Royal Netherlands Institute for Sea Research, Netherlands

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 (super 18) O excursions in marine limestones suggest that glaciation events may have punctuated this episode of extreme warmth. New delta (super 18) O data from the tropical Atlantic show synchronous shifts approximately 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 1to 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.