Time to chill

Well-preserved fossils deposited in the sediments of an Antarctic lake about 14 million years ago pin down when a large part of the now-icy continent most recently dipped below freezing. The new findings chronicle the changing climate of the region and answer long-standing questions about when and how quickly Antarctica cooled.

Antarctica holds about 90 percent of the world’s ice, which blankets the continent at an average depth of about 1.6 kilometers. Yet small regions, including the McMurdoDryValleys along the eastern side of the TransantarcticMountains, are bare, says David R. Marchant, a geologist at BostonUniversity.

Fieldwork in one of those ice-free valleys in 2004 and 2006 revealed a 50-centimeter-thick stratum of fossil-rich sediments that had been deposited in an ancient lake. Layers of volcanic ash below, within, and above these sediments indicate that the lake existed between 14.07 million and 13.85 million years ago, Marchant and colleagues report in the Aug. 5 Proceedings of the National Academy of Sciences .

“This is a great study which will increase our understanding of past climates in this region,” says Peter T. Doran, a paleoecologist at the University of Illinois at Chicago. “The preservation of the material collected in their samples is stunning.”

The lowermost deposits include an 8-centimeter-thick layer of moss, including a species extinct in the area but still living in lakes near the tip of the Antarctic Peninsula, about 2,000 kilometers north of the fossil site. Among the stems and leaves of the ancient mosses, the researchers found fossils of tiny crustaceans called ostracods, fragments of larval casings of mosquito-like flies called midges and a millimeter-wide bit of a beetle shell. Together, these fossils suggest that about 14.07 million years ago the average summer temperature around the lake was about 5° Celsius.

Overlying strata include fossils that suggest the lake deepened, possibly reaching a depth of 8 meters, during the millennia that followed. Those remains, including photosynthetic diatoms at the base of the lake’s food chain, also hint that the lake was a permanent body of water that was ice-free for at least part of the year, says Marchant. Then, a flush of glacial sediment filled the lake about 13.85 million years ago, when the average summer temperature there dropped to negative-3 degrees C, climate models suggest. Today, the average temperature in the region is negative-12 degrees C during the summer months.

The mosses unearthed from the ancient lake sediments are so well preserved — essentially, they’ve been naturally freeze-dried by the region’s cold, dry conditions — that they can be rehydrated. That’s a sign that the region’s climate hasn’t warmed since the lake filled in, the researchers propose. Otherwise, they say, decomposition would have ravaged the remains, as well as those of the other creatures entombed in the lake’s sediments.

Scientists have long debated whether East Antarctica’s ice sheet has remained intact for millions of years or whether it shrunk and expanded as climate elsewhere in the world warmed and cooled, says W. Berry Lyons, a geochemist at OhioStateUniversity in Columbus. The new findings “are compelling evidence that climate in this area has been stable for the last 14 million years,” he notes.