Jan 10 2008
Large ice-sheets existed on Earth about 91 million years ago, during one of the warmest periods since life began, an international team of scientists reports this week.
The findings, published in the journal 'Science', challenges the popular assumption that large glaciers could not have existed in the 'super greenhouse' climate, when tropical surface ocean temperatures reached as high as 35-37C (95-98.6F) and alligators lived in the Arctic.
Scientists from the USA, UK, Germany and Netherlands found evidence of an approximate 200,000 year period of widespread glaciation during the Turonian 'super-greenhouse' period of the Cretaceous, with ice sheets about 60 per cent the size of the modern Antarctic ice cap.
The team obtained their evidence from detailed analyses of sediments that were deposited in the western Equatorial Atlantic Ocean at that time.
The sediments, recovered by drilling into the ocean floor off Suriname in South America, contained fossil shells of tiny sea creatures, foraminifera, that lived in the Cretaceous seas.
These shells 'captured' chemicals that were present at the time, providing the researchers with clues about the temperature, composition and salinity of the seawater. Scientists at the Scripps Institution of Oceanography in the USA were able to use this information to reconstruct sea temperature, both at the surface and at depth.
Meanwhile, a European team at Newcastle University in the UK, the University of Cologne in Germany and the Royal Netherlands Institute for Sea Research (NIOZ), studied the composition of organic molecules in the same sediments, providing separate evidence about the temperature of the surface water during this period of time.
By combining these two lines of data, the team was able to identify temperature and chemical changes in the ocean that are consistent with periods of glacial formation.
Professor Thomas Wagner, of the School of Civil Engineering and Geosciences and the Institute for Research on Environment and Sustainability at Newcastle University, said: 'Speculation about whether large ice caps could have formed during short periods of the Earth's warmest interval has a long history in Geology and climate research, but there has never been final conclusive evidence.
'This uncertainty remained, as there is very little direct evidence from high latitude rocks supporting or disproving the concept; also computer simulations have difficulties to accurately model climate conditions at polar latitudes during past greenhouse conditions'.
'Our research from tropical marine sediments provides strong evidence that large ice sheets indeed did exist for short periods of the Cretaceous, despite the fact that the world was a much hotter place than it is today, or is likely to be in the near future'.
Professor Jaap S. Damste from the Royal NIOZ added: 'The results are consistent with independent evidence from Russia and the USA that sea level fell by about 25-40 metres at this time. Sea level is known to fall as water is removed from the oceans to build continental ice-sheets and to rise as ice melts and returns to the sea. Today, the Antarctic ice cap stores enough water to raise sea level by about 60 metres if the whole mass melted and flowed back into the ocean.'