Ice and Its Consequences:Glaciation in the Late Ordovician,Late Devonian Pennsylvanian-Permian,and Cenozoic Compared

摘要

Comparison of the duration,onset conditions,and biotic response to the four Phanerozoic glaciations suggests that there are two Phanerozoic glacial modes.Late Ordovician glaciation typifies short-duration,high atmospheric CO_2 events,characterized by cosmopolitan faunal distributions and two episodes of catastrophic extinction.Pennsylvanian-Permian and Cenozoic glaciation typify long-duration,low atmospheric CO_2 events,characterized by abundant bio-geographic differentiation and stable or rising biotic diversity.Late Devonian glaciation appears most similar to Late Ordovician glaciation:it had a short duration;Late Devonian biotas were cosmopolitan;and Late Devonian glaciation was associated with increased extinction at the Devonian-Carboniferous boundary.On the basis of biotic response,we would predict high atmospheric CO_2 levels at the time of Devonian glaciation.However,the Berner GeoCarb curve suggests relatively low atmospheric CO_2 levels in the Late Devonian.