Mike Casey
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The onset of the Younger Dryas, 12,900 years ago, may have been the result of an asteroid or comet impact. A layer of organic carbon produced by intense fires, a large number of magnetic microspherules, and a noticeable peak in the abundance of iridium point strongly to an impact which caused atmospheric cooling, the advance of glaciation, the termination of the Clovis culture, and the widespread extinction of Pleistocene megafauna, including mammoths and ground sloths. The link below points to a most interesting PDF file:
http://www.pnas.org/cgi/reprint/104/41/16016
http://www.pnas.org/cgi/content/full/0706977104/DC1
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Mike (tVA)
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LittleDob
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This hypothesis has received considerable interest and is not without skeptics. One reason for the skepticism is that the evidence for an impact is not found at other sites in North America - where the impact is postulated to have occurred. No impact site has been identified. While it could have been a Tunguska-like event, it is difficult to scale such an event to trigger a global response without more widespread evidence of the impact.
There is also pretty good evidence that the North Atlantic thermohaline circulation stopped during this time. How would an impact have contributed to this? Or is it purely a coincidence that the impact occurred around the time that Lake Agassiz was draining into the North Atlantic?
Speculation is that if an event occurred, it likely happened near the Great Lakes. Here is a funny thought... How unbelievably fluky would it be to have an impact hit the receding glacier near the Great Lakes, which then broke the ice-dam that was holding back Lake Agassiz from draining through the St Lawrence and then triggering the shutdown of the North Atlantic current!!!!! 
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Jason
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Mike Casey
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Quote:
This hypothesis has received considerable interest and is not without skeptics. One reason for the skepticism is that the evidence for an impact is not found at other sites in North America - where the impact is postulated to have occurred.
Jason, if you have Google Earth, go to 33°48'19.15"N, 78°51'43.55W. There you will find a purported strewn field identified as the Carolina Bays Impact Features. The dating indicates that they were created ~12,900 years ago. The field sure looks like it was created by a series of oblique impacts. Did you look at the page in the second link I posted? I think that the evidence is very strong that a group of asteroids or a cometary impact did occur in this area.
It is very interesting to consider that a serious impact sequence occurred only thirteen thousand years ago: a fraction of a second in geologic time. What happened then could happen again in our lifetime.
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Mike (tVA)
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LittleDob
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Hi Mike,
The Carolina Bays are an interesting feature. For years, multiple hypotheses were put forth to explain their origins, including impacts from a single major bollide that broke-up upon entry. Another plausible hypothesis is that they are a thermokarst feature developed in permafrost during the late glacial (the time of the YD). Similar features are known throughout Alaska and the Territories. The oblique geometry and uniform orientation may reflect the dominant winds - some of these features in Alaska show this and actually look square!
Don't get me wrong, I'm not dismissing the impact hypothesis. I accept it for what it is, a hypothesis. Hopefully more data and observations will become available to either support or reject it. Right now, the available data seems to support such an event. What is really interesting are the teleconnections or linkages of the event to a globally expressed climatic excursion. If we could unravel these mechanisms, we would certainly have a better understanding of global climate.
What also amazes me is the amount of data that has been collected on the Younger Dryas in just the last 10 years. Back then, there was still debate over whether this was a global event or a regional (North Atlantic) event. Now, there are high-resolution paleoclimatic records tracking this climatic event aroung the planet. Very interesting times for paleoclimatologits and Quaternary geologists indeed!!
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Jason
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Mike Casey
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Jason, I find it interesting that Lake Waccamaw, located at 34°17’10.73”N, 78°30’35.06”, seems to have the same directionally oblique symmetry as the Carolina Bays features, though quite a bit larger. Though dated at 15,000 years old*, I wonder if a search of the lake bed might turn up anything related to an extraterrestrial impact? The linked paper deals almost completely with the biology of the lake bed and does not specify whether a search for magnetic microspherules or iridium was included in the survey. Core samples were taken to a maximum depth of only 200 cm or a very shallow 6.56 ft. I wonder if deeper coring would find a layer of organic carbon.
* http://abob.libs.uga.edu/bobk/wacbay.html
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Mike (tVA)
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LittleDob
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From the link you provided, it would seem that Waccamaw may have initially formed by crustal rebound as the ice retreated and was then sculpted by wind processes:
"The unusually large size of Lake Waccamaw, its unique position astride a major river, and the presence of marine bluffs along the north shore of the lake argue strongly for a tectonic initiation of the lake near the close of the Pleistocene; wind and wave action subsequently led to the development of its elliptical shape as was probably the case with the other Carolina Bays (Kaczorowski, 1977)."
It would be good to know if anyone has detected the fingerprints of an impact in Agassiz sediments. I spoke to some folks at the U of Minn and they were not aware of any studies. Such a prominent event over northeastern NA should preserve some signature in the extensive catchment basin that was Lake Agassiz.
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Jason
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FirstSight
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I grew up near the Carolina Bays area, and whether correct or not, the meteorite theory of their formation was part of commonly accepted local lore.
Here's a website about the Carolina Bays and their origins, not only filled with lots of photographs and extensive substantive content on its own, but with plenty of links to other relevant sites.
http://www.georgehoward.net/cbays.htm
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Chris M., aka "First Sight"
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llanitedave
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Count me as a skeptic, not of the impact evidence itself, but of making it responsible for the Younger Dryas and the Pleistocene extinctions. Maybe my reading is outdated, but I understood the Clovis period to have ended hundreds of years after the time of the purported impact -- if not at the specific sites mentioned then elsewhere, and much of the Pleistocene megafauna also survived until more recent times.
The purported lack of "kill sites" mentioned by the PNAS paper doesn't bother me. That does not mean that hunting pressures by early humans were not severe. Habitation sites from this period are also lacking. But we know humans lived in the area.
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Mike Casey
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 Thanks for that great link, Chris. What a goldmine of information! I'll be there for quite awhile.
I came across this interesting little gem while browsing the many links available on the subject:
“Examination of impact mechanics and Carolina Bay morphometry eliminates traditional impact phenomena resulting from meteoroid swarms or asteroids. However, the unique orbital and physical characteristics of a comet favor a model in which a high velocity retrograde comet or a low velocity prograde comet collided with the Earth. The incoming nucleus approached from the northwest and fragmented. The fragments, diverging from the main trajectory, volatized and subsequently exploded in the atmosphere near the surface. The resultant shock waves created shallow elliptical depressions which are best displayed in the sandy sediments of the Coastal Plain.”
A RE-EVALUATION OF THE EXTRATERRESTRIAL ORIGIN OF THE CAROLINA BAYS*
by J. Ronald Eyton & Judith I. Parkhurst
http://abob.libs.uga.edu/bobk/cbayint.html
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Mike (tVA)
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D_talley
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This is just in the news. Great beasts peppered from space
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LittleDob
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That is amazing! I like the little photo showing an actual piece of a meteorite fragment embedded in the tusk. It is also quite interesting that tusks in both Alaska and Siberia show similar features. The timing of the event discords with the younger dryas, but serves as a reminder that impacts over these time-scales can and do have an effect on life!
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Jason
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Mike Casey
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That is indeed fascinating news, Dwight. Another interesting file to add to my Carolina Bays folder.
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Mike (tVA)
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D_talley
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Someone needs to develope a simple time line that shows all possible impact events for the past 50,000 years. This timeline shoud show recored as well as possible impacts.
I just read that the "flood" event may have been associated with a impact in the Indian ocean. The Dark ages and the Carolina Bay impacts and the last story I posted about the fragments in 35,000 year old tusks are all saying that there are more major impacts than we realize.
A time line will show if there is any pattern or cycle to the events, proven or not.
If we are due for an Impact, I want to max my credit cards out now on a vacation in the south of France.. 
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Mike Casey
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The basic problem with using any sort of timeline to predict future impact events is that asteroid and comet impacts are essentially random over time. Further, because of weathering and geological processes that tend to erase traces of impact events, the record will always be incomplete. Combine the changing crust of the earth with the randomness of impact events and predictability becomes impossible, except in some ambiguous statistical sense.
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Mike (tVA)
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D_talley
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I agree. I don't really mean to predict future events, just have a clear indicator of how many events have happened. I had no clue about these events until I found this thread. Most people only know about the impact site in Arizona. If there has been a sustained bombardment of the earth in the past 50,000 years, something not taught in schools, I would like to see timeline of the possible events.
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llanitedave
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The other complication is that most impacts happen over the ocean, and probably most bolides, like Tunguska, explode in the atmosphere, which minimizes surface traces while at the same time increasing the area suffering damage.
The best way to get an estimate, I think, is to count the smaller meteors that hit daily, try to get a distribution of sizes, and hope (I think evidence backs this up)that a power law describing that size distribution can be scaled up.
And then, of course, we need to be on the lookout for those bodies that we detect in advance.
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Mike Casey
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I find it interesting that Carolina Bays-type features extend above the coastal plains of Virginia. Whatever occurred there in the Late Pleistocene must have covered a rather large area of the eastern seaboard.
"Apparently the development of the bays is in no way related to the Piedmont bedrock which underlies the Midlothian gravels. Some of the bays have formed on gravels overlying granite, and some have developed on gravels overlying Triassic shale, sandstone, and coal measures (Fig. 4). To the author's knowledge, these are the first Caroline Bays reported west of the eastern Coastal Plain in Virginia and Probably are the first reported from an area outside of the sand-covered Coastal Plain or its erosional remnants."
http://abob.libs.uga.edu/bobk/cbayvirg.html
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Mike (tVA)
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D_talley
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Thanks for the link. Since I live in Richmond, I will try to check them out.
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Dwight
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Qkslvr
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Would you expect an impact to product rounded cobbles?
Having grown up in Ohio, I've always thought that the various rounded rocks found were dropped from glaciers as they retreated.
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Mike
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Mike Casey
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I would not expect an impact to produce rounded cobbles. They most certainly would be produced by fluvial action in a stream or riverbed, or by wave action on a beach.
The finding of cobble beds at certain Carolina Bays features would be by deposition before or after an impact event.
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Mike (tVA)
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