Ancient Ice Sheets Fell Like Dominoes

July 1, 2008 -- Tens of thousands of years ago, "armadas of ice" crumbled off of the ice sheet covering North America into the Atlantic Ocean. Meanwhile, thousands of miles away on the other side of the continent, icebergs calved off of another ice sheet into the Pacific.

Their synchrony -- just uncovered by new research -- suggests the events might be connected in a long-distance domino effect. The fact that melting at one location may influence ice sheets afar may be useful in understanding the behavior of ice today, according to the study's authors.

"What it is saying is that these ice sheets are connected," said lead author Ingrid Hendy of the University of Michigan. "If we melt Greenland, we could raise sea level and affect Antarctica. Or, if we melt Greenland, we can affect the tidewater glaciers up in Alaska."

Hendy and colleagues analyzed a sediment core collected off the western coast of Vancouver Island, in southern British Columbia. They analyzed the grain sizes of the sand and pebbles in the 130-foot core, using zooplankton remains to determine the date when the debris was deposited. The team published their results in Paleoceanography.

Because the core was taken offshore, past where waves could carry large particles of sand, large debris inside it is assumed to come from an iceberg -- laden with larger sand and pebbles trapped in the ice -- that floated out to sea and melted overhead, dropping the grains to the sea floor.

Such events in the sediment record indicate times when icebergs calved off a nearby ice sheet, in this case the Cordilleran Ice Sheet, which grew down from Alaska into northern Washington, reaching its maximum size 17,500 years ago.

Hendy identified three calving events, but they did not occur in synchrony with major climate swings in the Pacific.

"I was anticipating that I'd see the ice sheets responding to the rapid climate events," Hendy said. "What I found was that the ice sheet doesn't really care."

But the two major events both coincided with enormous so-called Heinrich events approximately 16,000 and 47,000 years ago, when huge numbers of icebergs broke off of the Laurentide Ice Sheet, which covered most of Canada and much of the northern United States, into the north Atlantic.