Ice and its consequences; glaciation in the Late Ordovician, Late Devonian, Pennsylvanian-Permian, and Cenozoic compared

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 (sub 2) events, characterized by cosmopolitan faunal distributions and two episodes of catastrophic extinction. Pennsylvanian-Permian and Cenozoic glaciation typify long-duration, low atmospheric CO (sub 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 (sub 2) levels at the time of Devonian glaciation. However, the Berner GeoCarb curve suggests relatively low atmospheric CO (sub 2) levels in the Late Devonian.