 By LEE BOWMAN Scripps Howard News Service April 08, 2005
The theory that our planet - oceans and land alike - was encased in ice several times from about 750 million to 580 million years ago has been explored since the 1960s. In the new study, geologists Bernd Bodiselitsch and Christian Koeberl from the University of Vienna in Austria analyzed cores drilled by copper miners in Zambia and the Congo, looking for signatures of ice. What they found was a clear spike in iridium - a platinum-like metal that's rare on Earth but fairly common in meteors and space dust - in sediments from about 635 million years ago and slightly less conclusive evidence from 710 million years ago. The work, published Friday in the journal Science, is already spurring other researchers to do similar research using other trace elements. Iridium is a good signal for extreme events on Earth because, normally, the material falls in a steady drizzle across the planet's surface. For instance, a big spike of iridium from 66 million years ago signaled the impact of a comet or meteor in what is now the Gulf of Mexico - a blast that spelled doom for the dinosaurs. The researchers suspected that dust falling on a so-called "Snowball Earth" would have accumulated on ice sheets worldwide, including areas near the equator. When the ice melted, the iridium would have been carried by water into riverbeds and oceans, leaving unusually high concentrations of the element. Earlier estimates for the length of the snowball periods ranged from several hundred thousand to 30 million years. Based on the amount of iridium found at the bottom of a layer of glacial material, the Austrian researchers determined that the glacial period that ended about 635 million years ago lasted at least 3 million but more probably about 12 million years. Deposits of iridium from the earlier glacial period didn't land quite as neatly among the glacial silt, which means, the researchers speculate, that either the meltdown from that glacial period was spotty, or that the ice didn't fully cover the oceans - a "Slushball Earth" situation. Although there's still much debate about what caused the glaciers to form in the first place - theories range from collisions with asteroids to change in the tilt of Earth's orbit - the events are important illustrations of how the planet regulates its temperature. Scientists believe that the increasing ice cover over the globe bounced more sunlight back into space, contributing to the freeze-up. However, the tectonic plates continued to move under the ice, and volcanoes continued to form and pump carbon dioxide into the atmosphere. With no liquid water to absorb the carbon dioxide, eventually greenhouse conditions developed, trapping heat and melting the ice rapidly. Just what brought the global freeze-thaw cycle to an end is also unclear. However, scientists do know that within 100 million years, there was a dramatic change in life on the planet from primitive single-celled forms living in the oceans to an abundance of complex, multi-celled animals and plants in the sea. Recent research around seafloor volcanoes and in modern icepacks makes it clear that many exotic strains of bacteria could have easily survived the "Snowball" episodes. Genetic evidence shows that the organisms that lived through the glacial times were part of the oldest lines of life forms on the planet.
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