In from the cold

日期:2018-01-12 06:29:08 作者:曲敲 阅读:

By Jeff Hecht in Boston EARTH’s oceans may all have completely frozen over at least four times between 750 and 570 million years ago, according to a geologist in Massachusetts. He says the repeated freezing and thawing may have triggered the evolution of the first animals bigger than microbes. The theory of a “snowball” Earth has been touted before. Paul Hoffman of Harvard University says climate models show that if polar caps become very large, they reflect far more solar energy than usual. “You get runaway cooling and the Earth freezes over,” he says. But sceptics have pointed to the lack of evidence for frozen seas in the warmest areas of the planet, such as the tropics. Now Hoffman and his colleagues have found new evidence that even tropical seas froze during Neoproterozoic times, more than 540 million years ago. They say rocks in Namibia in southern Africa indicate that at least four glaciations occurred during the Neoproterozoic, when the region was much nearer the equator. Hoffman says that before each proposed glaciation, the rocks contain high levels of carbon-13. This indicates a high rate of burial of organic carbon, and implies that lots of carbon dioxide had been sucked out of the atmosphere by algae, he says. Hoffman suggests that the removal of a lot of CO2 triggered the ice ages by reducing greenhouse warming. He also points out that glaciers may have formed more easily at that time than now, because the Sun was slightly fainter than it is today. On top of each “glacial” deposit, representing the time immediately after the proposed global ice ages, there were distinctive limestones containing less carbon-13 than the normal level in limestone. Hoffman says this shows that there was virtually no photosynthesis in the oceans for millions of years while ice sheets covered the water. No other such long-lasting nonproductive times are known in the past half-billion years. Hoffman suggests in last week’s Science (vol 281, p 342) that the break-up of a supercontinent called Rodinia about 750 million years ago triggered the glaciations by creating more coastlines, where organic matter is rapidly buried. As CO2 levels dropped, the planet cooled and glaciers spread to cover the globe. Some life would have persisted under the ice-covered oceans, Hoffman believes. He suspects that what finally thawed the planet was the slow accumulation of CO2 from volcanic eruptions, which acted as a greenhouse gas. He thinks the environmental changes brought about by the last snowball episode could have triggered the sudden appearance of animals larger than microbes. Experiments have shown that rapid evolution occurs when there is a population crash, so the planetary freeze-thaw cycles could have pushed life to evolve beyond the microbes and simple seaweeds that had dominated the oceans for nearly three billion years. Joe Meert of Indiana State University in Terra Haute, who has long doubted the possibility of global freezes, says the work amounts to interesting evidence of a snowball Earth. However, he is not yet convinced that the theory is the only possible explanation for the Namibian rocks. He says Hoffman needs to find glacial deposits of the same ages on different continents,