Pacific Atolls Are Just Young'uns As Islands Go
Bikini Atoll and the other idyllic, palm-covered atolls of the tropical Pacific Ocean are less than two thousand years old -- much younger than previously thought, according to a University of Arizona geologist.
The finding explains something anthropologists have been wondering about for a long time -- why, as the ancient Polynesians migrated east across the South Pacific, they took a break of about 1,500 years, sometimes referred to as "the long pause."
"I say they paused because there wasn't anything to go to!" said sedimentary geologist William R. Dickinson, professor emeritus of geosciences at UA. His presentation on the geological youthfulness of Pacific atolls will be given in Seattle at 4:30 p.m. Monday, Nov. 3, at the annual meeting of the Geological Society of America.
Although scientists, including Darwin, have been pondering atoll formation since the mid-1800s, how such landmasses are formed has been poorly understood. Darwin thought that atolls were formed over eons as fringing reefs grew on top of sinking volcanoes. It's not that simple, Dickinson said -- present-day atolls were formed from a complex interplay of fluctuations in sea level combined with changes in elevation of the sea floor.
Although atolls are built on top of ancient volcanoes, the classic atoll shape of a circular reef surrounding a central lagoon does not require an underlying volcanic crater. Before the beginning of the last ice age 115,000 years ago, corals built limestone reefs on top of barely submerged Pacific Ocean volcanoes. About 95,000 years later at the height of the last ice age, so much water was tied up in the continental ice sheets that sea level was 125 meters lower than it is today. As a result, those ancient volcano-top reefs stuck up out of the water, forming steep-sided, islands.
When rain fell on the islands, the water eroded the limestone in the center of the islands by dissolving it. Water carrying dissolved limestone then drained out of the sides of the island and evaporated, leaving behind a very hard, erosion-resistant mineral layer on the cliff edges. As the erosion continued, the islands ended up having a hard rim enclosing a depression in the center. Dickinson said that saucer-like shape is what set the stage for the formation of atolls.
When the glaciers began to melt about 18,000 years ago, sea level began to rise again. As the rising ocean reached the top of the islands, about 8500 years ago, the reefs that would end up forming atolls began to grow, Dickinson said. As those reefs grew, they followed the saucer shape that erosion had given the islands.
Once the tremendous pressure of the ice was removed, the continents and the sea floor slowly, slowly readjusted, with some parts rising and other parts sinking. Sea level continued to rise and in the tropics reached a highpoint of about 2 meters above current sea level approximately 4,000 years ago. Then about 2,000 years ago, the sea level in the tropics began to decline because the sea floor dropped in the temperate zones. By about 1,000 years ago sea level had declined to near current levels. At that point, the drop in sea level had left enough of the reefs high and dry to form atolls -- low islands encircling a lagoon. "The atoll islets are like little pearls on a necklace," Dickinson said.
This scenario for atoll formation had been proposed in the past, but without supporting data. Now Dickinson has confirmed the idea by comparing three independent lines of evidence -- cores from six different atolls that show the age of the post-ice-age reef, information on the changes in sea level in the last 125,000 years, and calculations about how much the original limestone plateaus had eroded. He said the three sets of numbers are "as nice a match as I've ever seen."
"Darwin thought the atoll shape was a direct result of the original fringing reef. This shows that's impossible," said Dickinson. "Considering the justifiably exalted status of people who have worked on this before, like Darwin, seeing how this all works is really one of the most exciting things I've ever done in science." He added, "One thing that has always been puzzling is why we find so little record of atolls in the geologic record. The answer is that they are very special things that need fluctuations in sea level to form."
* * *University of Arizona news is online @ http://uanews.org * * *
© 2003 The Geological Society of America