Pj. Brenchley et al., BATHYMETRIC AND ISOTOPIC EVIDENCE FOR A SHORT-LIVED LATE ORDOVICIAN GLACIATION IN A GREENHOUSE PERIOD, Geology, 22(4), 1994, pp. 295-298
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.