OXYGEN ISOTOPES FROM CONODONT APATITE OF THE MID-CONTINENT, US: IMPLICATIONS FOR LATE ORDOVICIAN CLIMATE EVOLUTION

To test competing interpretations of Late Ordovician climate evolution, sea surface temperatures were estimated from oxygen isotope ratios of single-species separates of conodont apatite from the mid-continent region of the United States. A major glaciation occurs at the end of the Ordovician, but disagreement exists over the timing of this event. This controversy stems in a large part from a lack of good constraints on the temperature history during the Katian (454-444 MA) and on relatively poor age control on putative glacial deposits from this Age. The end Ordovician glacial episode is accompanied by the 2^nd largest mass extinction event of the Phanerozoic and is a dramatic climatic shift from the ‘greenhouse’ conditions that prevailed for most of the Ordovician. Results from the sampled ~9 my long interval indicate significant fluctuations around a δ¹⁸O_VSMOW mean of 18.9‰; however, there is no evidence supporting progressive cooling during the Katian (454-444 MA). In fact, depending on assumptions about δ¹⁸O values of Late Ordovician sea water, the data suggest a slight warming trend (estimates ranging from 0.7 to 3.2°C) during the late Katian. These observations indicate that cooling and glaciation did not begin until the Hirnantian and are consistent with predictions for mid-Katian warming (the Boda event).