Monday, June 21, 2010

A More Disconcerting Cold Snap

It has been happily assumed in the past that the drainage of Lake Agassiz was through the St Laurence valley or possibly through the Mississippi.  I do not know of any evidence to support that at all.  At least this gives us plain evidence of strong currents carrying debris out onto the continental shelf in the Mackenzie delta.

When you consider the earlier claims, there should be a succession of lag deposits mapping the outlet channels in the same way the badlands of Oregon map a similar release of melt water.

Anyway, this will liven the debate and I am sure many eyeballs will be looking for confirmation of all three possibilities.  This at least ends the blind acceptance of the previous paradigm that filled textbooks.

A more disconcerting cold snap

cold snap that began about 6250 BC is attributed to catastrophic drainage of Lake Agassiz-Ojibway. Meltwater from the waning Laurentide Ice Sheet, ponded north of the Arctic-Gulf of Mexico drainage divide, was able to force open a channel beneath the ice and pour enough fresh water into the north Atlantic Ocean, via Hudson Strait, to weaken the meridional overturning circulation.

The palaeoclimatic records of the ice age tell us of other cold snaps lasting up to a millennium or two. The longest and best known, the Younger Dryas, began about 11,000 BC, lasted about 1,300 years, and is also thought to have been due to catastrophic drainage of North American meltwater.

The Younger Dryas is named after Dryas octopetala, a flower which is an indicator for the episode in records from Scandinavian peat bogs. And yes, there was an Older Dryas, and even an Oldest Dryas, but they are much less prominent and may not have been worldwide.

Dryas integrifolia, a cousin of the Younger Dryas’s eponymous D. octopetala, growing on Ellesmere Island in northern Canada. Both are also referred to as the “mountain avens”.

Now Julian Murton and co-authors describe what they believe to be evidence of the flood that triggered the Younger Dryas cooling. They worked on Richards Island, at the eastern edge of the Mackenzie River delta. The evidence consists in essence of an erosion surface truncating older glacial till and overlain by coarse gravel. The gravel is interpreted as a lag deposit, left over when the flood waters ceased to be capable of continuing to carry it.

The interpretation is rather persuasive. The dates bracket the events, and match the onset date of the Younger Dryas, about as closely as could be asked for. The gravel is pebbly to bouldery in size, not at all what would be expected in the delta of one of the world’s biggest rivers. The ice-sheet margin of the time lay 600 km to the south, ruling out a local source of glacial meltwater. And there is a possible candidate for the lake outlet from which the flood might have issued, near Fort McMurray in northern Alberta, where the ice sheet formed a dam by abutting on hills to the west.

Persuasive as it is, this may well prove controversial. I have some naive questions of my own. For example, even this putative flood might have had trouble carrying boulders the 2,000-plus km from Fort McMurray, at 370 m above modern sea level (lower at the time, because of differential rebound of the land surface) to Richards Island. And if the boulders didn’t come from the lake outlet or the uplands, where did they come from? And I am not clear at all about why the authors reject the Saint Lawrence as a possible outlet. They don’t seem to have evidence against it, just evidence in favour of a different outlet.

On a personal level, I can’t decide whether to be happy or sad about the relocation of the Younger Dryas flood to the Mackenzie and away from the Saint Lawrence. The building that houses my office sits on the floor of a spillway that drained meltwater from the ancestral Great Lakes, and perhaps even Lake Agassiz further to the north, for a brief period at about the right time. But the spillway’s cross-sectional area is only 50-100 times that of the modern river which occupies a small part of its floor. That is probably not big enough. The evidence offered by Murton and colleagues suggests a flood many kilometres wide and up to tens of metres deep. There are other spillways, to the north of mine, which also fed meltwater to the Saint Lawrence, but their carrying capacities, dates and durations of occupancy are not pinned down as well as they need to be.

A number of further questions arise. Is it just a coincidence that both this cold snap and that at 6250 BC show signs of having been double bursts? I can’t think of a plausible mechanism that would require these floods to happen in exactly two stages. But perhaps there is a prosaic explanation in terms of fluctuations of the ice-sheet margin. Repeated brief advances might close multiple spillways, not just two, and therefore produce multiple incarnations of the lake with its surface at different elevations at different times. Second, when is somebody going to find evidence of catastrophic drainage of the Eurasian equivalents of Lake Agassiz? Were there such outbursts? Finally and most generally, why does caprice, in the form of unpredictable outbursts of meltwater, seem to play such a substantial role in the evolution of past climates?
Posted by Graham Cogley on April 26, 2010

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