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IMPACT OF LATE ORDOVICIAN GLACIATION-DEGLACIATION ON MARINE LIFE 34 2 Impact of Late Ordovician Glaciation-Deglaciation on Marine Life W. B. N. Berry, M. S. Quinby-Hunt, And P. Wilde University of California, Berkeley ABSTRACT Sea-level fell at least 50 m during Late Ordovician continental glaciation which centered on the South Pole. Oxygen isotope analyses indicate that ocean surface waters cooled during glaciation. As sea-level fell and surface waters cooled, mass mortalities occurred among most marine benthic faunas, primarily brachiopods and trilobites. Carbon isotope analyses reveal a significant biomass loss at the time of the mass mortalities. The brachiopod-dominated Hirnantia fauna spread widely during glacial maximum. That fauna essentially became extinct during deglaciation. Cold, oxygen-rich deep ocean waters generated at the South Pole during glaciation drove a strong deep ocean circulation and ventilated the deep oceans. Potentially, waters bearing metal ions and other substances toxic to organisms were advected upward into ocean mixed layer during glacial maximum. Graptolite mass mortality apparently was a consequence. Mass mortalities took place among pre- Hirnantia brachiopod and trilobite faunas at the same time as the graptolite mass mortality. Reradiation among graptolites and benthic marine faunas followed after sea-level rise, and deep ocean circulation slowed as deglaciation proceeded. Initially, reradiation rates were slow as unstable environments persisted during the early phases of deglaciation. New colony organization developed among graptolites, but significant originations did not take place until habitats preferred by graptolites stabilized. Conodont mass mortality occurred at the onset of deglaciation. Originations of new taxa were slow initially, but the pace increased as shelf seas expanded and new environments became stable. Similarly, the pace of marine benthic faunal reradiation was slow at first but increased after shelf sea environments stabilized.