From: John Alroy <jack@homebrew.uchicago.edu>
>
> My point about sedimentation rates was that the MAXIMAL duration of
> the 2 meter "gap" was on the scale of 40,000 years, UNLESS there is a
> major hiatus in that interval. I have seen no indication that anyone
> believes there is such a hiatus, in which case my calculations are
> completely reasonable (they also agree with a separate but similar
> calculation made by Friesen). General words of caution about
> interpolating sedimentation rates don't address the fact that this is
> a valid UPPER estimate given the absence of a hiatus. My original
> purpose was to note that a 100,000 - 250,000 year figure for the 2 m
> "gap" was too high.
Well, I wouldn't say it was an *upper* estimate, I would put it
more at a *median*, but that is quibling, since it *does* appear
my initial estimate was too high.
>... I just came
> across an interesting quote in Raup's Nemesis book to the effect that
> 61% of 500 professional geologists and paleontologists in a 1984 poll
> accepted the evidence for an impact at the K-T.
Yep, and so do I, tentatively.
I, like Dr. Holtz, simply realize that the presence of an impact
at that time does not necessarily mean it was the cause of the
extinctions.
> This figure would
> surely be much higher in 1994, because there wasn't even a decent
> candidate crater 10 years ago.
And the candidate crater is now highly doubtful.
Right now the evidence appears contradictory, and it will take
some time, and some good field work, to resolve the nature of the
Chicxulub structure. (The evidence *supporting* an impact origin
for it seems strong, but so does the contrary evidence).
> I also just found a chapter in his
> book entitled "The Three Meter Gap and Other Evidence," which was
> written in 1985! We are not exactly going over new ground here.
True, but nobody has yet filled in the gap - except that
it is no longer a total gap, just a barren zone.
> Raup didn't come up with the Nemesis model for periodic mass
> extinctions; that's an astrophysicist's responsibility. He came up
> with the statistical data for the extinction pattern and wrote a
> popular book describing the course of the debate. The Nemesis model
> relied on 1) statistical analyses of extinction data, and 2) showing
> that many or most of the "right" extinction events were related to
> asteroid impacts. Because the data are hard to get, the latter is
> still an open question. The statistical analyses were equivocal and
> I'm actually a little surprised there hasn't been more on this now
> that the genus-level data are available (the original analyses were
> based on family-level data).
The problem is that the shortcoming of the original analysis was
due to the *temporal* resolution of the sampling, *not* the
taxonomic. That is the "periodicity" turned out to be a resonance
of the average interval dutation.
Fixing this requires substantially greater stratigraphic control
on the timing of the various extinctions, not splitting the taxa
further.
At present I consider there to be *no* evidence for regular
periodicity in the occurance of mass extinctions.
>
> I think the Kerr quote on the tsunami deposit in Mexico speaks for
> itself. The point is that a VERY HIGH-ENERGY mechanism would be
> needed to explain the deposit, and such a mechanism isn't provided by
> marine regression or Deccan Traps volcanism models.
So?
- Tsunamis caused by major earthquakes happen all the time.
- Even if the Deccan's and the marine regression were
involved in the extinctions, this does *not* mean an
impact didn't happen.
You keep trying to make these mutually exclusive alternatives.
They are not. All of the stuff could have happened - none of
the processes or events under discussion rule out any of the
others.
>
> The comment on Meyerhoff is hardly worth replying to. At this point I
> am starting to get the impression that Dr. Schwimmer believes that
> "expert opinions" are not to be questioned by anyone, regardless of
> how old the data are or whether they are even published!
No, I think that is not what he was saying.
What he *was* saying is that people with first hand experience
of the sediments in question should be listened to with respect.
It is often possible to see first hand a pattern that is difficult
to reduce to numbers in a clear way.
I agree, it would be nice if he put some numbers to his results,
but the lack of exact numbers doesn't make his *observations* any
less real.
> 2) are
> nearly 30 years out of date, and I say this because I have personally
> compiled 3000 faunal lists and attempted to bring them up to the most
> current taxonomy. What I find over and over again is that faunal
> lists from the 1960's and earlier are almost completely unreliable at
> the species level prior to being corrected.
Why do you keep bringing it back to faunal lists?
We are not talking about faunal lists, we are talking bone abundances.
>
> I agree that the K-T marine regression may have been an important
> contributing factor in the extinction event. However, it fails to
> explain the extremely rapid and severe extinction of terrestrial
> mammals and plants at that time.
Plants?
The last I read, land plants showed significant extinctions
*only* in North America. In other areas of the world it is
often impossible to recognise the K-T boundary by means of plant
fossils. (I.e. plants do not constitute index fossils for this
boundary except in North America).
> Regressions of similar magnitude
> occurred several times during the Cenozoic, and there is no evidence
> for a mass extinction of mammals ANYWHERE in the Tertiary record of
> North America! Why would marine regression cause such serious trouble
> for terrestrial communities at the K-T boundary but never again for
> nearly 65 million years? I genuinely find this intriguing.
>
Perhaps because mammals were already adapted to the highly seasonal
climates produced by regression, while dinosaurs, having evolved
in the equitable Mesozoic climates were not?
swf@elsegundoca.ncr.com sarima@netcom.com
The peace of God be with you.
---
HORTON@bcrssu.agr.ca writes:
> In my humble opinion, the evidence for dinosaur decline prior to
> the KT boundary layer is due to sampling effects.
Then you haven't read the latest article on that subject in
the literature. (I will try to bring the cite tomorrow).
In background, it has long been known that dinosaur bone
drops out about 3 meters below the K-T boundary in the
Lance/Hell Creek formations.
A recent intensive search of this 'barren interval' has only
yielded a few isolated dinosaur bones. Enough well preserved
one to rule out the idea that acid rain from the impact dissolved
them (since even surviving bones would be damaged by this),
but no where near as many bones as below the barren zone. Thus,
even after a very specific, very intense search of the beds
immediately preceding the impact layer, the barren zone remains
barren (even if no longer entirely empty).
The abundance of dinosaur teeth in channel sands also drops off
at about the same level, and then peters out *across* the boundary
layer (or at least up to it - the stratigraphic position of channels
is often hard to determine, since they erode into surrounding
sediment).
> The other theories, eg. disease,
> don't make much sense.
You seem to be unaware of the Deccan Trap volcanism which spanned
the boundary. This would have many of the same effects as an
impact, but spread out over a longer period.
Combine this with the cooling climate, the lowering shelf seas
(for marine organisms), and as a final blow, the Chicxulub impact,
and you get a fairly good multi-causal model for the extinction.
Almost all major extinctions are associated with several stress
factors, not just one. The Permo-Triassic extinction had lowering
sea levels, increased aridity, lowering temperatures, and flood
basalt volcanism similar to the Deccan series (in this case, I
believe it was the Siberian series) - but NO evidence of any impact.
And this was the greatest extinction in the Phanerozoic, and indeed
probably the greatest extinction since the oxygen crisis.
Most of the lesser extinction events of the Mesozoic are associated
with both flood basalts and with Ocean Anoxic Events - periods
in which the oxygen compensation depth decreased enough to put
most shelf areas under anoxic waters. (There may have been one of
these associated with the P-T extinction as well).
>. It has to explain
> not only this extinction event, but other mass exinction events.
Why? Why do all extintion events have to have the same cause,
or set of causes?
Actually, if they do, you have ruled out the impact, as impacts
are *not* associated with most extinction events.
In fact if you insist on one cause, then it has to be flood
basalt volcanism, which is the one thing that has the *highest*
correlation with extinction events. [The only extinction event
I know of that violates it for certain is the Pliestocene extinction]
> For an excellent discussion of the impact theory vs. other
theories, read
> "Nemesis: the Death Star" by Rich ...(I've forgotten his last name:
Mueller?).
> He was a friend of Louis and Walter Alvarez, two of the men who
proposed the
> impact theory. The author of this book takes the next step, to
propose a
> mechanism for periodic impacts.
This stuff is seriously out of date. Further studies have:
- cast doubt on the periodicity
- failed to find evidence of impacts in association with
most other extinction events.
- failed to find any evidence of any other planetary
body in the Solar System.
swf@elsegundoca.ncr.com sarima@netcom.com
The peace of God be with you.