New beat detected in the ice age rhythm.

If today's weather seems wacky, try to imagine conditions during the last ice age. A new study suggests that flotillas of icebergs flooded the North Atlantic Ocean every 2,000 years or so as temperatures repeatedly see-sawed from glacial to balmy and back again.

Such findings deepen the mystery of the massive climate shifts that made the last ice age such an unstable time. They also cause experts to wonder whether the modern climate can stage its own temperature flip-flops.

In 1988, German oceanographer Hartmut Heinrich discovered hints of massive iceberg armadas that sailed across the North Atlantic every 7,000 to 10,000 years during the last ice age. During these so-called Heinrich events, the melting icebergs dropped a trail of pulverized rock that accumulated in layers on the deep ocean floor (SN: 7/30/94, p.74).

When they looked in detail at records of sea sediments, Gerard C. Bond and Rusty Lotti of the Lamont-Doherty Earth Observatory in Palisades, N.Y., discovered evidence of subtle debris layers sandwiched in between the Heinrich events. Large numbers of icebergs must have plunged into the ocean at the relatively frequent interval of every 2,000 to 3,000 years. Then, after two or three such cycles, even greater floods of icebergs deluged the North Atlantic in the Heinrich events.

Bond and Lotti described their work in the Feb. 17 Science and in Atlanta last week at the annual meeting of the American Association for the Advancement of Science.

The layers differ in composition, according to Bond and Lotti. Heinrich event layers contain mostly white grains from carbonate rocks that have been traced to Canada's Hudson Strait. For that reason, scientists believe Heinrich events occurred when the giant Laurentide ice sheet on North America surged into the Hudson Strait, causing a record number of bergs to calve into the ocean there.

The intervening layers, however, contain predominantly black basalt and red hematite particles. Bond and Lotti match the basalt with rocks on Iceland, whereas the hematite debris bears the fingerprint of rocks from the Gulf of St. Lawrence. The researchers conclude that such layers record times of increased discharge from an ice sheet on Iceland and from a lobe of the Laurentide ice sheet that emptied into the Gulf of St. Lawrence.

The newfound layers provide a long-sought link between climate records in the ocean and those drawn from the Greenland ice cap. The ice cap holds evidence of climate swings every few millennia, but oceanographers had not previously found signs of similar climate oscillations in the North Atlantic. The sediment layers Bond and Lotti describe, however, line up with the cold spans recorded in Greenland.

"This makes me feel better," says glaciologist Richard B. Alley of Pennsylvania State University in University Park. "It worried me that we had this huge signal in the ice core that simply wasn't showing up in the ocean at all."

The findings also alter ideas about what caused the iceberg discharges. Some researchers had proposed that the waxing and waning of the Laurentide ice sheet controlled the timing of the Heinrich events. But this glacial model cannot explain how separate ice sheets could produce the layers between Heinrich deposits, Bond and Lotti say.

They suggest that variable currents in the North Atlantic may cause temperatures to swing every 2,000 years or so. "It may be a cycle that runs all the time, in which case it is going on today, but its amplitude is small," Bond says.

That possibility has got scientists prospecting for previously undetected climate shifts in the most recent part of the geologic record. "I don't think any one of us believes the Holocene [the last 10,000 years] is quite as boring as you've been led to believe," Alley says.