Science 319, 189–192 (2008)

Credit: ERIC CONDLIFFE

Massive glaciers, up to 60 percent of the size of the present-day Antarctic ice cap, may have existed during one of the warmest episodes on Earth. The Turonian period, 93.5 to 89.3 million years ago, when tropical sea surface temperatures were over 35 °C and alligators roamed the Arctic, was previously assumed to be ice-free.

But now a multinational team of scientists led by André Bornemann, then of Scripps Institution of Oceanography at the University of San Diego, has uncovered clues from the sea floor off Suriname in South America that challenge this supposition. Two separate lines of evidence point to widespread glaciation that lasted around 200,000 years during the 'super greenhouse' period. The first piece of the puzzle was garnered from chemical traces in the fossil shells of single-celled foraminifera recovered from deep-sea sediments. Combining the fossil evidence with an analysis of organic molecules in the same sediments, the team found past ocean temperature and chemical changes consistent with historic glacier formation.

The study suggests that ice growth occurred even during the hottest periods on Earth. But the researchers warn that this does not imply the same will hold in the future greenhouse world, as warming is now happening much more rapidly.