On six separate occasions during the past 60,000 years iceberg armadas streamed forth into the North Atlantic from the Labrador Sea. The icebergs carted boulders and rubble from Eastern Canada, which were deposited on the ocean bottom as they melted. The flood of meltwater in the North Atlantic was so great that it disrupted the circulation of the ocean, which affected climate globally. All this happened in only about 500 years.
Rest assured though: it’s only a theory, and these events occur only during ice ages. It was put forth by a German scientist named Heinrich, who fit together pieces of evidence from the climate record and from the bottom of the Atlantic Ocean. So-called “Heinrich events” occur when large icebergs drift down from the poles and disrupt the global climate.
But until now, no one could explain why most of the icebergs 60,000 years ago seemed to have come from the Labrador Sea.
In a paper recently published in the journal Nature, Dr. Brian Arbic, a research oceanographer at Princeton University, and a team of researchers that included Prof. Jerry Mitrovica in the department of physics at U of T, believe they have done just that.
“Big tides may explain why the ice came from that spot,” Arbic told The Varsity in a telephone interview. Ocean tides are Arbic’s area of expertise: he has developed a simulation that predicts the height of tides throughout the world with 92 per cent accuracy.
Tides are caused by the Moon’s force of gravity pulling on the Earth. “The part of the Earth facing the Moon is closer to it,” explained Arbic. “So it feels a stronger pull, whereas the opposite side of the Earth feels a weaker pull.” This difference in forces produces a wave on the side of the Earth that is being pulled more strongly-the ocean is literally pulled towards the Moon. This is known as high tide. On the opposite side of the Earth the opposite of a wave is produced-low tide. Since the Moon and Earth rotate around each other, the high tide constantly circles the globe as it follows the pull of the Moon.
Using estimates of global sea levels from Mitrovica, Arbic simulated the global height of tides during the times Heinrich events are known to have occurred. His simulation indicated that tides in the Labrador Sea were much higher than they are now, ranging from 2.7 to 3.9 meters. Presently, their height is only about 1.5 metres. Arbic believes these higher tides weakened the ice shelf -a tongue of ice on the surface of the ocean hundreds of metres thick-that covered the Labrador Sea.
The researchers were most surprised when they superimposed global tide levels onto a map. “You get this bulls-eye of incredibly large tides just over the area people had surmised was the source of the iceberg armadas,” said Mitrovica. “Those tides just pounded,” he said, and most likely broke off huge icebergs.
The important role ocean tides play in Heinrich events may lead scientists to consider the effect of tides in other climate enigmas, according to Mitrovica. “This is the first time, I think, that tides have been implicated in a climate story.”
A more complete understanding of Heinrich events is important as well. “It will help us understand what natural climate cycles the Earth can go through,” he said.
