From the University of Cincinnati: Long-Lost Lake Offers Clues to Climate Change
What caused water levels to drop in an immense yet long-vanished lake? Research by a University of Cincinnati geologist suggests that conditions 12,000 years ago encouraged evaporation.
Not long ago, geologically speaking, a now-vanished lake covered a huge expanse of today’s Canadian prairie. As big as Hudson Bay, the lake was fed by melting glaciers as they receded at the end of the last ice age. At its largest, Glacial Lake Agassiz, as it is known, covered most of the Canadian province of Manitoba, plus a good part of western Ontario. A southern arm straddled the Minnesota-North Dakota border.
Not far from the ancient shore of Lake Agassiz, University of Cincinnati Professor of Geology Thomas Lowell will present a paper about the lake to the Geological Society of America annual meeting in Minneapolis. Lowell’s paper is one of 14 to be presented Oct. 10 in a session titled: “Glacial Lake Agassiz—Its History and Influence on North America and on Global Systems: In Honor of James T. Teller.”
Although Lake Agassiz is gone, questions about its origin and disappearance remain. Answers to those questions may provide clues to our future climate. One question involves Lake Agassiz’ role in a thousand-year cold snap known as the Younger Dryas.
As the last ice age ended, thousands of years of warming temperatures were interrupted by an abrupt shift to cold. Tundra conditions expanded southward, to cover the land exposed as the forests retreated. This colder climate is marked in the fossil record by a flowering plant known as Dryas, which gives the period its name.
“My work focuses on abrupt or rapid climate change,” Lowell said. “The Younger Dryas offers an opportunity to study such change. The climate then went from warming to cooling very rapidly, in less than 30 years or so.”
Scientists noted that the Younger Dryas cold spell seemed to coincide with lower water levels in Lake Agassiz. Had the lake drained? And, if so, had the fresh water of the lake caused this climate change by disrupting ocean currents? This is the view of many scientists, Lowell said.
Lowell investigated a long-standing mystery involving Lake Agassiz – a significant drop in water level known as the Moorhead Low. It has long been believed that the Moorehead Low when water drained from Lake Agassiz through a new drainage pathway. Could this drainage have flowed through the St. Lawrence Seaway into the North Atlantic Ocean?
“The most common hypothesis for catastrophic lowering is a change in drainage pathways,” Lowell said.
The problem is, better dating of lake levels and associated organic materials do not support a rapid outflow at the right time.
“An alternative explanation is needed,” he said.
Lowell’s research shows that, although water levels did drop, the surface area of the lake increased more than seven-fold at the same time. His research suggests that the lower water levels were caused by increased evaporation, not outflow. While the melting glacier produced a lot of water, Lowell notes that the Moorhead Low was roughly contemporaneous with the Younger Dryas cold interval, when the atmosphere was drier and there was increased solar radiation.
“The dry air would reduce rainfall and enhance evaporation,” Lowell said. “The cold would reduce meltwater production, and shortwave radiation would enhance evaporation when the lake was not frozen and sublimation when the lake was ice-covered.”
Further research will attempt a clearer picture of this ancient episode, but researchers will have to incorporate various factors including humidity, yearly duration of lake ice, annual temperature, and a better understanding of how and where meltwater flowed from the receding glaciers.
Lowell’s efforts to understand changes in ancient climates have taken him from Alaska to Peru, throughout northern Canada and Greenland.
In Greenland, Lowell and a team of graduate students pulled cores of sediment from lakes that are still ice-covered for most of the year. Buried in those sediments are clues to long-ago climate.
“We look at the mineralogy of the sediments,” Lowell said, “and also the chironomids. They’re a type of midge and they’re very temperature sensitive. The exact species and the abundance of midges in our cores can help pinpoint temperature when these sediments were deposited.”
Lowell’s research was initially funded by the Comer Foundation. In recent years, the National Science Foundation has provided funding for this work.
When the Geological Society of America meets this year the University of Cincinnati will be well represented, with more than two dozen papers and presentations. Topics range from ice-age climate to the health effects of corrosion in drinking water pipes.
Photos By: Lisa Ventre
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See, the science is settled. Clearly the lake was destroyed by soccer moms driving their SUV’s. Stop the madness.
What? Younger Dryas you say? Ummm my name is not Dryas and I don’t have a little brother. You are clearly a mouthpiece for big oil, you should be ashamed of yourself.
Eh? 12,000 years ago? Well, of course this disaster happend then, Al Gore was not there to save those poor people. Just think about how much worse it would have been if western civilization had been around to destroy the environment then. Those were the days.
I hope I didn’t need a /sarc tag for that last post. 🙂
If this is truly the case, the mystery still remains as to what shut down the AMOC, and brought about the Younger Dryas period.
Very interesting! I had read that the outflow of Lake Agassiz as cause of the Younger Dryas was already problematic, as no outflow path of the right period for huge quantities in short time were found into the North Atlantic via the Hudson Bay or elsewhere. Remains the still open question what caused the Younger Dryas.
For a following sudden cooler period, the 8.2 kyr event, there are more indications that a sudden drainage of Lake Agassiz was the cause:
http://pangea.stanford.edu/research/Oceans/GES205/superlakes.pdf
I thought the lake drained south out of White Rock and ended up in the gulf. I live in the area and so this is a very Interesting article.
I’m not a quick study, but how does one lower water levels and increase the surface area by 7, isostatic tilting?
It was isostatic rebound that drained the lake
As a multi-kilometer thick ice cap melts away, the land rebounds significantly. This pushes the water around and any damming structures (terminal moraines, eskers or other fluvio-glacial deposits) can be circumvented or even washed away. The increase in area is understandable as flooding low-lying areas that were in the process of rebounding would further the process. It must have been one heck of a time to observe nature.
I’ve been a fan of Lake Agassiz since growing up in Minnesota. I actually find the whole idea of evaporation hard to grasp. The only present day equivalent I can think of is the Aral Sea, and it has been caused by humanoids!
R. Gates says:
October 5, 2011 at 2:02 pm
You are assuming that the shutting of the AMOC caused the Younger Dryas.
Steve R says: “It was isostatic rebound that drained the lake”
Sounds good to me.
It was my understanding that the AMOC shutdown is/was a proposed trigger for the Younger Dryas rather than a proven event for which the emptying of Lake Agassiz was proposed as a contributing cause.
I suspect that Lowell’s paper will not be universally embraced and that his reference to the need for additional research is warranted.
Interesting theories are proposed and rejected as the key elements of those theories are tested and fail.
Science plods on. Each study adding to or subtracting from our common understanding of how the world works.
Land rebounds – surely – isostatically.
But, I thought – slowly. The island of Great Britain – England Scotland and wales, is still rebonding – I understand – and levelling out [North end up, South end down], per my school history/geography.
london is going down – hence the Thames Barrier.
Crustal rebound would answer that question nicely. The Glacier melted and the depressed rock basin formed by the glacer filled with water, then as the crust rebounded as the ice load disappeared, and the water evaporated the depression would get more and more shallow but the water would spread out creating more surface area and faster evaporation. More water evaporates, more load is removed from the crust it rebounds more etc. etc.
Larry
“Tundra conditions expanded southward, to cover the land exposed as the forests retreated.”
“when the atmosphere was drier and there was increased solar radiation.”
So the mid latitude jets shifted equatorward leaving the area concerned under drier polar high pressure cells from a strongly negative AO and / or contracting equatorial air masses from cooler ocean surfaces. Most likely both as the negative AO increased global cloudiness and reduced solar input to the oceans.
Why not make the intuitive leap to latitudinally shifting climate zones as the regulatory process achieving a relatively stable global temperature for some 4.5 billion years ?
Such shifting climate zones represent changes in the speed and/or volume of the water cycle exerting a negative system response to any forcing whether it be an internal system forcing or an external solar forcing.
It really is that simple.
“And, if so, had the fresh water of the lake caused this climate change by disrupting ocean currents? This is the view of many scientists, Lowell said.”
In which Ocean? Lake Agassiz is a long way from any ocean.
R. Gates says:
October 5, 2011 at 2:02 pm
If this is truly the case, the mystery still remains as to what shut down the AMOC, and brought about the Younger Dryas period.
Whatever it was, it ran amoc. It sure wasn’t anthropogenic…..
To Auto:
There are two types of rebound, elastic and inelastic (or viscous, or plastic).. The former is instantaneous; the latter is gradual. –AGF
One way or the other, the water must have ended flowing through the St-Laurence and the Hudson Bay anyway… when water evaporates is snows or rains somewhere down East.
My geology is a bit rusty, but would not all that glacial melt include a lot of clastics that would help fill-up the lake?
Also is not the natural progression lake=>swamp=>water meadow=>prairie?
It is believed that at its higher water level points, the lake drained into the Glacial River Warren, now the Minnesota River. Evidence for this is the massive river valley that the relatively small Minnesota river lies in. We also have evidence near Bemidji (roughly at the northernmost point of the Mississippi River in north central MN and about 30 miles south of the old lake shore) that there were massive outflows.. though not necessarily from Agassiz. There is a tiny creek, Kabekona Creek, that lies in a huge river valley that would seem more akin to the Mississippi River than a small creek that you could probably lie across and touch both banks. It’s interesting how the area surrounding Bemidji is generally hilly… but you head north, and the pine and hardwood forest covered hills and lakes turn into a giant, flat, bog covered basin that encompasses nearly all of north-central MN. Red Lake is at the center of this. One more little tidbit: Lakes Manitoba and Winnipeg are the remaining bodies of glacial lake Agassiz.
I thought the mammoths drank it all up. Then they went south to pee.
@Steve R October 5, 2011 at 2:16 pm:
I’ve not heard anywhere of any rate of rebound, but I strongly suspect it was rather slow (in human terms), at the rate the ice was thinning.
From https://www.dmr.nd.gov/ndgs/ndnotes/Rebound/Glacial%20Rebound.htm, this from “GLACIAL REBOUND, WARPED BEACHES AND THE THICKNESS OF THE GLACIERS IN NORTH DAKOTA,” by John P. Bluemle:
If true, this says that not so much isostatic rebound happened after Lake Agassiz formed, perhaps only about one-quarter. And at what rate was it rebounding at its peak (during the thinning, I expect)? And how representative was N Dakota in terms of the ice sheet thickness and isostatic rebound, not to mention Lake Agassiz?
Bleumle’s unofficial opinion about the ice sheet thickness is that
If about 62% of Hudson Bay’s thickness, then one would assume the weight to be about 62%, suggesting the isostatic rebound was about 62%, too. And does he give any numbers on this?
Now, if even at its maximum the rebound was only 4 inches a year (33/100 = 1/3 of a foot per year), it brings up the possibility that Lake Agassiz slowly emptied itself. No outrush was required. And evaporation would not be a significant factor. 4 inches a year doesn’t even portend a dam eroding till it gave way. With Hudson Bay taking 5,000 years less time to rebound and Lake Agassiz straddling the distance between ND and Hudson Bay, the lake’s average would be about midway between the ND rate and Hudson Bay’s rate. Therefore, the average of the maximum rebound rate for Lake Agassiz would have been something on the order of half or less of that 4 inches per year.
Clearly, no Scabland-like wall of water was necessary to empty Lake Agassiz.
@A G Foster October 5, 2011 at 2:59 pm:
Not the whole story, AGF. If the stress is reduced gradually, the elastic rebound – though instantaneous – proceeds as gradually. Pound of pressure per pound of pressure, as I understand it. If it took 10,000 years for the ice sheet to melt, then the elastic rebound portion of it all took that long to drop to nil. 10,000 years (my number pulled out of my rear end, not theirs) would result in 8,000 ft/10,000 years = 0.8 feet per year less weight of ice.
I wonder if they figure the inelastic rebound from the end of the melting or the beginning? If not the beginning, I’d wonder why not.
I think Thomas Lowell should look at the geology and size of the river valley of the Minnesota River from Brown’s Valley to the confluence of the Mississippi. And then compare the sizes of the Upper Mississippi and Minnesota Rivers to the combined rivers known as the Mississippi River from Fort Snelling to the Gulf of Mexico. What caused the noticeable difference? Evaporation? Nah. I don’t think so. But I suppose I should read his thoughts before really “damming” him.
I don’t think this is correct.
As Bryan Short mentioned above, the Lake drained south through the River Warren until lake levels dropped enough so that the weight of the water was pushing against the glaciers over Hudson Bay and the Lake wasn’t high enough to overflow the continental divide at Traverse Gap Minnesota.
You can trace all these glacier meltwater outflows through the relief maps to this day. This is a nice one for the River Warren. Glacial Lake Superior and Glacial Lake Michigan also emptied through the southern part of this channel. At Chicago, they built a canal to move ships from Lake Michigan to the Mississippi over a similar divide.
http://en.wikipedia.org/wiki/File:River_Warren_Valley.gif
Traverse Gap, the north-south continental divide where Lake Agassiz literally ended.
http://en.wikipedia.org/wiki/File:Browns_Valley_flood_07.jpg
Once lake levels were below this level, it drained into Hudson Bay which is nothing but downhill and always has been. Once the glaciers melted enough, the Lake basically occupied Hudson Bay. Eventually, the fresh water broke out into the Atlantic but it was not the cause of the (much smaller than thought) Younger Dryas which has been way overblown by the usual suspects.
Other glacial meltwater outflow channels would include Lake Champlain (until the St. Lawrence opened up).
http://birrell.org/andrew/reliefMaps/image.php?zone=east&type=jpg&scale=25
Large size slow download relief map of the US which should be clear enough to see where the glacial outflow waters went when they were blocked by glaciers to the north.
http://birrell.org/andrew/reliefMaps/image.php?zone=all&type=jpg&scale=25