Deep ice stirs debate on climate stability.
Over the last four years, the U.S. and European groups have drilled separate holes through the Greenland ice sheet to collect samples containing clues about the last ice age and the interglacial warm period immediately preceding it. The ice cores can provide such information because the glacial cap built up layer upon layer over hundreds of thousands of years, trapping details about past temperatures, winds, greenhouse gas changes, and other aspects of climate.
The seven-nation European team made headlines in July when it reported that the last interglacial - called the Eemian stage - had a highly erratic climate that sometimes shifted abruptly into frigid ice age conditions and then snapped back into warmer weather (SN: 7/17/93, p. 36). The findings raise concern because scientists had long thought that interglacial spans -- such as the current one -- were immune from the unstable climate swings that characterize glacial epochs.
American researchers, in collaboration with several European scientists, now raise questions about the evidence of climate instability during the Eemian, which lasted from 135,000 to 115,000 years ago. At a meeting of the American Geophysical Union in San Francisco and in the Dec. 9 NATURE, the investigators report that although the two ice core records yield identical information for the glacial period, they disagree about the Eemian.
"There's clearly a discrepancy between the two cores. We have to sort out what's going on," says Kendrick C. Taylor of the University of Nevada at Reno.
The Europeans drilled their ice core at the summit of Greenland's ice cap, while the U.S. team worked on the flank, 30 kilometers away - an arrangement designed to capture the very differences now surfacing. Taylor and his colleagues announce that the upper 90 percent of the 3,000-meter-long ice cores match almost perfectly: Both show the same rapid shifts in climate during the last ice age and record the remarkable stability of the current interglacial period, which started 10,000 years ago.
But measurements of oxygen isotopes in the ice and of the ice's electrical conductivity do not match for the lower tenth of the cores. In the U.S. record, this deepest section contains folded patterns suggesting that layers of ice have overturned as the glacier flowed over hills in the bedrock. Any turnovers would break the chronological ordering of layers and alter the true climate record.
Researchers are divided on how to apply these findings to the European ice. That core also contains evidence of overturning, but such questionable layers appear in ice older than the Eemian period. As yet, the investigators have not found signs of scrambled layers within that crucial interglacial span, says Heinz Miller of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. The integrity of layering plus evidence from chemical and isotopic studies suggest that the climate actually did fluctuate dramatically during the Eemian, Miller told SCIENCE NEWS.
But some U.S. researchers say more work is needed to resolve the question of Eemian climate instability. "We're not saying they're wrong. We're simply saying it's more complicated than a nice layer-cake situation. Maybe we'll prove that they're right. We'll just have to wait and see," says Taylor.