BIKINI atoll and the other ring-shaped coral islands of the tropical Pacific Ocean are far younger than previously thought, contradicting Darwin鈥檚 ideas about how atolls form 鈥� and possibly solving a long-standing puzzle about one of the greatest human migrations in history.
Anthropologists have long puzzled over why, roughly 3000 years ago, the ancient Polynesians abruptly stopped their eastward sweep across the South Pacific, only island-hopping across the coral atolls about 1500 years ago. But if the atolls did not exist until then the mystery is solved.
鈥淭here鈥檚 been a lot of debate about what has been called 鈥榯he long pause鈥�,鈥� says Patrick Kirch, director of the Oceanic Archeology Laboratory at the University of California at Berkeley. 鈥淎toll existence or non-existence is critical.鈥�
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Darwin speculated that the atolls formed over millennia as coral grew around the edges of ancient volcanic craters while they slowly sank beneath the waves. But as William Dickinson, of the University of Arizona, told last week鈥檚 meeting of the Geological Society of America in Seattle, the age of the reefs based on atoll core samples and estimates of ancient sea levels suggest a more complex process.
According to Dickinson, coral-encrusted volcanoes emerged from the ocean during the last ice age, about 100,000 years ago, when much of the world鈥檚 water was in massive ice sheets. But rather than the volcanoes still having craters, the depressions were created as rainfall pelted the exposed portion of the coral reefs. Water carrying dissolved limestone drained off the edges of the islands, evaporating to leave an erosion-resistant mineral rim.
The islands became submerged once more, around 8500 years ago, when ice sheets melted and sea levels rose, allowing the coral reefs to grow again. This accentuated the initial depression. With the tremendous weight of the glaciers lifted from temperate regions, the continental plates began to shift and sea levels in the tropical Pacific fell again, reaching current levels between 1000 and 2000 years ago. Only then, says Dickinson, did the atolls appear.