A giant ice dam cracks in hundreds of places causing deafening roars like thunder. Chunks of shattered ice slide off the rim, plunging into a growing river of water. Behind the ice dam, a great lake swirls furiously, bearing enormous pressure against the frozen barrier, like a wild beast scratching against the wall of a cage. Ravens, ducks, and geese flee the scene. A sudden thunderstorm blackens the sky, bringing more thunder…and rain, the final blow. The precipitation heightens the water level which begins to flow between a submerged crack as well. Angry bubbles blow the fissure open. And the ancient lake rushes to escape its prison. The water explodes, now a waterfall not unlike Niagara. It’s a biblical deluge of icy water roughly following the Nelson River but overflowing its banks for miles. Big chunks of ice collapse into the deluge, carrying boulders, rocks, and topsoil. The water strips trees out of the ground, captures and drowns herds of bison and other poor beasts caught in its stormy path. The rush of water reaches the sea, an army of icebergs and an enormous quantity of fresh, cold, cold water hits the North Atlantic. This frigid water meets the warm gulf stream, and the cold water wins, forcing the warm salty water to sink. It shuts down this conveyor belt, the all important thermohaline circulation that keeps the northern hemisphere climate mild and warm. Earth plunges back into the depths of an Ice Age for another two-thousand years. Once again, the climate has suddenly become cold and arid, and glaciers advance rather than retreat as they had been doing for two-thousand years.
Side scan sonar image of iceberg keel scours off the coast of South Carolina. This image is from the paper referenced at the bottom of this blog entry.
My dramatization above is what scientists refer to as a Heinrich Event, also known as a meltwater pulse. Such events have occurred in regular cycles over the past 11,000 years, but on a much smaller scale than those of the Wisconsinian Ice Age. During Ice Ages, glaciers expand and dam rivers, creating enormous glacial lakes. Warm climate cycles melt these ice dams in a process that takes thousands of years. Interstadials result as warm rainy climate prevails when more and more water is transferred from ice to the atmosphere. Eventually, these ice dams would collapse and the frozen torrent flowed into the North Atlantic, shutting down thermohaline circulation, causing a reversal in climate phases. This explains why the Wisconsinian Ice Age (and all Pleistocene Ice Ages) had rapidly alternating interstadial and stadial phases (though orbital perturbations, the rise of the Himalayas, and other tectonic processes are the ultimate factors behind it all). Today, these cycles are not as dramatic because the north polar ice cap is much smaller and glaciers don’t stretch over all of Canada, like they formerly did.
Evidence of these astonishing Heinrich Events lie on the bottom of the North Atlantic. “Ice-rafted debris” (actually called Heinrich Layers) consisting of drop stones and boulders, as well as sediment, exist deep under the ocean. The rocks originated in what today is mid-western North America and could have only arrived at the bottom of the ocean here, if encased in icebergs because water can’t move heavy boulders, but it can float ice impregnated with these heavy rocks. Numerous furrows also line the North Atlantic floor. Geologists call these furrows keel scours, and they were made when the toes of icebergs scraped along the bottom of the ocean as the flow of water carried them south. The toes of icebergs flipped over rocks and boulders that were in their way, and these are visible as well.
Jenna Hill and five other scientists discovered keel scours off the coast of South Carolina about 660 miles south of where the ice margin was during the last glacial maximum (~29,000-15,000 BP). The scour marks are 500-660 feet under water, so the scientists had to use side scan sonar images to see them. They took these images from the Nancy Foster, a NOAA research ship. The scour marks range in width from 30 feet to about the length of a football field, and they’re about 30 feet deep. Some are as long as 6 or 7 miles, and they’re orented west/southwest which is in the opposite direction of where the gulf stream flows today. Near the end of these furrows are a series of circular pits. Apparently, as the icebergs hit shallow ground, they got stuck. Periodically, they melted, floated a while longer, and got stuck again in a mode of motion resembling a pogo stick.
Imagine vacationing on Myrtle Beach and seeing icebergs and smaller pieces of ice drift by, with seals and walruses sprawled on the latter. Of course, what today is Myrtle Beach was too far inland then for a person to view the ocean. Still, it’s amazing to think how much the world has changed since icebergs floated off the coast of South Carolina.
Hill, Jenna; et. al
“Iceberg scours along the southern U.S. Atlantic Margin”
Geology June 2008