Abstract
The end Ordovician glaciation is distinct among Phanerozoic glaciations in that CO 2 , levels were generally high, yet major continental ice sheets accumulated on the Gondwana supercontinent. New oxygen isotopic data indicate substantial changes in sea-water temperatures and ice volume coinciding with glacio-eustatic changes in sea level reflecting the growth and decay of the Gondwana ice cap. Major glaciation was apparently confined to the Hirnantian and was 0.5-1 m.y. long, rather than the 35 m.y. of earlier estimates. Carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep-water circulation and then back again. We believe that the changes in the carbon cycle effected a reduction in P CO 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world.
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