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Posted by baalke on August 16, 2007, 5:26 pm
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http://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=109768&org=NSF
Comet May Have Exploded Over North America 13,000 Years Ago
Caused wooly mammoth extinction, global cooling and end of early human
Clovis culture
National Science Foundation
August 14, 2007
New scientific findings suggest that a large comet may have exploded
over North America 12,900 years ago, explaining riddles that
scientists
have wrestled with for decades, including an abrupt cooling of much of
the planet and the extinction of large mammals.
The discovery was made by scientists from the University of California
at Santa Barbara and their colleagues. James Kennett, a
paleoceanographer at the university, said that the discovery may
explain
some of the highly debated geologic controversies of recent decades.
The period in question is called the Younger Dryas, an interval of
abrupt cooling that lasted for about 1,000 years and occurred at the
beginning of an inter-glacial warm period. Evidence for the
temperature
change is recorded in marine sediments and ice cores.
According to the scientists, the comet before fragmentation must have
been about four kilometers across, and either exploded in the
atmosphere
or had fragments hit the Laurentide ice sheet in northeastern North
America.
Wildfires across the continent would have resulted from the fiery
impact, killing off vegetation that was the food supply of many of
larger mammals like the woolly mammoths, causing them to go extinct.
Since the Clovis people of North America hunted the mammoths as a
major
source of their food, they too would have been affected by the impact.
Their culture eventually died out.
The scientific team visited more than a dozen archaeological sites in
North America, where they found high concentrations of iridium, an
element that is rare on Earth and is almost exclusively associated
with
extraterrestrial objects such as comets and meteorites.
They also found metallic microspherules in the comet fragments; these
microspherules contained nano-diamonds. The comet also carried carbon
molecules called fullerenes (buckyballs), with gases trapped inside
that
indicated an extraterrestrial origin.
The team concluded that the impact of the comet likely destabilized a
large portion of the Laurentide ice sheet, causing a high volume of
freshwater to flow into the north Atlantic and Arctic Oceans.
"This, in turn, would have caused a major disruption of the ocean's
circulation, leading to a cooler atmosphere and the glaciation of the
Younger Dryas period," said Kennett. "We found evidence of the impact
as
far west as the Santa Barbara Channel Islands."
The National Science Foundation's Paleoclimate Program funded the
research.
-- Cheryl Dybas, NSF (703) 292-7734 cdybas@nsf.gov
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Posted by George on August 16, 2007, 9:27 pm
Please log in for more thread options
> http://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=109768&org=NSF
>
> Comet May Have Exploded Over North America 13,000 Years Ago
>
> Caused wooly mammoth extinction, global cooling and end of early human
> Clovis culture
>
> National Science Foundation
> August 14, 2007
>
> New scientific findings suggest that a large comet may have exploded
> over North America 12,900 years ago, explaining riddles that
> scientists
> have wrestled with for decades, including an abrupt cooling of much of
> the planet and the extinction of large mammals.
>
> The discovery was made by scientists from the University of California
> at Santa Barbara and their colleagues. James Kennett, a
> paleoceanographer at the university, said that the discovery may
> explain
> some of the highly debated geologic controversies of recent decades.
>
> The period in question is called the Younger Dryas, an interval of
> abrupt cooling that lasted for about 1,000 years and occurred at the
> beginning of an inter-glacial warm period. Evidence for the
> temperature
> change is recorded in marine sediments and ice cores.
>
> According to the scientists, the comet before fragmentation must have
> been about four kilometers across, and either exploded in the
> atmosphere
> or had fragments hit the Laurentide ice sheet in northeastern North
> America.
>
> Wildfires across the continent would have resulted from the fiery
> impact, killing off vegetation that was the food supply of many of
> larger mammals like the woolly mammoths, causing them to go extinct.
>
> Since the Clovis people of North America hunted the mammoths as a
> major
> source of their food, they too would have been affected by the impact.
> Their culture eventually died out.
>
> The scientific team visited more than a dozen archaeological sites in
> North America, where they found high concentrations of iridium, an
> element that is rare on Earth and is almost exclusively associated
> with
> extraterrestrial objects such as comets and meteorites.
>
> They also found metallic microspherules in the comet fragments; these
> microspherules contained nano-diamonds. The comet also carried carbon
> molecules called fullerenes (buckyballs), with gases trapped inside
> that
> indicated an extraterrestrial origin.
>
> The team concluded that the impact of the comet likely destabilized a
> large portion of the Laurentide ice sheet, causing a high volume of
> freshwater to flow into the north Atlantic and Arctic Oceans.
>
> "This, in turn, would have caused a major disruption of the ocean's
> circulation, leading to a cooler atmosphere and the glaciation of the
> Younger Dryas period," said Kennett. "We found evidence of the impact
> as
> far west as the Santa Barbara Channel Islands."
>
> The National Science Foundation's Paleoclimate Program funded the
> research.
>
> -- Cheryl Dybas, NSF (703) 292-7734 cdybas@nsf.gov
This story has been posted elsewhere before, particularly in
sci.geo.geology. Problems:
If a dirty snowball four kilometers wide struck a 2 mile thick ice sheet,
where was the vegetation that was allegedly burned off (it is, after all, a
2 mile thick ice sheet), starving the Mammaths? And why did Mastodons
survive in some regions up to about 6,000 years ago? I read before that
this proposed comet allegedly struck the ice sheet in Nova Scotia. If so,
this would be deep into the heart of the ice sheet, well away from any
forested areas at the time. 1,000 miles away, the fireball would have been
below the horizon, and so there would have been no substantial direct
thermal affects at that distance. Whether or not it melted a sustantial
area of ice and released it into the ocean is another matter altogether.
Having said that, I'd like to see oceanic core samples from off the east
coast that show that such a large amount of water (and a resulting sediment
load) was released into the oceans at that time. I've yet to see anyone
present evidence of it. If large tracts of forest were essentially burned
instantaneously (where these forests were located is never mentioned), why
is there no continent-wide/region-wide carbon layer dating to the Younger
Dryas? Why did the Canadian flora extent in the Smoky Mountains and other
areas of the Appalachians, which date to the Pleistocene, survive? Also,
it doesn't explain the older dryas, a similar cold snap that preceded the
younger dryas by about 1,000 years. It also doesn't explain why South
America cooled first. Sorry, based on the evidence presented, I don't buy
it.
George
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Posted by George on August 16, 2007, 11:54 pm
Please log in for more thread options
>
>> http://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=109768&org=NSF
>>
>> Comet May Have Exploded Over North America 13,000 Years Ago
>>
>> Caused wooly mammoth extinction, global cooling and end of early human
>> Clovis culture
>>
>> National Science Foundation
>> August 14, 2007
>>
>> New scientific findings suggest that a large comet may have exploded
>> over North America 12,900 years ago, explaining riddles that
>> scientists
>> have wrestled with for decades, including an abrupt cooling of much of
>> the planet and the extinction of large mammals.
>>
>> The discovery was made by scientists from the University of California
>> at Santa Barbara and their colleagues. James Kennett, a
>> paleoceanographer at the university, said that the discovery may
>> explain
>> some of the highly debated geologic controversies of recent decades.
>>
>> The period in question is called the Younger Dryas, an interval of
>> abrupt cooling that lasted for about 1,000 years and occurred at the
>> beginning of an inter-glacial warm period. Evidence for the
>> temperature
>> change is recorded in marine sediments and ice cores.
>>
>> According to the scientists, the comet before fragmentation must have
>> been about four kilometers across, and either exploded in the
>> atmosphere
>> or had fragments hit the Laurentide ice sheet in northeastern North
>> America.
>>
>> Wildfires across the continent would have resulted from the fiery
>> impact, killing off vegetation that was the food supply of many of
>> larger mammals like the woolly mammoths, causing them to go extinct.
>>
>> Since the Clovis people of North America hunted the mammoths as a
>> major
>> source of their food, they too would have been affected by the impact.
>> Their culture eventually died out.
>>
>> The scientific team visited more than a dozen archaeological sites in
>> North America, where they found high concentrations of iridium, an
>> element that is rare on Earth and is almost exclusively associated
>> with
>> extraterrestrial objects such as comets and meteorites.
>>
>> They also found metallic microspherules in the comet fragments; these
>> microspherules contained nano-diamonds. The comet also carried carbon
>> molecules called fullerenes (buckyballs), with gases trapped inside
>> that
>> indicated an extraterrestrial origin.
>>
>> The team concluded that the impact of the comet likely destabilized a
>> large portion of the Laurentide ice sheet, causing a high volume of
>> freshwater to flow into the north Atlantic and Arctic Oceans.
>>
>> "This, in turn, would have caused a major disruption of the ocean's
>> circulation, leading to a cooler atmosphere and the glaciation of the
>> Younger Dryas period," said Kennett. "We found evidence of the impact
>> as
>> far west as the Santa Barbara Channel Islands."
>>
>> The National Science Foundation's Paleoclimate Program funded the
>> research.
>>
>> -- Cheryl Dybas, NSF (703) 292-7734 cdybas@nsf.gov
>
> This story has been posted elsewhere before, particularly in
> sci.geo.geology. Problems:
>
> If a dirty snowball four kilometers wide struck a 2 mile thick ice sheet,
> where was the vegetation that was allegedly burned off (it is, after all,
> a 2 mile thick ice sheet), starving the Mammaths? And why did Mastodons
> survive in some regions up to about 6,000 years ago? I read before that
> this proposed comet allegedly struck the ice sheet in Nova Scotia. If
> so, this would be deep into the heart of the ice sheet, well away from
> any forested areas at the time. 1,000 miles away, the fireball would
> have been below the horizon, and so there would have been no substantial
> direct thermal affects at that distance. Whether or not it melted a
> sustantial area of ice and released it into the ocean is another matter
> altogether. Having said that, I'd like to see oceanic core samples from
> off the east coast that show that such a large amount of water (and a
> resulting sediment load) was released into the oceans at that time. I've
> yet to see anyone present evidence of it. If large tracts of forest were
> essentially burned instantaneously (where these forests were located is
> never mentioned), why is there no continent-wide/region-wide carbon layer
> dating to the Younger Dryas? Why did the Canadian flora extent in the
> Smoky Mountains and other areas of the Appalachians, which date to the
> Pleistocene, survive? Also, it doesn't explain the older dryas, a
> similar cold snap that preceded the younger dryas by about 1,000 years.
> It also doesn't explain why South America cooled first. Sorry, based on
> the evidence presented, I don't buy it.
>
> George
Another problem is that one of the effects this hypothesis predicts, the
halting of the thermohaline, apparently has been based on data that is now
in dispute:
http://environment.newscientist.com/article/dn12494-fickle-ocean-current-foils-climate-modellers.html
a.. 14:46 16 August 2007
b.. NewScientist.com news service
c.. Stephen Battersby
The North Atlantic is stirring fitfully. A new monitoring system has shown
that the ocean's currents change rapidly, surging or slowing from one week
to the next. That makes it difficult to judge whether they really are
slowing down over the long term, as one study has suggested.
An overall slowdown might be bad news for Europe, which is warmed by a
current called the North Atlantic Drift, and it might be even worse for the
rest of the world because the North Atlantic forms a vital piece of
planetary plumbing. When the North Atlantic Drift reaches the Arctic, it
cools, sinks and flows back to the south, helping to drive global ocean
circulation.
The risk is that climate change will melt ice and increase rainfall around
the Arctic, flushing fresh, less dense water into the Arctic Ocean. That
could interfere with the crucial sinking process, slowing the current down
or even cutting it off. Some have even claimed this could plunge Europe
into a new ice age, though most climatologists now dismiss this extreme
scenario as just another climate change myth.
Most ocean models predict that a substantial slowdown is unlikely until
late this century, but in 2005 a team of oceanographers led by Harry Bryden
of Southampton University, UK, announced evidence that the vital
"overturning circulation" of the North Atlantic had already slowed by 30%.
However, they conceded that natural variability might explain their data,
because the only measurements available were intermittent ones made over
the past half century by oceanographers on ships.
RAPID measures
Now for the first time scientists can monitor the ocean continuously,
thanks largely to RAPID, an array of instruments strung on cables moored to
the seabed. RAPID measures the ocean's pressure profile, which scientists
can use to calculate how water is flowing.
An international team, including Bryden, has now taken the first year of
data from RAPID and combined it with two other sources: space-based
measurements of wind-blown surface currents, and the flow of the gulf
stream between Florida and the Bahamas, which is revealed by its electrical
effect on submarine telephone cables.
What they see is that the overturning circulation fluctuates wildly,
between 35 million tonnes a second and just 4 million tonnes a second. All
the earlier measurements lie within that range. "It is now going to be
difficult to make robust estimates of overturning based on earlier
results," says Stuart Cunningham, also of Southampton University and lead
author of the new study. "This defines an unambiguous baseline against
which future change may be gauged."
Cunningham still suspects that overturning circulation is slowing down,
pointing to an ocean model published last year by Carl Wunsch of the
Massachusetts Institute of Technology in which it fell by 14% between 1993
and 2004.
However, finding out for sure will take some time. At present RAPID is
funded until 2014, but it may be decades before scientists understand the
natural variability of the ocean and identify any effects of human-driven
climate change. "We hope to be able to explain all variability eventually,"
Cunningham told New Scientist.
|
|
Posted by George Dishman on August 17, 2007, 3:00 pm
Please log in for more thread options
>
>> http://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=109768&org=NSF
>>
>> Comet May Have Exploded Over North America 13,000 Years Ago
>>
>> Caused wooly mammoth extinction, global cooling and end of early human
>> Clovis culture
>>
>> National Science Foundation
>> August 14, 2007
>>
>> New scientific findings suggest that a large comet may have exploded
>> over North America 12,900 years ago, explaining riddles that
>> scientists
>> have wrestled with for decades, including an abrupt cooling of much of
>> the planet and the extinction of large mammals.
>>
>> The discovery was made by scientists from the University of California
>> at Santa Barbara and their colleagues. James Kennett, a
>> paleoceanographer at the university, said that the discovery may
>> explain
>> some of the highly debated geologic controversies of recent decades.
>>
>> The period in question is called the Younger Dryas, an interval of
>> abrupt cooling that lasted for about 1,000 years and occurred at the
>> beginning of an inter-glacial warm period. Evidence for the
>> temperature
>> change is recorded in marine sediments and ice cores.
>>
>> According to the scientists, the comet before fragmentation must have
>> been about four kilometers across, and either exploded in the
>> atmosphere
>> or had fragments hit the Laurentide ice sheet in northeastern North
>> America.
>>
>> Wildfires across the continent would have resulted from the fiery
>> impact, killing off vegetation that was the food supply of many of
>> larger mammals like the woolly mammoths, causing them to go extinct.
>>
>> Since the Clovis people of North America hunted the mammoths as a
>> major
>> source of their food, they too would have been affected by the impact.
>> Their culture eventually died out.
>>
>> The scientific team visited more than a dozen archaeological sites in
>> North America, where they found high concentrations of iridium, an
>> element that is rare on Earth and is almost exclusively associated
>> with
>> extraterrestrial objects such as comets and meteorites.
>>
>> They also found metallic microspherules in the comet fragments; these
>> microspherules contained nano-diamonds. The comet also carried carbon
>> molecules called fullerenes (buckyballs), with gases trapped inside
>> that
>> indicated an extraterrestrial origin.
>>
>> The team concluded that the impact of the comet likely destabilized a
>> large portion of the Laurentide ice sheet, causing a high volume of
>> freshwater to flow into the north Atlantic and Arctic Oceans.
>>
>> "This, in turn, would have caused a major disruption of the ocean's
>> circulation, leading to a cooler atmosphere and the glaciation of the
>> Younger Dryas period," said Kennett. "We found evidence of the impact
>> as
>> far west as the Santa Barbara Channel Islands."
>>
>> The National Science Foundation's Paleoclimate Program funded the
>> research.
>>
>> -- Cheryl Dybas, NSF (703) 292-7734 cdybas@nsf.gov
>
> This story has been posted elsewhere before, particularly in
> sci.geo.geology. Problems:
>
> If a dirty snowball four kilometers wide struck a 2 mile thick ice sheet,
I don't see anything in the above that suggests it
struck the sheet intact. The usual model is of a
comet fragmenting and perhaps exploding due to the
rapid heating in the atmosphere. Only much smaller
chunks would hit the sheet, tens of meters perhaps.
> where was the vegetation that was allegedly burned off (it is, after all,
> a 2 mile thick ice sheet), starving the Mammaths?
The fireball in the sky would cause radiative
heating over a long track, sufficient to ignite
forest fires at the right time of year.
> And why did Mastodons survive in some regions up to about 6,000 years ago?
They were visiting friends at the time ;-)
I read before that
> this proposed comet allegedly struck the ice sheet in Nova Scotia. If so,
> this would be deep into the heart of the ice sheet, well away from any
> forested areas at the time. 1,000 miles away, the fireball would have
> been below the horizon, and so there would have been no substantial direct
> thermal affects at that distance.
The fireball could have crossed most of the continent
before the remnants reached ground.
This is not my area but perhaps you need to visualise
a meteor track rather than the fireball from a nuclear
weapon to follow what is being suggested.
George (another one)
|
|
Posted by George on August 17, 2007, 4:32 pm
Please log in for more thread options
>
>>
>>> http://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=109768&org=NSF
>>>
>>> Comet May Have Exploded Over North America 13,000 Years Ago
>>>
>>> Caused wooly mammoth extinction, global cooling and end of early human
>>> Clovis culture
>>>
>>> National Science Foundation
>>> August 14, 2007
>>>
>>> New scientific findings suggest that a large comet may have exploded
>>> over North America 12,900 years ago, explaining riddles that
>>> scientists
>>> have wrestled with for decades, including an abrupt cooling of much of
>>> the planet and the extinction of large mammals.
>>>
>>> The discovery was made by scientists from the University of California
>>> at Santa Barbara and their colleagues. James Kennett, a
>>> paleoceanographer at the university, said that the discovery may
>>> explain
>>> some of the highly debated geologic controversies of recent decades.
>>>
>>> The period in question is called the Younger Dryas, an interval of
>>> abrupt cooling that lasted for about 1,000 years and occurred at the
>>> beginning of an inter-glacial warm period. Evidence for the
>>> temperature
>>> change is recorded in marine sediments and ice cores.
>>>
>>> According to the scientists, the comet before fragmentation must have
>>> been about four kilometers across, and either exploded in the
>>> atmosphere
>>> or had fragments hit the Laurentide ice sheet in northeastern North
>>> America.
>>>
>>> Wildfires across the continent would have resulted from the fiery
>>> impact, killing off vegetation that was the food supply of many of
>>> larger mammals like the woolly mammoths, causing them to go extinct.
>>>
>>> Since the Clovis people of North America hunted the mammoths as a
>>> major
>>> source of their food, they too would have been affected by the impact.
>>> Their culture eventually died out.
>>>
>>> The scientific team visited more than a dozen archaeological sites in
>>> North America, where they found high concentrations of iridium, an
>>> element that is rare on Earth and is almost exclusively associated
>>> with
>>> extraterrestrial objects such as comets and meteorites.
>>>
>>> They also found metallic microspherules in the comet fragments; these
>>> microspherules contained nano-diamonds. The comet also carried carbon
>>> molecules called fullerenes (buckyballs), with gases trapped inside
>>> that
>>> indicated an extraterrestrial origin.
>>>
>>> The team concluded that the impact of the comet likely destabilized a
>>> large portion of the Laurentide ice sheet, causing a high volume of
>>> freshwater to flow into the north Atlantic and Arctic Oceans.
>>>
>>> "This, in turn, would have caused a major disruption of the ocean's
>>> circulation, leading to a cooler atmosphere and the glaciation of the
>>> Younger Dryas period," said Kennett. "We found evidence of the impact
>>> as
>>> far west as the Santa Barbara Channel Islands."
>>>
>>> The National Science Foundation's Paleoclimate Program funded the
>>> research.
>>>
>>> -- Cheryl Dybas, NSF (703) 292-7734 cdybas@nsf.gov
>>
>> This story has been posted elsewhere before, particularly in
>> sci.geo.geology. Problems:
>>
>> If a dirty snowball four kilometers wide struck a 2 mile thick ice
>> sheet,
>
> I don't see anything in the above that suggests it
> struck the sheet intact. The usual model is of a
> comet fragmenting and perhaps exploding due to the
> rapid heating in the atmosphere. Only much smaller
> chunks would hit the sheet, tens of meters perhaps.
>
>> where was the vegetation that was allegedly burned off (it is, after
>> all, a 2 mile thick ice sheet), starving the Mammaths?
>
> The fireball in the sky would cause radiative
> heating over a long track, sufficient to ignite
> forest fires at the right time of year.
>
>> And why did Mastodons survive in some regions up to about 6,000 years
>> ago?
>
> They were visiting friends at the time ;-)
>
> I read before that
>> this proposed comet allegedly struck the ice sheet in Nova Scotia. If
>> so, this would be deep into the heart of the ice sheet, well away from
>> any forested areas at the time. 1,000 miles away, the fireball would
>> have been below the horizon, and so there would have been no substantial
>> direct thermal affects at that distance.
>
> The fireball could have crossed most of the continent
> before the remnants reached ground.
>
> This is not my area but perhaps you need to visualise
> a meteor track rather than the fireball from a nuclear
> weapon to follow what is being suggested.
>
> George (another one)
>
If you plug in their suggested numbers into the following program, it's
clear that a comet of the size they suggest would not break up in the
atmosphere, but would strike the ground intact.
http://www.lpl.arizona.edu/impacteffects/
George
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