Ordovician using oxygen isotopic analysis of conodont apatite

LAURA E. SPEIR; FERNANDA SERRA; NICOLÁS A. FELTES; GUILLERMO L. ALBANESI; LINDA C. KAH; R. L. ETHINGTON; J. F. MILLER; K. G. MACLEOD

Washington

Jornada; American Geophysical Union Fall Meeting 2018; 2018

American Geophysical Union

The Ordovician Period is marked by the greatest interval of sustained biodiversification of the Phanerozoic (the Great Ordovician Biodiversification Event), as well as one of the Phanerozoic's "Big Five" mass extinction events. Climate change has been proposed as a primary forcing mechanism for both of these events. Over the last decade, Ordovician proxy temperature data, particularly the number of oxygen isotopic analyses of conodonts, has increased markedly. Resolution is now sufficient to begin to discriminate between alternative hypotheses regarding the relationships between variation in climate and subsequent observed changes in ocean chemistry and biological diversification. Most of these efforts, though, have been concentrated on the last 15 million years of the Ordovician, and data for the early parts of the interval remain sparse.In this study, we present new conodont δ18Ophos data from the late Cambrian through the Middle Ordovician. Samples from Nevada and Texas suggest that late Cambrian through Early Ordovician climates were very warm. Apparent temperature differences of 5°C exist between sections and could reflect temporal and/or regional differences. Analyses of samples from the Middle Ordovician of Argentina suggest cooler temperatures than those interpreted from Early Ordovician sections (assuming all specimens were mineralized in waters of the same isotopic composition) but are still warm relative to existing Middle Ordovician estimates. Our results indicate the Dapingian and early Sandbian may have been times of relatively rapid climatic change. Comparing paleontological and isotopic data, inferred temperature trends suggest diversification began before evidence for apparent cooling, and temperatures remained warm enough through the Middle Ordovician to be problematic for models suggesting growth of ice sheets at that time.