COLUMBIA UNIVERSITY RECORD May 27, 1994 Vol. 19 No. 30 SCIENTISTS THEORIZE GLOBAL CLIMATE CHANGES LAUNCHED 'ICEBERG ARMADAS' Massive icebergs burst from the glacier-covered North American continent seven times during the last ice age and drifted across the North Atlantic. Did expanding ice sheets suddenly collapse or did a global climate change launch the iceberg armadas? Scientists from Columbia's Lamont-Doherty Earth Observatory Monday reported new evidence that glaciers in Iceland and two different regions in eastern North America all may have discharged icebergs at nearly the same times. The finding, announced at the American Geophysical Union's meeting in Baltimore, challenges the theory that glacial dynamics triggered the iceberg armadas, since it is unlikely that unconnected and distant ice sheets could have behaved so synchronously. Instead, the Columbia researchers offered a new explanation-- global sea-level rises. Ice that had advanced onto continental shelves in many parts of the globe could have been lifted simultaneously by rising seas, which would have reduced friction at the base of the glaciers and allowed them to cascade freely into the ocean, they say. The finding provides an intriguing new clue in ongoing efforts to piece together the causes of abrupt climate changes during the last ice age and the potential for dramatic change in the future. "We're unraveling how Earth's complex climate system works," said Gerard Bond, senior scientist at Lamont-Doherty. "We're learning about how unstable and delicately balanced the climate system is and the potential for upsetting the balance in the future." Bond conducted the research with Rusty Lotti, curator of the Ocean Sediment Core Laboratory at Lamont-Doherty, Columbia's earth science research center in Palisades, N.Y. Evidence for the iceberg armadas first appeared in 1988 when Hartmut Heinrich of the German Hydrographic Institute in Hamburg found six layers of tiny stones in Atlantic Ocean sediment cores. The stones, which had been scraped off by the advancing Laurentide Ice Sheet in what is now eastern Canada, were frozen in the ice, carried out to sea by icebergs, deposited on the ocean floor and buried by subsequent sediments. Each layer represented an iceberg discharge, which has come to be known as a Heinrich event. Then last year, Bond and colleagues linked Heinrich events to rapid climate shifts. They found that between 70,000 and 10,500 years ago, air and ocean temperatures in the North Atlantic region repeatedly and dramatically rose and fell--within centuries or decades. Each roller-coaster temperature cycle culminated in the launching of a great iceberg armada, followed by a rapid warming. "The research documented a previously unrecognized link between how the ice sheet behaved and how the ocean and atmosphere behaved," Bond said, leading to the next unsolved puzzle: What ultimately caused the ice sheets to launch the armadas? One leading explanation is the so-called "binge-purge" model, in which the Laurentide Ice Sheet is thought to have gradually thickened, or "binged," until heat and pressure began to melt the glaciers' base. That formed a mushy layer on which the ice sheet could slide easily, causing dramatic "purges" of icebergs through the Hudson Strait and into the Labrador Sea. The ice sheet, the theory says, then returned to a "binge" phase, expanding over several thousand years. In their new research suggesting the role of sea-level rises, Bond and Lotti analyzed three cores from Lamont-Doherty's core laboratory, which houses the world's largest collection of ocean sediment cores. One core, from the Denmark Strait between Greenland and Iceland, contained volcanic rock from Iceland in layers that corresponded closely with the first three Heinrich events. That indicates that Icelandic glaciers "calved" icebergs into the sea in step with the Laurentide Ice Sheet. In another core from south of Iceland, slight increases in red hematite-coated grains corresponded with the first three Heinrich events and the Younger Dryas period, a long-documented cold spell between 10,000 and 11,000 years ago that is now considered by many scientists to be another Heinrich-like event. The scientists suggest that the source of the hematite-coated grains is the Scotian Shelf near Newfoundland. That indicates that the southern portion of the Laurentide Ice Sheet also launched icebergs through the St. Lawrence Seaway during these same periods. "These new data suggest nearly synchronous increases in discharges of icebergs from the Hudson Strait and St. Lawrence portions of Laurentide ice and from Iceland, a degree of synchroneity within and between ice sheets that is difficult to explain with the binge-purge model," Bond said at the AGU meeting. Preliminary analysis of a third core from off the Scottish coast shows evidence that yet another unconnected ice sheet in Scotland disgorged icebergs concurrently with the Younger Dryas event. The scientists are continuing to analyze the cores near Iceland and Scotland to determine if the ice sheets in those regions exhibited the same degree of synchroneity during the remaining Heinrich events. Bond told the AGU that a global sea level rise would have affected all the ice sheets relatively synchronously, perhaps "lifting and breaking away loosely pinned, unstable portions of the ice shelves." At a conference on abrupt climate change at Lamont-Doherty on May 12, Douglas Mac-Ayeal, a glaciologist at the University of Chicago, said that the new evidence casts doubt on the idea that the Laurentide Ice Sheet acted as a "lone gunman," and instead points to "a conspiracy theory," in which many North Atlantic ice sheets participated. Revising his model of ice-sheet dynamics, MacAyeal said that ice sheets may binge while their bases are frozen and then begin to flow as lubrication increases. But small bumps, cleats or adhesions at the glacial bases may be enough to stop the ice sheets, leaving them poised in a latency period. An external mechanism--like the global sea-level rise suggested by Bond--"could push the ice sheet over the edge, reducing friction enough to overwhelm any sticky spots and tumbling the ice sheets to purge," he said. Bond said the next step will be to date sea-level rises precisely and see whether they precede Heinrich events. If so, scientists will seek to explain how ice may have melted in lower latitudes while glaciers advanced in higher latitudes. The Lamont-Doherty research was supported by the National Science Foundation and the National Oceanographic and Atmospheric Administration.