Something very unusual happened about 80,000 years ago, as Earth's last ice age was getting started. Sea levels that had been dropping for thousands of years--as more and more water became trapped in expanding glaciers--suddenly rose, according to a new study. Then after a few thousand years, the levels fell again. Although the researchers haven't found the cause of this phenomenon, they say the findings could force at least a partial rethinking of the mechanisms governing Earth's climate.
For the past several hundred thousand years, our planet seems to have followed a fairly regular climate cycle. About every 100,000 years, kilometers-thick ice sheets form atop the northernmost reaches of North America, Europe, and Asia and extend well into the mid-latitudes. The ice sheets also tie up so much seawater that ocean levels can drop by over 100 meters. Then, after about 90,000 years, the glaciers retreat and land reappears, until the next ice age begins. The last ice age ended about 11,000 years ago.
Various researchers going back nearly a century have attempted to isolate the cause of the ice-age cycle. They have uncovered several factors--periodic changes in Earth's orbit around the sun, a wobble in our planet's axis of rotation, for example--that seem to be in play. But so far no one has presented the definitive answer.
Tomorrow in Science, a team doesn't so much solve the riddle as reinforce the uncertainties about how the ice-age cycle behaves. What they've found is very strong evidence that when Earth should have been hunkering down for the last ice age, the seas rose.
In coastal caves on the Spanish island of Mallorca in the Mediterranean Sea, the team studied stalactites encrusted with calcite. They measured the elevation of those encrustations, which were deposited like bathtub rings that mark high- and low-water levels, and then dated those deposits using the radioactive decay of traces of uranium into thorium isotopes. Based on those calculations, the researchers found that sea level 80,000 years ago had rebounded to the point where it rose 1 meter higher than it is today. And it could have risen quite quickly, as much as 2 meters per century, says geochemist and lead author Jeffrey Dorale of the University of Iowa, Iowa City.
That suggests the glaciers were melting at a tremendous rate. Even half that rate would still be "a major finding," Dorale says. So it "has major implications for future concerns with sea-level change."
Geologist and co-author Bogdan Onac of the University of South Florida, Tampa, says he and his colleagues Angel and Joaquin Gines of the University of the Balearic Islands had suspected the implications of the Mallorca encrusted stalactites back in the 1970s, when they first discovered them, but they lacked the technology to date them accurately. So when they could finally use the uranium-thorium ratio for a precise measurement, "we realized that these encrusted [stalactites] hide information related to ice sheets coming and going."
The study "shows persuasive evidence" for a surge in sea level about 80,000 years ago, says geologist Daniel Muhs of the U.S. Geological Survey in Denver. Muhs, whose own work on Bermuda had suggested the same spike in sea level, says the Mallorca findings help make a "powerful" argument for the phenomenon. Still mysterious, however, are other data, taken from Barbados and New Guinea, that also suggest rising sea level about 80,000 years ago but not nearly as much of a change. It means, Muhs says, "that there is much about interpreting the geologic record of sea-level variation that we still do not understand."