Posted on 06/08/2012
Pebbly glacial deposits, about 715 million years old, in layers from northern Namibia. The pebbles were deposited as “dropstones” from floating ice
Researchers from Royal Holloway, University of London have discovered that ice retreated completely through the glaciations.
A team led by Dr Daniel Paul Le Heron from the Department of Earth Sciences at Royal Holloway have found evidence revealing the ice was much more dynamic than ever expected on the frozen planet.
Some 715 million years ago, thick ice sheets grew on each continent, as far as the tropics, in an event termed snowball Earth. Glaciers in this icy period have long been thought to be stable throughout the glaciation, with ice acting like a lid for greenhouse gases that built up from volcanoes under the ice. Once volcanoes punctured the icy veneer, an unstoppable, catastrophic melt of ice around the globe would occur, and the Earth would warm very rapidly.
Studying spectacular sedimentary rocks in northern Namibia, where a thick pile of debris was laid down by glaciers, the research team discovered non-glacial shale rocks sandwiched by glacial sediments above and below. Dr Le Heron says “The evidence suggests that ice retreated completely during the glaciation, melting away in a warmer interglacial period, then regrowing before finally disappearing completely. This is a very exciting finding because it shows that these ancient glaciers behaved exactly like ice sheets today”.
The research is published online in Sedimentology.