Missoula floods and an energy problem

[Andre]

As I was reading Hanson et al 2012...

Glacial Lake Missoula formed when the Purcell Trench lobe of the Cordilleran ice sheet dammed Clark Fork River in Montana during the Fraser Glaciation (marine oxygen isotope stage 2). Over a period of several thousand years, the lake repeatedly filled and drained through its ice dam, and floodwaters coursed across the landscape in eastern Washington.
(...)
The floods date to between approximately 21.4 and 13.4 cal ka ago based on regional chronological data. ...

That made me think of another recent contemplation about energy required and available for all that melting as per http://dl.dropbox.com/u/22026080/Hare1976.QR.PersistentProblemsEnergytoMeltLIS.pdf .

So Hare triggered me to do some very rough order of magnitude calculations on http://dl.dropbox.com/u/22026080/icemeltenergy.xlsx to find that the melting of the ice sheets in the last glacial maximum would have required some ROM of 30 W/m².

Now looking at the July insolation graph on 65 North latitude lowest one here we see insolation variations of 50-80 W/m², so that seemed not unreasonable initially.

However in combination with Hanson et al 2012 and the earliest dating of the Missoula floodings after 21,400 years ago, a question arises. If you have a flood, you need to melt the ice first and looking at the insolation graph, we were at a minimum at that time and the ice should have been accumulating - insolation wise. So where did the energy come from? What is wrong here?

 

[Barakn]

If you have a flood, you need to melt the ice first

That's the problem right there. If you look at a map of the Lake and surrounding area, you'll realize that what really happened was that the glacier blocked the course of an existing river, the Clark Fork of the Columbia. The Clark Fork (as it is generally referred to) has a drainage area mostly to the south and east of the Purcell lobe, so the water that filled the lake was not necessarily ice melt but whatever kind of water a Montana river normally receives.

 

[Andre]

I meant to return here sooner, but some study needed to be done first, anyway.

With repeated floodings you would expect an ice sheet advancing stage required to restore the ice dam after the breach.

The problem here is that several studies suggest a rather early retreat for the Cordilleran like this here suggesting 16 Ka BP calibrated to about 19.3 Ka Cal BP, so there are several thousand years that an ice dam appeared to have restored by advancing in a generally retreating scenario.

I'll study some more.