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.
"Heinrich events are armadas of ice," said John Clague of Simon Fraser University in Burnaby, British Columbia, who was not a part of the study. "They are massive discharges of ice."
Hendy proposes two explanations for how the Heinrich events in the north Atlantic may have influenced melting in of the Cordilleran Ice Sheet in the Pacific.
One possibility is that the Heinrich events triggered sea-level rise, which caused the margins of the Cordilleran Ice Sheet to float up and destabilize.
However, there is no strong evidence that a change in sea level occurred at these times. "But there's something going on then, because the corals don't seem to be growing," Hendy said.
Another possibility, she said, is that continental temperatures had increased, especially in summertime, leading to calving from both ice sheets.
"If you warm up the North American continent, you connect the Laurentide to the Cordilleran," she said.
"The relation between the events in the west and the east is good enough that coincidences and accidents won't work," said Richard Alley of Pennsylvania State University in State College, "but I'm not positive whether one can tell whether the [ice debris] is a warming or a cooling signal in the west, or maybe a sea-level signal."
Alley thinks sea level is least likely.
Understanding the connection between the ice sheets could be helpful for predicting what will happen under today's climate change.
"We know that our climate models now can't predict the full amount of climate change that we see," Hendy said, "If we know what the connections were in the past, we could say whether they would happen again."
credited to discovery.com