Comets Over Canada...Maybe Not!

Re. the Younger Dryas Impact theory.

Since layers of charcoal in the fossil record exist in Northern Europe from around the onset of the Younger Dryas, and since this charcoal can be explained as a result of trees killed by the onset of cooler temperatures serving as fuel for wide-spread forest fires, there is no reason to postulate an "explosion of extra-terrestrial material over northern North America" to account for a similar charcoal layer in North America.

H/T Jeff Hecht, who has a story on the impact theory in last week's New Scientist (from which I have copped, hopefully legally, the picture above-left).

Comets Over Canada...or Not [Again]

ScienceDaily (Jan. 27, 2009) — New data disproves the recent theory that a large comet exploded over North America 12,900 years ago, causing a shock wave that travelled across North America at hundreds of kilometres per hour and triggering continent-wide wildfires.

Dr Sandy Harrison from the University of Bristol and colleagues tested the theory by examining charcoal and pollen records to assess how fire regimes in North America changed between 15 and 10,000 years ago, a time of large and rapid climate changes.

Their results provide no evidence for continental-scale fires[*]...

[*] This being one of the predictions made by the Younger Dryas impact theory.

Ah well, Pre-History just got a bit more boring.

Killer Comet Theory Crosses Important Threshold!

I've written a number of times re the theory, proposed by scientists like Allen West and others, that

...an extraterrestrial impact, possibly a comet, set off a 1,000-year-long cold spell and wiped out or fragmented the prehistoric Clovis culture and a variety of animals across North America almost 13,000 years ago.

Well, the theory achieved an important milestone in the last couple of days: a peer-reviewed paper on the topic has come out in the highly regarded journal Science, and the authors have been kind enough to make the .pdf available here. Specifically, the multi-institutional team behind the paper have discovered

A carbon-rich black layer, dating to
12.9 ka, has been previously identified at
50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at
12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at
12.9 ka.

Tiny diamonds, in other words, of the kind that tend to be associated with ET impacts.

Now, the theory is still a long way from proven. As one of the paper's referee's has noted:

...though the scientists [have] not proved their case, they [have] offered enough evidence that the idea warranted a closer look by others.

Several objections have been lodged against the hypothesis. For one thing, it is not entirely clear that the Clovis Culture collapsed at the appropriate moment. One recent paper found no evidence of a population decline among the Paleoindians at 12,900 ± 100 calBP .

Another issue is the lack of a crater:

“A body of this size does not just blow up without a trace in the atmosphere,” Dr. Koeberl said. “Physics won’t have it.”

Proponents have suggested that the meteor hit an ice sheet a couple of miles thick or that there was a series of smaller objects that exploded in the air. But Dr. Koeberl said something hitting an ice sheet would still generate a hole in the ground underneath, and he questioned whether smaller impacts or air explosions would produce the shock waves needed to make diamonds.

An impact should also have left remnants of melted rocks and shocked minerals, Dr. Koeberl said.

I actually raised this point in an earlier post on the topic. Dr. West's response, which obviously Mr. Koeberl has heard and thus far found unconvincing, can be found here.

Incidentally, Dr. West's description of the event

"Imagine these fireballs exploding in the air. A Clovis hunter standing and looking at these things would have seen a canopy of fire as these things came in and exploded," said Allen West, a geophysicist and one of the paper's co-authors. "There would have been no sound. There would have been massive explosions. Brilliant light, brighter than the sun. There would have been radiant heat -- it would have been capable, at the very least, of giving him serious burns and, at the maximum, of incinerating him."

...seems to call into question of the more intriguing recent findings that, as of last year at least, was cited in favor of the hypothesis: the mammoth corpses that appear to have been pelted with microscopic with iron fragments. It was argued that these fragments were bits shed from the impacting object. If so, however, should these mammoth corpses not have been "seriously burned" or perhaps even disintegrated?

Meanwhile, special exploding bonus: did a meteorite trigger a tsunami off New York City?

Comets Over Canada...Or Not?

I have written previously about Allen West's theory re a comet or other extraterrestrial object exploding over Canada 13,000 years ago and bringing an end to the Clovis Culture, triggering the Younger Dryas, and causing the extinction of the mammoth.

Well, science marches on. A new paper (Paleoindian demography and the extraterrestrial impact hypothesis) has appeared testing the first of these three conclusions:

The impact is also claimed to have caused major cultural changes and population decline among the Paleoindians. Here, we report a study in which ≈1,500 radiocarbon dates from archaeological sites in Canada and the United States were used to test the hypothesis that the ET resulted in population decline among the Paleoindians.

[...]

The results of the analyses were not consistent with the predictions of extraterrestrial impact hypothesis. No evidence of a population decline among the Paleoindians at 12,900 ± 100 calBP was found. Thus, minimally, the study suggests the extraterrestrial impact hypothesis should be amended.

Ah well. Pity. I am partial to any theories that involve stuff exploding in the sky.

And I know nobody reads these sciency-type posts. I still enjoy doing them, especially when writing about politics starts to make me feel unclean.

H/t Afarensis.

Comets Over Canada: Part II

Ever since Geophysicist Allen West and others put it forward last year, I've been interested in the theory that a comet or other ET object exploding somewhere over Canada might have been the event that ended the Clovis Culture, triggered the Younger Dryas, and brought about the demise of the Mammoths.

Evidence seems to be mounting in support of the theory, but the latest brick in the wall

Samples of diamonds, gold and silver that have been found in the [Ohio] region have been conclusively sourced through X-ray diffractometry in the lab of UC Professor of Geology Warren Huff back to the diamond fields region of Canada.

The only plausible scenario available now for explaining their presence this far south is the kind of cataclysmic explosive event described by West’s theory. "We believe this is the strongest evidence yet indicating a comet impact in that time period," says Tankersley.

...raised a few problems for me. I wrote:

I cannot get this to jibe with earlier articulations of the theory. Specifically, geophysicist Allen West has previously suggested that the impactor exploded before touching ground (a la the Tunguska Event), this to explain the absence of a crater. But how could diamonds originating in the Canadian diamond fields have turned up so far South if not as ejecta from a (still undiscovered) crater?

I fired off an e-mail re these concerns to some of the theory's architects, and yesterday Mr. West himself was kind enough to respond and address this point:

The hypothesis is fairly complex. We believe the impact was a collision with a cloud of debris, much like Comet LINEAR [see above left]. The smaller objects would have detonated in the atmosphere, much like Tunguska. Depending on the angle of entry, the largest objects could have impacted the 3-km-thick ice sheet, creating ice-walled craters that left no lasting imprint on bedrock.

There are multiple processes that could account for the diamonds: 1) some diamonds appear to have condensed inside burning biomass due to the heat and pressure of the impacts/airbursts; 2) diamonds are known form from shock impact upon terrestrial materials, such as coal, peat, carbonate rock, which would have been entrained as detritus in the ice sheet or would have been near the surface under the ice; 3) diamonds may have been formed by carbon vapor deposition (CVD) in the plasma of the fireball; 4) pre-existing kimberlite diamonds could have been ejected into the air along with other detritus entrained in the ice sheet; and 5) the impacts/airbursts most likely created immense meltwater surges both from above and below the ice and which would have carried pre-existing diamonds and other debris along with the meltwater.

Thank you Mr. West.

While I still had his ear, I asked Mr. West about my favorite part of the theory, where several of his co-workers had produced evidence that, at around the time of the onset of the Younger Dryas

mammoth[s] and other great beasts from the last ice age were blasted with material that came from space.

Eight tusks [...] all show signs of having being peppered with meteorite fragments.

In other words, when the comet crashed over Canada, its as though every mammoth in proximity to the event took a shotgun blast full of metal tiny pellets to the head!

Back in December, I put this part of the theory to the lads and lasses of the Dinosaur Mailing list. Gregory S. Paul gave it a thumbs down:

Some in this discussion still seem to imagine that sand sized blast debris can be imbedded in bone surfaces or skin at substantial range from an meteoritic explosion. Tiny particles can travel at high velocities if they are being carried along by air that is itself an equally fast moving part of the supersonic shock wave (shock waves are shock waves because they move faster than sound) produced by the explosion, which are limited to the region immediately surrounding the point source. Anything hit by high velocity microdebris in this zone will be so severely damaged by even more obvious shock and heat that the sand impact will be incidental. The supposedly impacted tusks and bones should be shattered and scorched. Any living animal will be killed outright, the debris will not be the killing agent. Once the micro-debris hits stable air it slowsdown to harmless terminal velocity in well under a kilometer. Even pebble sized objects will slow down to a 100 mph in a few kilometers. That is why being hit by a round musket ball or grape shot at long range was not lethal.

Dr. West was kind enough to respond to this as well:

Most discussions cite the usual explanation for impacts, which is true in most cases. However, that theory fails to explain the Carancas impact (http://www.universetoday.com/2008/03/18/peruvian-meteorite-may-rewrite-impact-theories/) or the particles in mammoth tusks. New research hints at a "tunneling" effect for some impactors, particularly fragmented ones. In those cases, the leading objects tunnel into the atmosphere and explode, apparently opening the way for trailing objects to reach ever closer to the surface. Some bunker-buster bombs work on the same principle. The first bomb opens a crater, the 2nd digs deeper, and subsequent ones even deeper.

In simulations, the initial explosions of a highly fragmented comet appear to be able to push aside the atmosphere and allow the particles from subsequent explosions to travel with almost no atmospheric friction. This is all theoretical but explains some seemingly inexplicable phenomena.

So there you have it!!!

Comets Over Canada

I have written here about the theory that the Younger Dryas, an 1,500 year long period of extreme cold that took place between about 13,000 and 11,500 years ago, was triggered by an "impact event" occurring somewhere over Canada. Well, more evidence in favor of this new theory has recently turned up:

Samples of diamonds, gold and silver that have been found in the region have been conclusively sourced through X-ray diffractometry in the lab of UC Professor of Geology Warren Huff back to the diamond fields region of Canada.

The only plausible scenario available now for explaining their presence this far south is the kind of cataclysmic explosive event described by West’s theory. "We believe this is the strongest evidence yet indicating a comet impact in that time period," says Tankersley.

Fascinating stuff. But I cannot get this to jibe with earlier articulations of the theory. Specifically, geophysicist Allen West has previously suggested that the impactor exploded before touching ground (a la the Tunguska Event), this to explain the absence of a crater. But how could diamonds originating in the Canadian diamond fields have turned up so far South if not as ejecta from a (still undiscovered) crater?

The Younger Dryas, Celestial Impacts, and Scarborough Bluffs

When scientists, even pretend ones like Dennis Quaid in "The Day After Tomorrow", talk about Global Warming leading to a new Ice Age, they usually refer to the 1,500 year period between about 13,000 and 11,500 years ago, known as the Younger Dryas, as evidence. During this time the mean annual temperature, which had been gradually increasing for several thousand years, dropped suddenly to levels more typical of the Last Glacial Maximum (see graph above).

What caused the Younger Dryas? The conventional wisdom is that, as the planet gradually warmed and the glaciers retreated, proglacial lakes were formed by the water melting from them. The largest of these was Lake Agassiz, which is theorised to have been at the very center of the continent (around present day Lake Winnipeg). Another was Lake Algonquin, which comprised most of the current great lakes, Georgian Bay, and parts of Northern Michigan. And a third was Lake Iroquois, which was an enlarged version of Lake Ontario.

For most of its existence, drainage from Lake Agazziz was Southward into the proto-Mississippi. Lake Algonquin flowed South towards the Gulf of Mexico, and Lake Iroquois drained South into the Hudson River. However, as the glaciers retreated (so the story goes), the ice-dam which prevented water from these lakes from flowing Eastward collapsed abruptly. An immense pulse of cold, fresh water flowed down the St. Lawrence Valley and out into the Atlantic, where it caused a shut-down of the North Atlantic thermocline (sometimes called the great ocean conveyor) which carries warm water Northward via the Gulf Stream. This in turn caused Northern Hemisphere temperatures to plunge, glaciers to advance again, and so on and so forth.

If you live in the Greater Toronto Area you can actually find evidence of the aftermath of this ice dam collapse in the form of Scarborough Bluffs. These bluffs are made of packed clay soil and in fact constitute the shore-line of old Lake Iroquois; previous to the great Eastward outflow which drained all of these pro-glacial lakes, the waters stood about 100 feet higher than today.

However, a new theory has come onto the scene. A recent paper has suggested (backed by some pretty good evidence) that an "impact event" occurred somewhere near the Great Lakes at the beginning of the Younger Dryas:

Iridium peaks, and spikes of ammonium and nitrate which could have been produced by extensive burning of biomass, are also found in the Greenland ice cores. The authors therefore propose that the black horizon is the result of massive forest fires raging across the whole North American continent, triggered by the fiery impact of a meteorite or comet - or possibly a low altitude explosion like the Tunguska event. Enough soot from the fires, and dust from the impact, would have been thrown into the atmosphere to significantly cool the climate, causing enough environmental stress to wipe out both the Clovis people and the mammoths.

So did the cooling associated with this impact trigger the Younger Dryas? Probably not on its own. Though enough soot from the fires, and dust from the impact, would have been thrown into the atmosphere to significantly cool the climate, these effects would only have lasted several years. It is more likely that the impact caused a "destabilization/melting" of North America's interior ice sheets--including the bust up of that ice dam--which triggered that massive Eastward outflow from our pro-glacial lakes, and so on and so forth.

Now, why did I just write all this? Well, if you encounter a skeptic who tells you that its all bullshit because how can warming produce colder temperatures? you can tell them about the Younger Dryas. Of course, when scientists talk about the possibility of a Gulf Stream shut-down caused by modern day warming, they are looking at an entirely different mechanism: cold fresh water melting from the Greenland Ice Sheet, for the most part, bringing about the same effect.

Furthermore, and luckily enough, this outcome does NOT seem imminent.