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Re: K/T impact



The big post about the impact scenario...

There is no doubt whatsoever about the age and nature of the Chicxulub
impact crater. It has been dated by U/Pb from zircons to 64.98 +- 0.05 Ma
(1). This fits damn well. The "undisturbed Cretaceous sediments" that lie in
the crater have very simply fallen into it as ejecta and/or when the primary
crater collapsed.
        The impact produced an earthquake that is calculated to have been
12.4 on the Richter scale. This scale is logarithmic -- 8 is 10 times as
strong as 7 --, and there is no reason to assume that the Earth can produce
anything even at 10 on that scale. (Few recorded earthquakes were over 8.)
This shattering alone may explain why no terrestrial animal heavier than ~
25 kg survived. (1)
        The impact site was in water. For the physical effects to the sea,
see http://www.dinosauria.com/jdp/impact/breccia.htm and
http://www.dinosauria.com/jdp/impact/wham.htm. Truth is better than
fiction -- impacts are much more terrible than Hollywood shows.
        The heat of the impact (100s of 1000s of degrees) ignited everything
within 100s of km. The fireball needed only hours to spread all over the
globe.
        However, the heat had a preferred direction because, as the crater
shape indicates, the planetoid came in at a low angle and from the
Southeast: http://www.dinosauria.com/jdp/impact/impact.htm. Shudder.
        The impact site contains sulfates that produce sulfur dioxide when
vaporized, and in all that heat nitric oxides were produced. Add water
vapor, and voilà, acid rain with a pH of -0.5 or so. Additionally, all these
gases are very poisonous. Nitric dioxide is deeply red-brown, and sulfuric
acid makes reflecting aerosols in the air -- it became even darker than by
the dust alone. Dinitric monoxide, aka laughing gas, is a good greenhouse
gas.
        The nitric oxides immediately destroyed the ozone layer.
        Evidence for the acid rain exists. For example, around the K-T
relative lots of strontium are found in marine sediments. A simple
explanation for how the strontium came into the sea from the continents is
that the rainwater suddenly became very acidic.
        "Gregory Retallack [.] has found evidence in the boundary clay in
Montana of severe acid leaching, possibly enough to have dispersed the
iridium and dissolved the shocked minerals and spherules. Thus, Retallack
says, some impacts might be 'self-cleaning', eliminating traces of their own
existence. Because some soils naturally buffer acids and others do not, acid
rain might also explain some of the K-T selectivity. For example, the
floodplains of ancient Montana would have remained above a pH of 4 [...]"
(2)
        After that business it got hot because of the greenhouse gases.

Before the planetoid hit, it may have revolved around Earth a few times in
low height. This produces tides 30 m high (forgot the ref)... the Niobrara
Sea is thought to have been 40 m deep, just for comparison (it was largely
dry by the time of the impact).

Plants -- "Dr. Kirk Johnson claims that there are no grasses, oaks, maples,
or willows in the Hell Creek Formation. Ferns are uncommon in the *majority*
of the formation, however there is a great increase in the abundance of
fossil fern spores in the two centimeters of rock that directly overlies the
impact fallout layer. This increase in fern spore abundance is commonly
referred to as "the fern spike" (meaning that if the abundance of spores as
a function of stratigraphic position were plotted out, the graph would show
a spike just above the impact fallout layer). Johnson also found that the
majority of the angiosperm genera in the Hell Creek Formation are now
extinct. He also believes that, very roughly, 80% of the plant life died out
in what is now Montana and the Dakotas at the K/T boundary." See ref at
http://www.dinosauria.com/jdp/misc/hellcreek.html.

New plants continue to be discovered below, but not above the K-T, so (2)
writes, IIRC, that 90 % of all pollen species died out within the last 2
CENTIMETERS of the Hell Creek Formation (and even there there is still the
Signor-Lipps effect...).

Planktonic species (including those with planktonic larvae) were
particularly hard hit. This explains why the reef-forming clams and
ammonites (with planktonic larvae) died out while nautiloids (non-planktonic
larvae) didn't. The nearly sterile result is called a Strangelove ocean.
        There is a marine equivalent to the fern spike: In the early
Paleocene of New Jersey animals that were rare before and afterwards
blossomed on the barren sea floors, namely brachiopods, sponges, bryozoans
and solitary corals. These groups have evolved at times when there wasn't
much plankton, in the Cambrian or before: Brachiopods have the lowest
metabolic rates and the lowest food requirements of all living multicellular
shelled marine animals and lay yolk-rich eggs from which fully formed little
brachiopods (no larvae) hatch, and one brachiopod species forms whole layers
in New Jersey. Sponges, bryozoans and corals can reproduce by budding, means
that their population sizes rebound very quickly.
        The immediate winners in all waters were the crocodiles (now we know
why crocodiles grin...). All over the Paleocene they were the biggest
animals and could eat all others. Sea crocs (Dyrosauridae etc.) didn't die
out before the end of the Eocene when 25-m-long whales had been in place for
considerable time. The sharks also grew (2 relatives of the Great White are
known from the Paleocene of New Jersey). (3)

Refs:
(1) Frederik DeWiel: Die Erde als Ziel extraterrestrischer Boliden [ = The
Earth as a target of extraterrestrial bolides], Naturwissenschaftliche
Rundschau, 50. Jahrgang, 436 -- 439, 11/1997
(2) James Lawrence Powell: Night comes to the Cretaceous: dinosaur
extinction and the transformation of modern geology, W. H. Freeman 1998
(Highly recommended!!!)
(3) William B. Gallagher: When dinosaurs roamed New Jersey, Rutgers
University 1997

Hey! This was really short! :-)