[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Extinction (was: Religion)
> Just because Williams paper is more recent than Sheehan et al.'s
> doesn't mean it's "outdated" and therefore irrelevant. Quite the
> contrary: Sheehan's paper at least makes an attempt to deal with the
> problem statistically, whereas Williams' includes no original data
> whatsoever, much less an analysis (whatever else you might say about
My main objection to Sheehan's paper is that resolution the is
too course. One third of the Late Maastrichtian is too large
a piece to resolve the issue.
> I honestly believe that the now-infamous "2-3 m gap" is irrelevant to
> this debate. I already went over the argument that this interval
> represents no more than 40,000 years, unless there is a major hiatus
> within it that nobody has detected (as far as I know), and Friesen
> agrees with this figure. If we are talking about dinosaur extinctions
> in the context of the Maastrichtian world, we should remember that
> this interval includes less than 0.4% of the record for the
And about 1% of the Late Maastrichtian. And ...
I think it is relevent, since it seems to persist despite
hight sampling rates in the Hell Creek.
> Dinosaurs do NOT have a good record, and having EITHER
> a sampling- or taphonomy-produced gap of this length is absolutely
Taphonomic is possible, sampling was possible, but is no longer.
If it is taphonomic, then at least some sections of the Hell
Creek should lack the gap (as the Hell Creek covers several
thousand square miles of exposed sediments). I cannot imagine
a flood basin so uniform over so large an area as to have a
uniform taphonomic bias over the whole area at same time.
Now, since only a few localities in the Hell Creek have been
well sampled, it *is* still possible that the barren zone is
local in occurance, and disappears in some less sampled locality.
But so far nobody has found such a locality.
> As far as I know the age of the Mexican tsunami deposit has not been
> called into question (the Brazos River section is another issue).
OK, I have not followed the literature on that part of the world
very closely, so I can well believe that.
> The end Triassic IS one of the Big 5, the others being the end
> Ordovician, late Devonian, Permo-Triassic, and K-T. The original
> reference on this is Raup and Sepkoski (1982: Science 215,1501). I
> suspect this is just a lapse on Friesen's part, unless, of course,
> there is a problem with R & S' analysis (entirely possible but it
> sure looked good to me).
Hmm, the list I am familiar with puts in the terminal Cambrian
and leaves out the Late Triassic.
> I think Friesen and I agree that the "world gone to hell" model may
> work for the Permo-Triassic, although I don't think it's quite
> necessary for the K-T (not that the late Maastrichtian was the nicest
> time in Earth history). However, the timing of the glaciations is way
> off from the boundary (see Erwin's recent book).
True, but the extinctions also peaked before the official boundary,
and in fact coincide rather closely with the glaciations, *and*
with the flood basalt volcanism (I think the Siberian Traps).
[The extinctions mostly occured one epoch *prior* to the P-Tr
> I agree that sea
> level change may have made things tough at the K-T, but it's not
> clear to me how cause and effect worked here - there was an abrupt
> and major climate change at the K-T (not BEFORE the boundary, at or
> after the boundary), and that could have had an affect on sea level.
The sea level was dropping from well before the boundary, in fact
over most of the Late Maastrichtian. (The Hell Creek beds overly
regressive beach sandstones).
And the climate of the entire Late Maastrichtian was more
continental than the preceding times, at least in the western
North American section.
True, there was another, rather abrupt, change of climate in the
vicinity of the boundary, though it seems to have been quite
some years *after* the actual boundary, as the Hell Creek
Formation to Tullock/Fort Union transition is some distance
above the K-T boundary as now defined, and it is that transition
that corresponds to the largest change in climate - with a
substantial increase in effective rain fall.
> The article cutting the Hell Creek into three units and showing there
> were no differences in the dinosaur faunas in those three units was
> indeed the same as the Sheehan et al. article I have been talking
> The Hell Creek doesn't include a complete K-T boundary section in
> EVERY exposure. If you look at Archibald's strat sections (1982:
> UCPGS 122) you will see that the situation is extremely complex, with
> an awful lot of lateral variation. Many of the "boundary" sections
> are badly messed up by channel complexes that cut down into or
> completely obliterate the boundary. Furthermore, the main evidence
> for the location of the boundary in the first place is the location
> of the "lowest" coal, wherever that may be.
That is the Hell Creek/Fort Union boundary, not the K-T boundary.
Where the section is complete, the boundary clay, with its Iridium
and other siderophiles is *below* the lowest coal by some distance.
[Lowest "continuous" coal, that is, local coal beds are known
from within the Hell Creek].
This is something I remember quite well, as it caught me by surprise
once when I read an article that talked about the Paleocene
portion of the Hell Creek Formation. The *old* treatment was
to simply *assume* the top of the Hell Creek was also the K-T
boundary. Alvarez's results changed that.
>. Since Friesen has
> already made a good point about this being a phenomenon entirely
> local to the Hell Creek, I think we should call a truce on it.
At least so far. It has not actually been *looked* for elsewhere
yet. (Actually, there are relatively few places with a sufficiently
complete section across the boundary to do so).
> The record for extinctions other than the K-T is far, far worse in
> all respects (excluding the end Pleistocene, which is restricted
> almost entirely to mammals and birds). I am including the other four
> of the Big 5 here; the lack of data extends to biostratigraphy,
> absolute dates, mechanisms like flood basalts, glaciations, impact
> craters, you name it. If the K-T impact story is unconvincing, I
> guess we should just give up completely trying to explain large-scale
> patterns in the fossil record (of course, that would put me out of a
I guess I just like to see confirmation of models.
Just because the record is currently less clear for the others
does not mean it cannot be made better.
I am suggesting that one way to resolve this is to concentrate
research on the other extinctions, particularly the P-Tr and Late
> A really huge volcanic eruption could change atmospheric chemisty and
> therefore climate in a big way, but to be honest this seems to me
> like a much less efficient way to cause massive extinctions in the
> oceans than slamming a gigantic asteroid into a carbonate shelf.
Not one huge volcanic eruption - nearly continuous, massive lava
flows and lesser eruptions for several hundred thousand to a few
This is *continuous* disruption of the atmosphere for many, many
The sort of volcanism seen in the deposition of flood basalts
is not found anywhere on Earth today, so we have no intuitive
model of what it would have been like.
The closest thing I know of is the Iceland Hot Spot, but that
is not really the same at all.
> course, this depends on what you are putting in the atmosphere, how
> much of it, how fast, and how it effects ocean chemistry, and
> possibly these things could be worked out. I will try to get ahold of
> the Rampino refs - are any of the in Geology? That would make things
I don't know. The main one I have access to is the one in
the symposium I mentioned (as I *own* a copy of that).
Talking about references, however, I located the Brazos article,
and it *is* in "Extinction Events in Earth History". The
Hanson, T.A. & B. Upshaw, 1990. Aftermath of the Cretaceous-
Tertiary extinctions: rate and nature of the early Paleocene
So, you were right, it is molluscs.
In the section in question, 15+ species out of ~55 species
of mollusc disappear in about 10 cm, about 0.5 meter below
the tsunamite. This is followed by a 10+ cm in which no
species disappear, and then the number of species starts to
drop off gradually over the last third of a meter. This last
gradual drop off certainly looks like a sampling effect, and
probably represents an abrupt extinction of most species in
the interval destroyed by the tsuname (assuming it was a
This looks like two nearly instantaneous extinction pulses,
each smeared out a little by sampling effects. (That is, the
10 cm width of the first pulse is also probably artifactual).
> Checking the Williams article again - he repeats the critical data on
> dinsaur tooth abundances - I see that the drop in dinosaur abundance
> starts in the Bug Creek Anthills fauna. This is the same fauna where
> we first see (to my mind) completely unambiguous Paleocene mammals
> mixed up with presumably reworked Cretaceous mammals.
The citation I have (which is incomplete in this regard) shows
18 Lancian species and only *4* non-Lancian species in the Bug
Creek Anthills channel. It does *not* say whether these four
species continue into the undoubtedly Puercan channels.
[I seem to remember another, more detailed, article by Archibald
which does go into that, but that would be in a journal, not
in any of my home references].
This does not look to me much like a Puercan assemblage with
reworked Lancian teeth. Though if the 4 "new" species show
up higher in the section, I might be willing to consider that
> Note the
> monotonic decrease in the density of dinosaur remains going up into
> higher and higher channels, exactly as predicted by a taphonomic
> "moving window."
Hmm, actually, it looked stepped to me, an abrupt drop between
the KK and BCA localities, and then essentially constant throughout
the next 4 or so channels, and then nothing at all.
[At least the small variations between samples did not reach
The peace of God be with you.