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Re: SPECULATION: pterosaur extinction versus bird survival
At 6:20 PM -0500 2/5/01, Marilyn Wegweiser wrote:
Sorry..... but recent discussions I have been having with several
astrophysicists lead me to currently say, despite popular
depictions, that there are good chances are that "it" probably
didn't "impact" at all. I recognize that this could be perceived as
out of the box thinking, but then, I'm not in sarcophagus yet. "It"
probably vaporized some distance above the earth, large chunks of it
falling out spectacularly, etc. (e.g., Tunguska)
Two things weigh quite heavily against thinking that the KT impactor
did not reach the surface if it was the canonical 10-km asteroid.
Several models have looked at the dynamics of large impacts,
including atmospheric effects. Most show that 10-100 m objects are
likely to break up in the atmosphere, but that larger ones will hit
the surface. A 10-km asteroid is big relative to the thick layer of
the atmosphere, and it's moving fast.
Second, at least one crater has been identified on the deep ocean
floor, which survived a fluid considerably thicker than the
atmosphere -- namely, the ocean. I'm pasting in my report from the
1997 GSA meeting, an edited version of which appeared in New
ID: Deep-sea impact
Ocean basins cover about two-thirds of the earth, so most comet and asteroid
impacts should make big splashes. However, geologists have spotted only one
crater in the deep sea, and only now are taking a close look at it. It
produced a splash felt around the world 2.2 million years ago which may have
fooled glacial researchers into thinking much of the Antarctic ice sheet
melted at that time , says Ranier Gersonde of the Alfred Wegener institute
for Polar and Marine Research in Bremerhaven. Careful analysis of a deep-sea
core also could explain puzzling features of other impacts.
The Eltanin impact structure in the Bellingshausen sea off southern Chile was
spotted in 1981, but only in 1995 were the first deep-sea cores collected.
Gersonde told a meeting of the Geological Society of America in Salt Lake
City that the cores show a chaotic sequence of deposits that settled to the
bottom after the impact, a mixture of bottom sediments of various ages
covered by slow-settling fine-grained material. He says up to 5 percent of
the impact ejecta is unmelted asteroid fragments, including one
50-millimeter chunk, which he called "the biggest fragment of a large-scale
meteorite ever found."
Gersonde believes a one-kilometer landed in the five-kilometer ocean basin,
creating a transient 20-kilometer crater in the water, and raising
four-kilometer waves. Sea water splashed high into the atmosphere, with some
reaching the upper atmosphere and spreading across the world. The dust and
salt caused short-lived cooling. By the time the tsunami waves neared the
Pacific coast, they were 20 to 40 meters high over deep water, but rose to a
kilometer high at the shoreline.
"There is some evidence of disturbances in New Zealand and Australia" at the
time of impact, says Gersonde. However, he told the meeting that the dry
valleys of Antarctica "might be the only place" the impact debris survives as
it settled to the ground. He suggests that could explain how marine diatoms
came to be in the dry valleys. Their discovery a couple of years ago [check,
I think we reported] led other scientists to conclude that up to 70 percent
of the Antarctic ice sheet had melted during the warm spell of the Pliocene 3
to 5 million years ago [check date if you find story]. "We think a more
elegant explanation was that the material came from an impact," Gersonde
Analysis of cores from the Eltanin impact site may help solve the mystery of
excess iridium just below the iridium-rich layer deposited by the Chicxulub
impact blamed for killing the dinosaurs, Frank Kyte of the university of
California at Los Angeles told the meeting. Skeptics had said the puzzling
iridium "halo" might indicate a dual impact. However, Kyte says the Eltanin
core shows iridium diffused 8 to 10 centimeters below the impact layer.
Severe global climate change due to so many particles in the
atmosphere would have lasted for years, not just a few months. There
is "lag time" in global climate. For instance - even if ENSO-like
events did occur in the Cretaceous paleo-Pacific, what would have
been the effect on them by an impact scenario, and would those
effects have propagated well northward in the area influenced by the
Cretaceous Interior Sea? Just one aspect to consider......
Impact aftereffects on climate definitely would have lasted a long
time, but we seem to understand them about as well as we do other
weather and climate effects.
-- Jeff Hecht
Jeff Hecht Boston Correspondent New Scientist magazine
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