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Great Barrier Turned up the Heat New Scientist 31 May 1997

Lou Bergeron, Santa Cruz

ONE well-kept secret of Australia's Great Barrier Reef is out: it's a lot younger than scientists thought. And this means that it may have been part of the driving force behind a mysteriously hot period on Earth around 400 000 years ago, an international team of geologists told this week's meeting of the American Geophysical Union in Baltimore. For years, most researchers assumed that because the reef lies close to the reefs of Papua New Guinea, which have been dated at 3 to 4 million years old, it must be similar in age. To test this, a team of geologists led by Peter Davies of the University of Sydney decided to date the reef. Last year, Davies's team drilled two holes into the reef and dated the corals at the bottom of the reef as well as the material below. The results indicate that the reef began growing between 300 000 and 400 000 years ago. That was a time when the sea level was up to 20 metres higher than today-the highest level in almost half a million years.

There have been numerous fluctuations in sea level, caused by warming or cooling of the Earth. Most are put down to the Milankovitch cycle, a small wobble in the Earth's axis that alters the amount of sunlight reaching northern latitudes. But the high sea level 400 000 years ago is difficult to explain, as the tilt of the Earth would have favoured a cool and stable climate.

Davies says that the formation of the Great Barrier Reef may, at least in part, resolve this mys tery. Coral reefs form as calcium carbonate precipitates out of the sea. And as the sea loses calcium ions, it becomes more acidic and less able to retain carbon diox ide, which is released into the air. "For every mole of carbonate you precipitate, you produce 0.6 moles of CO2" says Davies. The release of CO2 a greenhouse gas, would have warmed the Earth and caused a rise in sea level, says Davies. This process can be self-sustaining, he adds. A slight increase in sea level would flood dry land, turning it into new habitat for corals, which would release yet more C02s rose Davies suspects that other major reefs, such as those in the Bahamas, began forming around the same time. "There was a global tum-on of carbonates about 400 000 years ago," he says. Wolfgang Berger of the Scripps Institute of Oceanography in La Jolla, California, agrees that precipitation of carbonates may explain the warm spell. "The Great Barrier Reef gives us the possibility of filling in that particular missing link," he says. F7