BATHYMETRIC AND ISOTOPIC EVIDENCE FOR A SHORT-LIVED LATE ORDOVICIAN GLACIATION IN A GREENHOUSE PERIOD

The end Ordovician glaciation is distinct among Pharnerozoic glaciatio ns in that CO2 levels were generally high, yet major continental ice s heets accumulated on the Gondwana supercontinent. New oxygen isotopic data indicate substantial changes in sea-water temperatures and ice vo lume coinciding with glacio-eustatic changes in sea level reflecting t he growth and decay of the Gondwana ice cap. Major glaciation was appa rently 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 sign ificant changes in carbon cycling as the oceans changed from a state w ith warm saline bottom waters to a state with cold deep-water circulat ion and then back again. We believe that the changes in the carbon cyc le effected a reduction in P(CO2) levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse condi tions in a greenhouse world.