Tuesday Morning: Thundering Down the Hudson Strait

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Enlarge ImageIn Winsor's model, fresh water (aqua) flowing out of the Hudson Strait meets but doesn't mix much with salty ocean currents (blue and red). (courtesy Peter Winsor, WHOI)

Fresh Water: Does Not Mix Well with Others

Among people who study the waxing and waning of ice ages (paleoclimatologists), there’s a bit of puzzlement about how two transient cold periods came about. The two periods are the Younger Dryas, when the climate got really cold about 12,000 years ago, and another slightly less cold period about 8,200 years ago.

A favorite explanation for these cold periods is that massive releases of fresh water, freed from huge inland lakes as glaciers retreated, gushed into the North Atlantic and weakened that ocean’s currents. Weaker currents brought less heat into northern latitudes, and the climate got frosty.

The puzzlement comes because best estimates indicate that more freshwater was involved with the weaker, 8,200-year-old cooling than in the stronger one 12,000 years ago. How can this be, the paleoclimatologists wonder. It should have been colder 8,200 years ago.

On Monday WHOI Assistant Scientist Peter Winsor suggested an answer. Winsor and his colleagues modeled the way water poured out of a transient glacial lake (called Lake Agassiz) and, as he put it, “thundered down the Hudson Strait” and out into the North Atlantic. (A similar phenomenon took place 13,350 years ago with glacial Lake Iroquois.)

Their results suggest that as the water poured passed the Grand Banks east of Canada, it would have hit a wall of much denser seawater and abruptly turned south, to trickle along the continental shelf toward Cape Hatteras (in present-day North Carolina). Only a little of the fresh water in the model mixed into the North Atlantic, so it had little effect on the strength of the ocean currents. During the Younger Dryas flood, fresh water took a different route to the ocean, so mixing with salt water would have happened much differently.

As the Earth warms, melting ice from arctic North America and Greenland is pouring vast amounts of fresh water into today’s oceans. So knowing precisely how fresh water mixes with salt water, and how it may have affected climates in the past, may help us plan for our own future. Winsor's model calculated water flow at a new degree of resolution for the size of the area modeled, but he said even finer resolution will be needed to fully understand the problem.

After a long session on ice ages - coincidentally in an excessively air-conditioned room - I was ready for the next research topic: new findings from hot, deep-sea vents.