Isotopic evidence for glaciation during the Cretaceous supergreenhouse

The Turonian ( 93.5 to 89.3 million years ago) was one of the warmest periods of the Phanerozoic eon, with tropical sea surface temperatures over 35 degrees C. High- amplitude sea- level changes and positive delta O-18 excursions in marine limestones suggest that glaciation events may have punctuated this episode of extreme warmth. New delta O-18 data from the tropical Atlantic show synchronous shifts similar to 91.2 million years ago for both the surface and deep ocean that are consistent with an approximately 200,000- year period of glaciation, with ice sheets of about half the size of the modern Antarctic ice cap. Even the prevailing supergreenhouse climate was not a barrier to the formation of large ice sheets, calling into question the common assumption that the poles were always ice- free during past periods of intense global warming.