Oxygen isotopes from conodonts of the mid-continent, US : implications for Late Ordovician climate evolution
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] 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 from a lack of good constraints on the temperature history during this period and relatively poor age control on evidence of possible Katian (454-444 Ma) glaciation. The end Ordovician glacial episode is accompanied by the 2nd largest mass extinction event of the Phanerozoic and marks a dramatic transition out of the 'greenhouse' conditions dominating the Ordovician. Results from the sampled [about]9 my long interval indicate significant fluctuations around a [delta]18OVSMOW mean of 18.9%, but no general trends during much of the Katian (454-444 Ma). A gradual warming trend began in the mid-Katian ([about]449 Ma), though, and continued through the late Katian. Depending on assumptions about [delta]18O values of Late Ordovician sea water, the estimated warming ranges from 0.7 to 3.2[degrees]C. 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).
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