[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Fw: Keller 2003

This is my summary and critique of the paper that I presented to another
list (which fortunately allows HTML; that makes a few things easier). I have
the pdf of the paper. I've taken the radiometric dating of the impact melt
at Chicxulub (64.98 +- 0.05 Ma ago) from

James Lawrence Powell: Night Comes to the Cretaceous. Dinosaur extinction
and the transformation of modern geology, W. H. Freeman 1998

which is a must read -- but I don't have it at home, so I can't cite the
primary literature where the date comes from.

----- Original Message -----
From: David Marjanovic
To: paleo_bio_dinosaur_ontology@yahoogroups.com
Sent: Thursday, September 18, 2003 9:10 PM
Subject: Keller 2003

Gerta Keller: Biotic effects of impacts and volcanism, Earth and Planetary
Science Letters 6771, 1 -- 16 (2003, in press)

I recommend this paper [...]. It presents some good science, even though the
conclusions do not have much connection with the data.

The seeding research was done on a deep-sea drillcore from the Ninetyeast
Ridge in the Indian Ocean. This ridge is volcanic, it was formed when the
Indian plate moved over a hotspot, and some of this volcanism took place in
the late Maastrichtian, some hundred thousand years before the K-Pg
boundary. The drillcore contains sediments from that time. It is shown that
there is a very good correlation between episodes of volcanism and disaster
paleobiocoenoses of planktonic foraminifera (which are similar to those that
are found worldwide after the K-Pg boundary).
        After this, Keller looked what happened at the same time in other
(marine) places, focusing completely on the forams as always*. A section in
Madagascar, although plagued by 3 hiatuses caused by sea-level lowstands,
shows the same succession of disaster and recovery. A site in Israel shows
the same, but weaker. Not much is recognizable in Qreiya/Egypt, even less in
El Kef and Ellès/Tunisia, and nothing at all in Spain (not figured). Fig. 1,
a map of the time in question, shows why this is no surprise (even though it
doesn't show the Seychelles and may therefore put India and Madagascar too
far apart).
        Lesson: Big volcanism produces changes in planktonic forams very
similar to that of the K-Pg -- but confined to an ocean or two, not all over
the globe.
        Unlike the sites in the Indian Ocean, Qreiya preserves a K-Pg
boundary layer (clay with iridium anomaly). Therefore I do not understand
why Keller puts a hiatus there, too. In fig. 4, note the total extinction of
large, specialist subsurface forams and the abrupt drop in species richness
at the K-Pg boundary. Species richness does not rise again afterwards, but
stays at a low level throughout the pictured part of the sequence.
        Mishor Rotem in Israel has a hiatus above the K-Pg boundary, but a
beautiful 3 mm thick boundary layer (red clay with "altered impact
spherules" [Keller, p. 8 -- see below] and Ir anomaly). Interestingly, there
are 3 more red clay layers with anomalies of iridium, palladium and
platinum, pointing to more impacts. As an alternative Keller suggests
volcanism, but not any specific volcano -- Deccan can't have been it, after
all. A field for further study. The mass extinction can easily be found, but
the drop in species richness is small, perhaps due to the hiatus, what do I
know. As always, large specialists disappear precisely at the boundary.
        El Kef, the stratotype of the K-Pg boundary, and Ellès, which ought
to be the stratotype because it's even better, are not separated by Keller.
The fused fig. 6 is nevertheless impressive. There is no hiatus. The K-Pg
boundary marks the extinction of the last gradualists -- _get a look at that
figure_. :-o Especially at the line of pixels directly above the boundary. I
state that Keller is not a gradualist. -- Disaster species stay pretty rare
during the volcanic disaster intervals, but explode after the K-Pg boundary,
showing that something different in magnitude was going on at the K-Pg

And then the weirdness starts. Fig. 7 attempts to correlate the sections
with each other, the volcanic episodes at the Ninetyeast Ridge and Deccan,
and assorted impacts. The volcanism correlates wonderfully with the episodes
of disaster -- but I wonder how this correlates with the fact that in one
place in India the K-Pg boundary layer is in the middle of an intertrappean
bed, not at the base of a trap. Chicxulub, Boltysh (in Ukraine) and
Silverpit (in the North Sea) are confidently placed at 65.3 Ma ago, and
impact layers from Haiti, Guatemala and Mexico are confidently assigned an
age of about 64.8 to 64.9 Ma, leaving the K-Pg boundary (which is followed
by the shortest disaster period and the longest recovery period, indicating
its severity) without any known crater. The Discussion ought to explain how
come. Here are the relevant parts (p. 14):

> What sets the KT boundary and basal Danian apart from these other high
stress periods is the mass extinction of all specialized larger species

Yeah. Among other things, described above.

> The coincidence of this mass extinction and the impact event is the
primary evidence that supports a cause and effect scenario.

Apart from all the meteorite remains in the boundary layer itself!!! Coupled
with the absence of evidence for volcanic influence in that layer!

> However, the current data indicate that the mass extinction was likely
caused by the coincidence of intense volcanism and a large impact.

No further elaborations on the basis of this statement.

>         To separate the biotic effects of impacts from those of volcanism
documented in this study is difficult, if not impossible.

Then take the geochemic effects, for example. Or take the absence of
evidence for such a gigantic volcanic eruption that could produce the
oceanwide disaster worldwide.

> This task is further complicated by the recent discoveries of multiple
impacts across the KT boundary [30]. The new core Yaxcopoil-1 drilled inside
the Chicxulub crater revealed late Maastrichtian planktonic foraminiferal
assemblages of zone CF1 in dolomitic limestones overlying the impact breccia
and underlying a KT hiatus [31].

When it's a hiatus, how can it be dated so precisely?

< [Sorry! Wrong question, as nobody on paleo_bio_dinosaur_ontology found
out. Right question: Was the hiatus just assumed because no preserved K-Pg
boundary was found above the impact breccia?]

> This reveals that Chicxulub is not the KT boundary impact crater, but an
earlier pre-KT impact, and the crater was probably much smaller ( < 120 km)
than previously estimated [32]. An alternative interpretation explains these
late Maastrichtian dolomitic limestones as backwash after the impact and
cratering event [32].

Indeed, incredible heaps of "backwash" must be expected in such a big
crater. Kilometer-high vertical walls are not something stable. Why does
Keller _completely ignore_ the radiometric date of _64.98 +- 0.05 Ma ago_
for the impact melt in the crater??? It was, after all, produced by the
uranium-lead ratio in zircon crystals -- the most precise method in
existence. It's not some potassium-argon date that becomes too young when
the noble gas argon starts to escape.

> However, this interpretation cannot explain the absence of reworked
breccia clasts, or clasts from shallow water limestones, gypsum and
anhydrite underlying the breccia,

What about: That huge chunk they were boring through broke out off the wall
of the primary crater, and lay below the anhydrite etc.? Why should
"reworked breccia clasts" result from such a violent event?

> or the absence of size grading and sorting,

I'd expect complete chaos when km-high walls of rock full of cracks collapse
in a matter of hours. "Size grading and sorting" results from much gentler,
more gradual processes, such as the sedimentation of clasts that were
distributed in the upper water layers by a storm at sea.

> or the origin of pelagic planktonic foraminifera which did not exist in
the lagoonal to subtidal Yucatan platform prior to the impact [31].

What do I know... I expect that sooner or later lots of water gushed into
the crater from far away. It's possible that the impact emptied the entire
Gulf of Mexico (e. g. see here
http://www.dinosauria.com/jdp/impact/wham.htm). Unfortunately [31] is an
in-press paper.

>         The recent discovery of three or four microtektite layers
interbedded in up to 10 m of late Maastrichtian marls (zone CF1) in numerous
sections throughout northeastern Mexico also indicates that the Chicxulub
impact predates the KT boundary [33].

Wrong. This indicates that there was an impact before the K-Pg boundary -- 
but by no means that this was Chicxulub!

> The oldest microtektite layer is near the base of zone CF1 and indicates
the impact occurred about 300 kyr prior to the KT boundary.

So _this_ is where her date for Chicxulub comes from?!?!? See above for
radiometry -- Keller agrees with, meanwhile, everybody else that the
boundary was 65.0 Ma ago.

> [...] Although these new findings are still controversial, perhaps mainly
due to the entrenched view that Chicxulub is the KT impact crater,

Hear, hear. Keller argues from psychology. I will not point out that this is
a broadside ad homines argument. Instead, I will speculate that this means
that Keller never shared the view that Chicxulub was the K-Pg boundary
crater, even when the "evidence" she cites now was still undiscovered.

> they reveal a consistent pattern of a pre-KT age that is difficult to

Yes. A pre-K-Pg age of some impact. Which need not be Chicxulub.

>         There is mounting evidence of multiple impacts in Late
Maastrichtian and KT sediments. In Oman [34] two distinct Ir anomalies mark
the KTand pre-KT impacts.

Then so be it. On geologic time scales, big impacts (as opposed to gigantic
ones like Chicxulub) are not rare.

> In addition, there is evidence for two small impact craters during the
late Maastrichtian.

During? Or at its end?

> One is the 24 km wide Boltysh crater of Ukraine dated at 65.2 +- 0.6 Ma
[35] and the other is the 12 km wide Silverpit crater of the North Sea [36].
Neither of these smaller craters likely had long-term biotic effects,

No wonder at that size -- but considering the possibility that their age is
65.0 Ma, it is possible that they contributed to the K-Pg mass extinction.
Which is quite a long-term biotic effect!

> though they suggest a period of comet showers associated with the KT mass
extinction and volcanism (Fig. 7).

Except that Chicxulub was made by a planetoid, not by a comet. And so was
the K-Pg impact, if it really was different from Chicxulub. :-| -- 
"Associated"? Does she just mean they coincided in time, or that there was a
causal association?

Fire away! :-)

* Although she ignores the tiniest forams, those under 38 µm in diameter.
They fell through her sieve.