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Re: impact extinctions

 > >Finally, at this time there was a major "Ocean Anoxic Event".
 > ...
 > If impacts are periodic, and not random, in nature, then it has been
 > that the mechanism that causes the periodicity (eg. a companion
 star) sends in
 > a shower of comets over a period of hundreds of thousands or
 millions of years.

Yes, this has been suggested.  However, the supposed evidence for
periodicity is invalid.  It turns out to have been an artifact of
the method of analysis. That is, because the data were collected
at the epoch level, and because of the way we have divided the
past, with epoch lengths clustering around a very few values,
multiples of those durations show up as peaks in the periodicity
analysis regardless of any actual periodicity.

Thus there is *no* evidence for periodicity.

Also, there is no real evidence for high rates of impacts
at or around the K-T boundary.  So, if that case has only one or
two major impacts, it is unlikely that the other, less studied
cases, have more.

 >... Stan
 > provided no explanation for what caused eg. anoxia, and I am
 wondering if some
 > of the effects noted above MIGHT BE CAUSED BY impacts.

There have been several suggested explanations, including changes
in rates of geochemical weathering, especially of carbonate rocks,
which in turn could cause changes in abundance of phytoplankton.

Another possiblity is basically 'overturn', that is a profound
increase in upwelling rates bringing anoxic bottom waters up to
near the surface.

 > Yes, there may be problems to be solved in the impact hypothesis.
 The reason I
 > still support it is that there are even bigger problems with all the
 > alternatives that I have seen proposed to replace it.

Even the multi-causal model I have been discussing?
What is the problem with three major sets of environmental
stresses (for the P-Tr) causing a major extinction?

The current round of ice ages certainly seems to have been
a partial cause of the minor (so far) extinction now going

And volcanism on the flood basalt scale would cause many of the
same environmental effects as an impact, but continuously over
a much longer time period.  (Indeed it is suggested that the
P-Tr ice age was *caused* by the Siberian Trap volcanism).

It is interesting to note that the Late Maastrichtian, the
time of the Deccan Trap volcanism, was a time of lowering
global temperatures, reaching a minimum at around the K-T
boundary (plus or minus a million years or so).  That was
also a time of lowering ocean levels unparalleled since the
Late Permian, with the increased seasonality that increased
continentalism implies.

The main reason I even still consider the possibility that an
impact was a *contributing* cause to the K-T extinctions is that
there were no other major environmental stresses at the time
except the Deccan volcanism.  The K-T extinction is unusual
in that there is *no* Ocean Anoxic Event associated with it.
Almost all of the others have *both* an OAE *and* flood basalt

swf@elsegundoca.ncr.com         sarima@netcom.com

The peace of God be with you.