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Sep 03 2013

Pleistocene may have ended with a bang

The end of the most recent ice age is an intriguing time in natural history, particularly in the northern hemisphere. Evidence shows a sudden savage cold snap lasting centuries affecting a region stretching from the upper eastern seaboard of the US to the British Isles and beyond beginning about 12,900 years ago. A number of Ice-age mega-fauna which had withstood multiple cycles over millions of years also went extinct around the same time.

Scientists refer to this period as the Younga-Dryas, named after a tell-tale flowering plant that flourished in colder conditions. The prime suspect was the melting of ice sheets in North America leading to a release of fresh water in the North Atlantic causing a temporary shut down in the top of Atlantic conveyor. A new study turned up another possible factor, a big rock from space:

Red Orbit — A new study, funded by the National Science Foundation and to be published online in the Proceedings of the National Academy of Sciences (PNAS) Early Edition, has found that a cataclysmic asteroid or comet impact in the Canadian province of Quebec nearly 13,000 years ago wiped out many of the world’s large mammals and may have prompted early humans to move from a hunting lifestyle to start growing and gathering some of their food.

Dartmouth College researchers explain that this impact, which occurred at the beginning of the Younger Dryas period, marks an abrupt global shift to colder climes that had far-reaching effects on both animals and humans. In North America, large mammals such as mastodons, camels, giant sloths and saber-toothed cats all disappeared due to the cataclysmic event. Because of this, human hunters, known as the Clovis people, started taking a hunter-gatherer lifestyle, subsisting not only on smaller game, but also roots and berries.

So far scientists do not have a candidate crater for such an event. But fooling around with an impact simulator may give us grounds to speculate why. For starters, this wouldn’t be a dino killer or anything close to it — if that had happened 13,000 years ago the dramatic effects would still be with us and visible world-wide! This would more likely be a smallish asteroid as big impacts go, perhaps in the neighborhood of 250m to 1000m in diameter. Second, objects like this can often calve into two or more pieces when they pass a large object like a planet and, depending on composition and other factors, it will break up in the atmosphere to some degree on top of that. The end result could be a sort of scatter-shot impact zone instead of a single distinct impact. Lastly, if it hit 13,000 years ago in Quebec, there’s a good chance it hit thick surface ice or a big, fat ice shelf.

I couldn’t find an impact simulator that took a thick icy surface into account, our best bet would to model the effects would then be deep water. The Laurantide ice sheet which covered much of Quebec was up to two or more miles thick in some places, I used 3000 meters to simulate a full mile of packed glacial ice. Assuming an object 500 meters in diameter made of intermediate to low density rock hitting at a 45 degree angle at 25 k/s, and using the Earth Impact Effects Program here, we get the following hypothetical results.

The result of the impact is a crater field, not a single crater. The following dimensions are for the crater in the sea floor produced by the largest fragment: Final Crater Diameter: 131 meters (428 feet), Final Crater Depth: 27.8 meters (91.1 feet).

That’s a hell of an impact for anyone near the region, about 1000 to 10,000 times more powerful than the biggest hydrogen bomb ever detonated. But the largest scar is noticeably smaller than the famous impact crater in Arizona. It’s small enough that subsequent movement of ice and water on the now chaotic surface could smooth it out and fill it in quickly, making it hard to find unless you know exactly where to look. In addition, much of the eastern edge of that region, areas that were dry land due to lower sea levels during the end of the Pleistocene, are now covered by water.

It is estimated an impact of that magnitude occurs on average about once every 100,000 to one-million years. But not all large impacts are equal, depending on the conditions on the ground, surface geology, and what’s going on in the local ecology, their effects can be amplified or dampened. It seems reasonable that an impact like this might trigger the collapse of nearby preexisting ice dams leading to the release of vast quantities of fresh water into the Northern Atlantic, in turn causing a conveyor shutdown as already theorized, as well as causing localized nuclear winter scenarios, torrential acid rainstorms and blizzards, or other chain reactions we have not yet thought about.

4 comments

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  1. 1
    StevoR : Free West Papua, free Tibet, let the Chagossians return!

    Great post DarkSyde – shared. Cheers.

  2. 2
    David Marjanović

    Hm, I wonder. The idea is a few years old and has been generally rejected because there’s basically no evidence of an impact. I’ll have to read the PNAS paper when it actually comes out.

  3. 3
    OldEd

    Just a minor quibble: it is the YOUNGER DRYAS, not the Younga-Dryas.

    A major quibble: the evidence for an impact is extremely thin, and the hypothesis has little support from the geophysical community.

  4. 4
    lpetrich

    DarkSyde’s video is about older and bigger ones, the biggest of them being the 300-km/200-mi Vredefort Dome in South Africa about 2 billion years ago. Back then, the continents were smaller and unrecognizable, cyanobacteria had just started pumping oxygen into the atmosphere, and eukaryotedom had either just originated or was as much as a few hundred million years in the future.

    There’s evidence of an even older one: The remains of a gigantic, three-billion-year-old meteorite impact discovered in Greenland

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