Re: endless K-T drivel

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John Alroy <jack@homebrew.geo.arizona.edu> wrote:

> Sorry, folks. Got carried away. This is yet another reply to MacLeod.

Just so everyone on PALEONET knows, there have been similar threads
of discussion on this general topic in both sci.bio.evolution and
sci.geo.geology over the past weeks.  I've put in a few words, but
John Alroy is the only other PALEONET subscriber who, as far as I
know, has been reading and posting in Usenet.  The discussions have
been marked by a greater diversity among the participants than is
seen here, with several flavors of paleobiologist and geologist, as
well as numerous articulate participants from outside these fields.
I am looking forward to the creation of sci.bio.paleontology in a
few weeks!


Elliot, D.H., R.A. Askin, F.T. Kyte, and W.J. Zinsmeister (1994)
"Iridium and dynocysts at the Cretaceous-Tertiary boundary on 
Seymour Island, Antarctica:  Implications for the K-T event"
Geology 22(8):675-678

...reports the presence of the iridium signal in a shallow marine
formation in the high southern latitudes.  They write "deposition
across the boundary was apparently continuous, a well-defined Ir
horizon is present, and the macrofauna does not reflect a mass
extinction event."  They also say "fossils of most invertebrate
groups diminish markedly in abundance in uppermost unit 9."  The
Ir signal is in a "glauconitic interval" between units 9 and 10.
Friends of volcanism note:  no sign of ash at the Ir horizon. 
Because the usual marker forams and calcareous nanofossils are
missing in the boundary beds [I assume these are units 9 & 10],
a "carbonate dissolution interval" was previously suggested.

I am not competent to evaluate the last observation, but I would
like to know whether it is in accord with the "blank meter" idea
that was brought up in the discussion of the Signor-Lipps effect.


> [can] the marine record ... definitely rule out a macro-interval
> for the extinctions...?"

How might the marine record be used to address the question of the
interval of extinction of terrestrial animals?


> I realize now that so far both of us have been conflating the
> duration of a causal event and the duration of its biotic sequelae.

Ecologists don't have this problem.  Volcanoes and forest fires rage
for days or weeks;  full biotic recovery often takes many decades.
Yes, a brief recovery probably implies a brief disturbance, but a
long recovery tells us little about the duration of the disturbance.


> ocean anoxia ... should have no major impact on terrestrial organisms.

Don't the global circulation models include several cycles between 
the oceans and atmosphere?  What sorts of impact might anoxic oceans
have on CO2 and O2 content in the atmosphere, and pH of rainfall?


> offering an alternative model that accounts for _all_ of the major empirical  
> observations (such as the floral extinctions, or the timing and size of the  
> crater vs. its supposed lack of sequelae).

Turn it around.  Can evidence of floral extinctions *disprove* any of
the various K/T boundary causal hypotheses?  Can any biotic evidence
help us pick out the One True Cause?  I believe it was Norm McLeod who
argued here that paleontologists can't solve this problem because the
evidence does not come from the biosphere.  Given a bolide impact (A),
what is implied about (B) extinction of X fauna?  A lot.  What does the
extinction of X fauna (B) imply about either an impact (A) or massive
volcanism (A')?  Not very much.


> Perhaps I am being uncharitable and all of us really agree that the
> impact (or impacts) did have major biotic consequences, but that other
> interesting things were going on before and after the impact that also
> had major consequences.  

Sure, why not?  Isn't that usually how things work?


	Una Smith			una.smith@yale.edu

Department of Biology, Yale University, New Haven, CT  06520-8104