Chicxulub Debate
"MORTAL BLOW TO K/T-CHICXULUB THEORY?" I DON'T THINK SO!
CCNet 2 June 2005
(Smit) Dear Benny,
Gerta Keller and colleagues again think they have dealt "... the mortal
wound for the
Chicxulub theory" on the basis of new cores taken at Brazos River,
Texas.
See
http://www.nature.com/news/2005/050523/full/050523-2.html
Hermann Burchard, not a geologist, says he believes her now (CCNet 2
June 2005).
Yet the only evidence we have seen is an abstract at the AGU New
Orleans 2005 meeting,
where is stated that between the event sandstone beds (marked by coarse
Chicxulub ejecta)
and the K/T boundary (marked by an iridium anomaly) a normal claystone
layer would exists
(labelled as interval F and G by Thor Hansen, who established the
classic stratigraphy of
the Brazos K/T section). The cornerstone of Keller's arguments is that
deposition of this
F/G claystone would have lasted 300kyr.
I have analysed the grainsize distribution of the same F/G claystone
interval several
times, and every time I got the same results: the entire claystone
layer is size graded,
with the largest grainsize at the bottom and the finest at the top near
the iridium anomaly
(Smit et al 1996). This stands in stark contrast with the normal
claystones above the
iridium anomaly and below the event beds, that are not graded at all.
Grading is due to settling of silt and clay-sized material out of a
suspended sediment cloud
in the Sea, in our view stirred by the mega-tsunamis that resulted from
the Chicxulub impact.
Such settling takes days to perhaps week, not 300.000 years.
Other arguments, same as used last year in the GeolSoc debate (
http://www.geolsoc.org.uk/template.cfm?name=NSG2349857238495)
include burrowed layers,
magnetostratigraphy and Cretaceous foraminifers. These arguments are
easily countered.
Burrows can originate between the individual tsunami waves surges, that
easily last 1 hour,
plenty of time for oprooted organisms to dig in the seafloor again (to
be uprooted again by
the next wave!)
Foraminifers in F/G are all of Cretaceous affinity, and like the silt
and mud stirred by
the tsunamis, will slowly settle to the seafloor in interval F/G. Thus,
no evidence for
indigenous foraminifers!
Magnetochron 29Reversed straddles the KT boundary from roughly 350 kyrs
before to 350 kyr
after the K/T boundary. So anything happening in that 700Kyr interval
has a reversed
magnetic signature, including the Chicxulub impact, K/T boundary, and
claystone F/G.
Thus nothing argues for either pro or contra Chicxulub being the KT
crater.
J. Smit et al., in The Cretaceous-Tertiary Event and Other Catastrophes
in Earth History G. Ryder, D. Fastovski, S. Gartner, Eds. (Geol. Soc.
of Amer., Boulder, 1996), vol. Sp. Pap. 307, pp. 151-182.
So, once again, this is not a "remakable discovery", but a rehash of
the same, erroneous arguments.
Sincerely,
Prof. Dr. J. Smit
Department of Sedimentology
Faculty of Earth and Life Sciences
Vrije Universiteit, de Boelelaan 1085
1081HV Amsterdam, the Netherlands
(Geller) Dear Benny
There is nothing better in science than to have the weight of empirical
evidence proof or disproof a popular theory.
But the hardest thing in science is to convince the true believers and
main proponents of a popular theory that the evidence doesn't support
it.
Frequently, there is gut reaction denial of the existence of the
evidence,
expressions of disbelief and some go as far as discrediting the
messenger.
But every once in a while some scientists realize that the weight
of the evidence is lined up against the popular theory that the
Chicxulub impact is the KT killer that caused the mass extinction.
Hermann Burchard is such a scientist and it took some courage to
publicly
retract his former doubts on CCNet.
Keller, Adatte, Stinnesbeck and others (2003, 2004) have shown that the
Chicxulub impact predates the KT boundary by about 300,000 years based
on
the Chicxulub impact crater core Yaxcopoil-1, KT sections throughout NE
Mexico
and now also in Texas along the Brazos River (AGU, 2005, see write-up
by Rex
Dalton in NATURE NEWS.
http://www.nature.com/news/2005/050523/full/050523-2.html
New drilling by DOSECC this spring and investigations of new outcrops
along tributaries of the Brazos River by Thierry Adatte, Tom Yancey,
Jerry Baum and myself have uncovered outcrops that detail the KT
boundary,
storm event beds (formerly called KT impact tsunami deposits), and the
original
Chicxulub impact ejecta layer. The three events are separated by
laminated
fossiliferous shales with the KT boundary and Ir anomaly up to 1.6 m
above the top of the storm event beds and the original Chicxulub impact
ejecta layer at least 45 cm below the base of the storm event beds. The
storm
event beds have been studied extensively by Tom Yancey (l996) and Andy
Gale
(2005); both came to the conclusion that these bioturbated sandstone
layers were deposited in separate storm events over an extended time
period and not related to the KT boundary impact.
The evidence indicates that two major impacts occurred about 300,000
years apart: the one at the KT boundary is marked by a global Ir
anomaly.
The earlier Chicxulub impact is never associated with an Ir anomaly,
but is
known by its breccia in the crater and glass shards and spherule ejecta
throughout the Caribbean, Central America and southern US. There is a
precedent
for multiple impacts in the late Eocene (originally discovered by
Keller et
al.,1983) with large craters known from Popigai and Chesapeak Bay. The
end-Cretaceous appears to have been a time of multiple impacts and
massive
volcanism.
Gerta Keller
References:
Gale, A.S., 2005 Proceedings of the Geologists Association (in press)
Keller et al., l983, Science 221, 150-152.
Keller et al., 2003, ESR 62, 327-363.
Keller et al., 2004, PNAS 101(11), 3753-3758.
Yancey, l996, Gulf Coast Assoc. of Geol. Societies 46, 433-442.
Gerta Keller
Department of Geosciences
Princeton University
Princeton NJ 08544, USA
Website:
http://geoweb.princeton.edu/research/Paleontology/paleontology.html
The
Chicxulub Debate:
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