[meteorite-list] Dryas all wet

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Tue, 13 Oct 2009 00:32:46 -0500
Message-ID: <3348d5p8ou2gap452ra73tj29ve6jqe9pf_at_4ax.com>

http://www.nature.com/news/2009/091012/full/news.2009.997.html

North America comet theory questioned

No evidence of an extraterrestrial impact 13,000 years ago, studies say.

Rex Dalton

An independent study has cast more doubt on a controversial theory that a comet
exploded over icy North America nearly 13,000 years ago, wiping out the Clovis
people and many of the continent's large animals.

Archaeologists have examined sediments at seven Clovis-age sites across the
United States, and did not find enough magnetic cosmic debris to confirm that an
extraterrestrial impact happened at that time, says the report in the
Proceedings of the National Academy of Sciences (PNAS)1. It is the latest of
several studies unable to support aspects of the impact hypothesis.

In 2007, a team led by Californian researchers announced a theory2 that a comet
or asteroid had exploded over the North American ice sheet, creating widespread
fire and an atmospheric soot burst followed by a cooling period known as the
Younger Dryas. Sometime after this, the Clovis people, sophisticated
large-animal hunters known for their spear points, mysteriously disappeared; the
team linked their vanishing to the environmental effects of the proposed impact.

Key evidence came in the form of magnetic microspherules discovered in sediments
at 25 locations, including eight Clovis-age sites. Richard Firestone, of
Lawrence Berkeley National Laboratory in California, and his colleagues argued
that the microspherules were remnants of cosmic debris from an explosion.

But in more than 18 months of sedimentary analysis, a team led by Todd Surovell,
an archaeologist at the University of Wyoming in Laramie, was unable to detect
microspherule peaks. Two of the seven sites the group studied were places where
Firestone's team identified spherule peaks.

"I spent hundreds of hours at the microscope examining sediment samples," says
Surovell, "and I didn't find any physical evidence to support their theory."
Standing firm

The other team isn't backing down. "Their study doesn't negate our hypothesis,"
says James Kennett, a palaeoceanographer at the University of California at
Santa Barbara and one of Firestone's co-authors. Another co-author, avocational
geophysicist Allen West of Prescott, Arizona, says that Surovell's group didn't
use the correct technique to extract, identify and quantify the microspherules.

Several other groups have been unable to support important aspects of the comet
theory.

In a PNAS article published in February3, Jennifer Marlon, a doctoral geography
student at the University of Oregon in Eugene, and her colleagues found no
systematic burning of biomass ? as would have occurred if continent-wide fires
had happened ? at the time of the Younger Dryas in pollen and charcoal records
at 35 sites. And at the Ecological Society of America meeting in Albuquerque,
New Mexico, in August, Jacquelyn Gill, a palaeoecology doctoral student at the
University of Wisconsin-Madison, reported finding no evidence of massive burning
in sediment cores taken from lake beds in Ohio and Indiana.

Kennett, however, calls these studies "flawed". In August, his team published a
report4 saying they had found nanometre-sized diamonds, purportedly created
during an impact, and soot in sediments dated to the Younger Dryas on Santa Rosa
Island, off the coast of California.

More studies of the theory ? both critical and supportive ? are in the
publishing pipelines at other journals.

Surovell's co-author Vance Holliday, an archaeologist at the University of
Arizona in Tucson, and his colleagues have an article in press at Current
Anthropology that says the archaeological and geochronological records don't
support a collapse of Clovis people at the time of the purported impact.

    *
      References
         1. Surovell, T. A. et al. Proc. Natl Acad. Sci. USA advance online
publication doi:10.1073/pnas.0907857106 (2009).
         2. Firestone, R. B. et al. Proc. Natl Acad. Sci. USA 104, 16016-16021
(2007).
         3. Marlon, J. R. et al. Proc. Natl Acad. Sci. USA 106, 2519-2524
(2009).
         4. Kennett, D. J. et al. Proc. Natl Acad. Sci. USA 106, 12623-12628
(2009).





http://arstechnica.com/science/news/2009/10/irreproducible-results-raise-doubts-about-ice-age-impact.ars


Asteroid impact-driven climate change called into question

In recent years, a team of researchers has been gathering evidence of an
extraterrestrial impact that occurred precisely as the Earth plunged into a cold
snap. Now, other researchers have tried to reproduce a key piece of that
evidence, and apparently failed.

By John Timmer | Last updated October 12, 2009 3:11 PM CT

The end of the last ice age was a busy time in North America. As a whole, the
world seemed to be in the process of exiting the long cold snap nearly 15,000
years ago, but there was a sudden return to icy conditions that lasted nearly
2,000 years. At about the same time, the first humans made their presence felt
in North America, and many of the larger species of fauna went extinct. Over the
past few years, a series of papers have built the case that everything other
than the arrival of humans could be tied together nicely by the impact of a
series of carbon-rich meteors or comets that burst in the air over North
America, setting off massive fires locally, and blocking the sun globally.
Today, PNAS is releasing a paper that calls all of that into question for a very
simple reason: they tried to reproduce some of the results, and failed.

The events in question take place during a period called the Younger Dryas,
named for a plant that did well during the cold. As the graphs below show,
temperatures had been rising steadily from the depths of the ice age until
12,900 years ago, at which point they quickly reverted to their previous, frigid
levels. It took well over 1,000 years before they'd return to the sort of warm
that we've enjoyed for the last 10,000 years or so. But climate wasn't the only
thing that was changing; North America saw major species losses, with a lot of
its megafauna going extinct within a relatively short period of time.

Traditionally, that has been ascribed to the combination of the arrival of the
Clovis culture's hunters and climate change. The climate change itself was a bit
harder to pin down, but a number of people have pointed a scientific finger at
the draining of massive lakes, such as Lake Agassiz, that had built up behind
glacial dams. During the first warming, these dams broke, releasing massive
amounts of fresh water into the North Atlantic, and altering the ocean's
circulation in a way that returned the planet to its icy state.

The new papers, generally published by a relatively small set of researchers,
challenged those interpretations. They focused on the presence of dark organic
sediments, called "black mats," that are associated with many of the first
Clovis sites. The collaborators have argued that these dark sediments literally
represent burnt material, with massive fires set off by the impact of
extraterrestrial objects. Reports linked the black mats to magnetic and metallic
grains, chemicals specific to soot, and, perhaps most tellingly, nanodiamonds,
which can only form under a limited number of high-temperature, high-pressure
conditions, such as those created by an impact.

The new paper challenges only one aspect of that data: the presence of high
levels microscopic magnetic spheres in the same sediment layers as the other
signs of a potential impact. These items do form at a constant rate from the
impact of micrometeors, but may appear at higher numbers during a period of more
intense bombardment. The authors tracked the levels of these magnetic
microspheres at a series of seven Clovis-era sites, including two used by the
authors to argue in favor of an impact. In one case, they obtained samples only
a few centimeters away from the previous ones.

What they found, however, is nothing like the results that appeared in the
previous papers. "Although concentrations of magnetic grains vary by more than
two orders of magnitude among all study sites, no individual site shows clear
evidence of uniquely enhanced levels of magnetic grains in YDB samples," the
authors state, noting that the majority of their sites actually show a decrease
at this time. They term this "a discrepancy between the two studies that is
particularly troublesome." They suggest that local conditions, such as the
presence of flowing water, might dominate a site's properties and cause
extensive variability.

Obviously, that's just one piece of evidence out of a more extensive list that
has been used to argue in favor of an impact. But the authors note that a
separate study has raised questions about how extensive the evidence for massive
fires is. In addition, the radically different results may suggest that there's
a problem with the method of assigning samples to specific dates, which might
call into question some of the more compelling findings, like the presence of
nanodiamonds.

In any case, it's an interesting scientific controversy that doesn't appear to
be going away anytime soon, as those who favor the impact idea will undoubtedly
have more to add. It doesn't really say much about our understanding of climate
change, as either or both of the events?an impact and the draining of Lake
Agassiz?seem more than sufficient to have global consequences.

But the controversy also says important things about science in general. Various
critics of science argue that contrary views don't get published, or complain
that historical science is immune to the sort of reproducibility that takes
place in laboratory-based science. The back-and-forth here shows that neither of
those appears to be especially true.
Received on Tue 13 Oct 2009 01:32:46 AM PDT