Imagine your favorite beach or seaside retreat disappearing under a rapidly rising ocean within the next two generations. Indiana University Bloomington geologist Paul Blanchon has found evidence off the island of Grand Cayman indicating that such a catastrophic rise in sea level occurred roughly 8,000 years ago, and that the cause of the rise could be an omen for a warming planet. He presented his findings at the annual meeting of the Geological Society of America Oct. 20.
 1997 was declared the International Year of the Reef in response to the growing threats to coral reefs around the world. Launched at the 8th International Coral Reef Symposium in Panama last June, the program promotes sustainable management.
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"This increase must have been very rapid indeed, because it shifted these early reefs out of the shallow surf-zone into deeper water so quickly that it prevented the corals on the reef from recovering both during and after the rise," Blanchon said.
The reefs Blanchon studies contain elkhorn coral (Acropora palmata), which grows as fast as 15 millimeters a year and only thrives on the reef crest down to a depth of five meters. But the newly discovered drowned reef now lies under 21 meters of water and has been concealed by layers of living, deep-water coral and sediment. Blanchon and his colleagues drilled into the reef and removed samples of elkhorn coral which were then dated to confirm their antiquity.
Blanchon's discovery is only the fourth drowned elkhorn reef ever to be found and the first whose location was predicted in advance. Other drowned reefs have all been found by chance, in locations stretching from Florida to Barbados, and all died at approximately the same time and same depth.
"Previous explanations for the sudden demise of these other reefs focused on changes in the nutrient levels of Caribbean waters," Blanchon explained. "Yet the dead elkhorn reefs were quickly colonized by deeper water corals, and new elkhorn reefs were quickly reestablishing themselves further up-slope in shallower water, indicating that conditions for coral growth did not deteriorate."
At the time of the sea level jump that Blanchon is studying, ice sheets had all but disappeared from the Northern Hemisphere, and the only region of potentially unstable marine-based ice lay over Antarctica.
Then, as now, the western half of this ice sheet was grounded below sea level and buttressed by floating ice shelves pinned precariously on bedrock pinnacles. With such an unstable configuration, and in the absence of other sources, it seems likely that a collapse of the West Antarctic Ice Sheet produced a catastrophic rise in global sea level that drowned fast growing elkhorn reefs in the Caribbean.
"This raises the disturbing possibility that the ice sheet is inherently unstable and might be prone to further collapse," Blanchon said. "The sea level jump 8,000 years ago also coincided with a period when global temperatures were approximately 2 degrees Celsius warmer than present. Any similar warming that affects the underpinnings of the Antarctic ice shelves could, therefore, have the same disastrous effect in the future."
Related Link:
http://www.indiana.edu/~reefpage
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