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Sea Caves Reveal Rapid Rise in Ancient Ocean Levels

Sea levels can rise and fall fast, even during an ice age, according to new research
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© Bogdan P. Onac]

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Mallorca, Spain's largest island, is not just a desirable place for a Mediterranean vacation; it's also a treasure trove of the geologic record. That's because of coastal caves that precisely record in stone formations sea level thanks to the island's long-term geologic stability; it has been relatively unaffected by tectonics or glacial uplift or subsidence. Plus, these caves have a series of formations, known as speleothems, like stalagmites, scattered at various levels, both above and below present-day sea level, thereby offering a record in the carbonate crust left on them by the lapping waters of sea level over time.

So a team of geologists set out to identify sea level changes over the course of the past 135,000 years by collecting rock samples from six formations at various levels in five different caves. The researchers found that sea levels were roughly 1 meter higher than present 81,000 years ago when the world was thought to be experiencing an ice age that should have locked up water in glacial ice, thereby lowering sea level as much as 30 meters.

More disturbingly, the record suggests that sea level can rise or fall as fast as two meters a century—nearly 12 times as fast as sea level rise in the past 100 years and indicating the potential for a meter of sea-level rise within one human lifetime. "This has major implications for future concerns with sea-level change," says geoscientist Jeffrey Dorale of the University of Iowa, lead author of the new research published in the February 12 issue of Science. "Our study indicates rapid rates of ice melting and ice formation. The mechanisms underlying these dramatic changes need further consideration as we look to a future of impending climate changes."

By measuring the decay of radioactive uranium and thorium present in the encrusted speleothems, the geologists dated the layers. Because water levels in the cave precisely track sea level today—and Mallorca is generally geologically stable—this record provides a more precise measure of historic sea level than the records left in coral reefs or ancient shorelines, which had been used in the past to estimate primordial sea levels. In fact, the speleothem record backs findings from Caribbean islands as well as the U.S. east and west coasts that sea level was at least as high 81,000 years ago, and probably higher.

By tracking sea-level change over the entire period, the geologists were able to rule out shifting land levels as the cause of this finding. That's because sea level in the caves remained stable over the last 3,000 years, even as the Mediterranean basin as a whole has sunk by roughly 60 centimeters—and the record matches sea levels found in samples elsewhere in the world for other periods.

It remains unclear what might have caused the sea-level rise, although it is coincident with an increase in the amount of sunlight hitting the Northern Hemisphere due to slight variations in Earth's orbit, known as Milankovitch cycles. "Maybe unstable ice was involved in some of the rapid rise," Dorale speculates. "But we don't even know for sure what the ice configuration was at this time."

Yet, the record in rock also suggests that sea level can change aside from these orbital cycles, notes geologist Bogdan Onac of the University of South Florida, who collaborated in the research—and that Milankovitch cycles cannot entirely explain ice growth or melt over the last 100,000 years. As it stands, the rock record points clearly to higher sea levels. "There are other lines of evidence that come from corals and marine terraces pointing toward a high sea-level stand," Onac says. "It must have been a dramatic melting event. What exactly caused it is hard to tackle with just this set of data."

Glaciologist Richard Alley of The Pennsylvania State University, who was not involved in the research, calls the findings "solid" and "careful," and notes that this research confirms that ice-sheet changes can happen quickly. "It points to rather rapid shrinkage and growth of ice," he says, while cautioning that further research will be needed to confirm this finding. "The growth rates are surprising, but not impossible."

One thing is clear, however, the finding points up how complex Earth's climate is. "Greenhouse gases are clearly important to climate," Dorale says, "but just as clearly they are not the only major factor at work."

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