Lake Superior did not Cause 10.6 kya Cooling Event

An analysis of the late glacial lake levels within the western Lake Superior basin based on digital elevation models

Author:

Andy Breckenridge

Abstract:

This study establishes a detailed lake-level history for the Lake Superior basin by mapping strandlines from 10-m and 3-m digital elevation models. There are 24 levels above the mid-Holocene Nipissing level, and elevations increase along a direction of 23.1° due to post-glacial rebound. The highest level, the Epi-Duluth, is steeper than subsequent levels and may pre-date the Lake View ice advance into the western Lake Superior basin at the end of the Younger Dryas stade. The most prominent level is the Duluth, ca. 10,800 cal yr BP. Ice retreat exposed successively lower outlets, routing overflow to the Lake Michigan and Huron basins. By 10,600 cal yr BP, lake levels in the western Superior basin had dropped almost 200 m. This transformative period is complicated by multiple basin-wide events: the influx of glacial Lake Agassiz overflow, the creation of three sub-aqueous moraines, and a red to gray color transition in basin sediments. A later drawdown event has been hypothesized to have initiated the 9300 cal yr BP cooling event, but this flood was much smaller than estimated previously. If freshwater triggered the 9300 cal yr BP event, the source of the water must have been Lake Agassiz, not Lake Superior.

What Caused the Low-water Phase of Glacial Lake Agassiz?

What caused the low-water phase of glacial Lake Agassiz?

Authors:

Lowell et al

Abstract:

First-order modeling suggests that a low-water phase in late-glacial Lake Agassiz can be explained through changes in the balance between evaporation, precipitation, and runoff, rather than drainage. The low-water Moorhead Phase is often attributed to drainage through outlets opened by isostatic depression and retreat of the Laurentide ice margin. However, new data indicate that the proposed outlets were ice-covered during the Moorhead Phase. Instead, the lake water levels dropped to the Moorhead Phase before the start of the Younger Dryas chronozone and remained there until 11.3 ka. Thus, drainage seems to be an implausible explanation for Younger Dryas-aged low water levels in Lake Agassiz. An alternative explanation is that evaporation equaled or exceeded water inputs from the adjacent ice margin and the deglaciated parts of the drainage basin. To evaluate whether this hypothesis is plausible, we constructed a simple model that considers the paleo-basin geometry, hydrology, and meltwater production from the adjacent ice margin. Modest hydrologic changes (within the range of present-day variability), coupled with low meltwater production, produce a closed basin. Shifts in the location of the polar jet, driven by increased Arctic albedo, may explain our inferred hydrologic changes.

Draining of Lake Agassiz Caused Younger Dryas Snap Freezing

Caption: A new model of flood waters from melting of the Laurentide Ice Sheet and large glacial lakes along its edge that covered much of North America from the Arctic south to New England over 13,000 years ago, shows the meltwater flowed northwest into the Arctic first. This weakened deep ocean circulation and led to Earth’s last major cold period. A new model of flood waters from melting of the Laurentide Ice Sheet and large glacial lakes along its edge that covered much of North America from the Arctic south to New England over 13,000 years ago, shows the meltwater flowed northwest into the Arctic first. This weakened deep ocean circulation and led to Earth’s last major cold period.

For more than 30 years, climate scientists have debated whether flood waters from melting of the enormous Laurentide Ice Sheet, which ushered in the last major cold episode on Earth about 12,900 years ago, flowed northwest into the Arctic first, or east via the Gulf of St. Lawrence, to weaken ocean thermohaline circulation and have a frigid effect on global climate.

Now University of Massachusetts Amherst geoscientist Alan Condron, with Peter Winsor at the University of Alaska, using new, high-resolution global ocean circulation models, report the first conclusive evidence that this flood must have flowed north into the Arctic first down the Mackenzie River valley. They also show that if it had flowed east into the St. Lawrence River valley, Earth's climate would have remained relatively unchanged.

"This episode was the last time the Earth underwent a major cooling, so understanding exactly what caused it is very important for understanding how our modern-day climate might change in the future," says Condron of UMass Amherst's Climate System Research Center. Findings appear in the current issue of Proceedings of the National Academy of Sciences.

Events leading up to the sharp climate-cooling period known as the Younger Dryas, or more familiarly as the "Big Freeze," unfolded after glacial Lake Agassiz, at the southern edge of the Laurentide ice sheet covering Hudson Bay and much of the Canadian Arctic, catastrophically broke through an ice dam and rapidly dumped thousands of cubic kilometers of fresh water into the ocean.

This massive influx of frigid fresh water injected over the surface of the ocean is assumed to have halted the sinking of very dense, saltier, colder water in the North Atlantic that drives the large-scale ocean circulation, the thermohaline circulation, that transports heat to Europe and North America. The weakening of this circulation caused by the flood resulted in the dramatic cooling of North America and Europe.

Using their high resolution, global, ocean-ice circulation model that is 10 to 20 times more powerful than previously attainable, Condron and Winsor compared how meltwater from the two different drainage outlets was delivered to the sinking regions in the North Atlantic. They found the original hypothesis proposed in 1989 by Wally Broecker of Columbia University suggesting that Lake Aggasiz drained into the North Atlantic down the St. Lawrence River would have weakened the thermohaline circulation by less than 15 percent.

Condron and Winsor say this level of weakening is unlikely to have accounted for the 1,000-year cold climate event that followed the meltwater flood. Meltwater from the St. Lawrence River actually ends up almost 1,900 miles (3,000 km) south of the deep water formation regions, too far south to have any significant impact on the sinking of surface waters, which explains why the impact on the thermohaline circulation is so minor.

By contrast, Condron and Winsor's model shows that when the meltwater first drains into the Arctic Ocean, narrow coastal boundary currents can efficiently deliver it to the deep water formation regions of the sub-polar north Atlantic, weakening the thermohaline circulation by more than 30 percent. They conclude that this scenario, showing meltwater discharged first into the Arctic rather than down the St. Lawrence valley, is "more likely to have triggered the Younger Dryas cooling."

Condron and Windor's model runs on one of the world's top supercomputers at the National Energy Research Science Computing Center in Berkeley, Calif. The authors say, "With this higher resolution modeling, our ability to capture narrow ocean currents dramatically improves our understanding of where the fresh water may be going."

People ask me what we run on our dayjob supercomputers. Well. There ya go.

Lake Agassiz Megaflood Caused Younger Dryas?

The main cause of a rapid global cooling period, known as the Big Freeze or Younger Dryas - which occurred nearly 13,000 years ago - has been identified thanks to the help of an academic at the University of Sheffield.

A new paper, which is published in Nature today (1 April 2010), has identified a mega-flood path across North America which channelled melt-water from a giant ice sheet into the oceans and triggering the Younger Dryas cold snap.

The research team, which included Dr Mark Bateman from the University of Sheffield's Department of Geography, discovered that a mega-flood, caused by the melting of the Laurentide ice sheet, which covered much of North America, was routed up into Canada and into the Arctic Ocean.

This resulted in huge amounts of fresh water mixing with the salt water of the Arctic Ocean. As a result, more sea-ice was created which flowed into the North Atlantic, causing the northward continuation of the Gulf Stream to shut down.

Without the heat being brought across the Atlantic by the Gulf Stream, temperatures in Europe plunged from similar to what they are today, back to glacial temperatures with average winter temperatures of -25oC. This cooling event has become known as the Younger Dryas period with cold conditions lasting about 1400 years. The cold of the Younger Dryas affected many places across the continent, including Yorkshire in the Vale of York and North Lincolnshire which became arctic deserts with sand dunes and no vegetation.

Before now, scientists have speculated that the mega-flood was the main cause of the abrupt cooling period, but the path of the flood waters has long been debated and no convincing evidence had been found establishing a route from the ice-sheet to the North Atlantic.

The research team studied a large number of cliff sections along the Mackenzie Delta and examined the sediments within them. They found that many of the cliff sections showed evidence of sediment erosion. This evidence spanned over a large region at many altitudes, which could only be explained by a mega-flood from the over-spilling of Lake Agassiz, which was at times bigger than the UK, at the front of the Laurentide Ice-sheet rather than a normal flood of the river.

Dr Bateman, who has been researching past environmental changes both in the UK and elsewhere in the world for almost 20 years, runs the luminescence dating lab at Sheffield. The lab was able to take the MacKenzie Delta sediment samples from above and below the mega-flood deposits, and find out when the mega-flood occurred, enabling its occurrence to be attributed to the start of the Younger Dryas.

no time, but it fits, in my mind at least, for the base cause of the Younger Dryas better than comet impact hypothesis.