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Extinction scenarios



Peter Buchholz wrote:
>Another thing I would like to point out to everyone is the size of the rock
>that fell from the sky.  Get a globe, your standard American 12" diameter
>globe will be fine for my example.  I have heard estimates of the size of this
>rock from between 6 and 10 miles across.  That means that with the 12" globe
>the rock that killed the dinosaurs would be between 1/110 and 1/66 of an
>inch!!!  This is bordering on microscopic.
>
As Allen said, it's more realistic to think of the crater left behind -- a
hole the size of Connecticut, roughly (by my calculations) 1.4% of the
Earth's diameter. It landed in anhydrite, ejecting large amounts of sulfate
as well as CO2, stuff that didn't do the atmosphere any good. It may have
hit at a steep angle, ejecting even more material than it would have if it
hit the Earth straight-on. It is the biggest well-documented impact of the
past half-billion years. (There's a crater in South Africa that _may_ be
bigger, but is not well characterized.)

You're right in saying that the chain of events from impact to extinctions
is not well documented. But there are many possibilities. Perhaps we
haven't fully appreciated just how much a big dose of sulfuric acid (which
the surfates become in moist air) can devastate the global ecosystem. Or
perhaps we're missing something terribly obvious. Suppose, for example,
that the impact ignited wildfires taht spread across North America or the
world, and the fires filled the air not just with smoke but with toxins
such as carbon monoxide, nitrogen oxides, and sundry nasty hydrocarbons,
and those toxins killed off the dinosaurs? (The danger threshold for CO is
50 parts per million, according to the Handbook of Chemistry and Physics.)
Normal-scale fires don't pose serious threats because their fumes are
quickly diluted, but you can't do that if you're burning a large fraction
of the terrestrial biosphere. Maybe the real threshold for extinction was
tolerance to CO or some other toxin from the wildfires?


-- Jeff Hecht