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:
Update on the possible Falkland
craters.
In his book "Rogue asteroids and doomsday comets" Duncan Steel
refers
to investigations by Michael Rampino (then at the Goddard
Institute of
Space Studies): "The appropriate antipodal point to Siberia
(Siberian
Traps 250Ma) is
the Falkland plateau off Tierra del Fuego... on that plateau
Rampino
has identified two sub-oceanic circular basins with diameters of
about
300km and 200km respectively. Dating the rocks indicates the same
age
as the Late Permian Extinction." He cites a Rampino paper in the
book
"Hazards due to Comets and Asteroids" called "Extraterrestrial
impacts
and mass extinctions" - this may contain more information.
I cannot find any recent references to this subject. It seems that
the
same issues that delayed discovery of Chicxulub are occurring in
the
case of the Falkland Plateau - commercial and political
sensitivities.
The area has potential
oil reserves and is also very sensitive politically. Altimetry
map of the Falkland Plateau from Delft Institute for
Earth-Oriented
Space Research.
CCNet
21 Oct
1999
(1)With reference to Michael Paine's communication (CCNet
20.10.1999),
several indicators exist for extraterrestrial impacts
contemporaneous
with the Permian-Triassic boundary - which saw the largest
extinction
recorded in Earth history - including:
1. Araguainha impact structure, Brazil - ~247 +/-5.5
Ma;
D=40 km (see R.A.F. Grieve's crater listing).
2. Lorne Basin - New South Wales - a candidate P-T
boundary
impact
structure, 35x30 km large (Tonkin, P.C., 1998, Aust. J. Earth
Sci.
45, 669-671).
3. shock features in quartz (PDF) along the P-T boundary
in
Antarctic
and New South Wales (Retallack G.J., Geology, Jan. 1999; for
other
references re-P-T boundary conditions refer to Retallack and
Krull,
1999,
Aust. J. Earth Sci. 46:785-812.
4. Weak Ir anomalies reported from China and Japan,
remaining
unconfirmed due to possible analytical problems.
On the basis of known stratigraphic constraints, more than
one
impact structures may prove to be of a P-T boundary age by
future
isotopic age studies. The Falkland structure (M.R.
Rampino)
and Bedout structure
(off
NW Australia, J.D. Gorter) are only candidate P-T impact
structures
inferred from geophysical and in the latter case drilling
data, as yet
unconfirmed and undated. As yet the magnitude of the confirmed
impact/s
is not large enough to link them to the P-T boundary
extinction and/or
as
triggers of the Siberian volcanic traps (248.4+/-2.4 Ma),
although it
is definitely possible further crater/s identification and
isotopic
dating may shed light on these questions.
Andrew Glikson, Research School of Earth Science,Institute of
Advanced Studies, Australian National University
(2) POSSIBLE FALKLAND IMPACT STRUCTURE
There is a large, circular gravity anomaly on the Falkland
Plateau that
resembles anomalies associated with large impact
craters. It is
quite large; greater than 200 km in diameter.
The basin that is indicated could be Late Paleozoic or Early
Mesozoic in age, but not much more is known about it. Recent
papers
have suggested that it is of tectonic origin, but more study
is needed.
I suggested that it might be an impact structure, and should
be
more closely studied back in 1992.
Dr. Michael R. Rampino
Update 15 Aug 17 Terra Nova ($): Geophysical
evidence for a large impact structure on the Falkland
(Malvinas) Plateau by Rocca, Rampino and Presser
See also Duncan Steel's book "Rogue
asteroids
and doomsday comets"
Impacts and vulcanism
CCNet 15
Feb 2001
P/T BOUNDARY: NO IRIDIUM
From Hermann Burchard <burchar@mail.math.okstate.edu>
Dear Benny,
In Permian/Triassic boundary strata in South China, the element
iridium is not present or at most only in trace amounts,
according to
Doug Erwin, who kindly responded to my e-mail question. This can
be
understood, as
I would like to suggest, by noting certain connections with the
iridium-rich Hawai'i hotspot, which has been moving in a SE
direction
across the Pacific for >100Ma, probably 225Ma, starting off
from
Sibiria.
As mentioned by Victor Clube and Bill Napier in their book
"Cosmic
Winter", magmas from the great Hawai'i volcanoes are rich in
iridium.
They discuss this, because it's an argument against cometary
impact as
a cause of the abundance of the element in extinction layers,
such as
the famous K/T-boundary.
There is a clear trace on the floor of the Pacific ocean
beginning
with the Emperor Seamount chain from the Kamchatka Peninsula to
Midway
Island, then angling off in a slight left turn along the
Hawai'ian
island chain. Although the trace possibly is now partly
subducted in
the Kamchatka -
Aleutian trench, it seems clear enough that the hotspot was
originally
positioned in Eastern Sibiria.
Underlying the hotspot is a mantle plume which presumably was
created when a cosmic body hit Sibiria and created the vast
flood
basalts of Yakutia (Sakha). See the article by Renne et
al. in
"Science", 1995, 269:1314, for a map of the conjectured extent
of the
original lava beds, which may not have been fully
explored. These
cover Yakutia (Sakha), bordering directly on the Sea of Okhotsk
near
Magadan, immediately adjacent to the present day NW-terminus of
the
Emperor Seamount chain. From my less than
adequate maps, the basalt beds seem to abut on or even include
the
Kolyma
gold and diamond fields; diamonds have been studied in
connection with
impact
sites e.g. by Christian Koeberl.)
Therefore, little doubt can exist concerning the essential
identity
of the following events:
1.
Inception of Hawai'i hotspot in Sibiria.
2.
Sibirian flood basalt eruption.
3.
Cause
of P/T mass extinction.
We owe the identity of 2. and 3. to the work of paleobiologists
like Doug Erwin. Here, we wish to explain that event 1. probably
was a
cosmic body impacting in Sibiria - more precisely a spot in
Gondwana-land which became present-day Eastern Sibiria.
Much of the meteoritic material from the comet or asteroid,
that
struck Earth at the P/T transition, appears to remain still in
the hole
punched in the upper mantle by the cosmic impact body, the
Hawai'i
hotspot (I sincerely doubt that this will seem like a very novel
idea
in the minds of many
geologists).
Hence we may conclude:
[A] Iridium continues to be pumped upward with
deep
mantle material in Hawai'i volcanoes to this day.
[B] Little of the cosmic material was thrown into
orbit at
impact time, because of uniquely deep penetration of the giant
P/T
impactor.
[C] Iridium cannot be traced in the layers
separating
Paleozoic and Mesozoic rocks, never having been
dispersed
to a great extent.
[D] Rather than refute it, as Clube-Mapier feared,
abundant ir in the magmas from Hawai'i confirms the impact
theory
of mass
exinctions.
The relationship between impacts and hotspots is perhaps still
somewhat controversial, so I will attempt to elaborate on this.
Hotspot
physics
and geology is probably not a perfect science. If I
understand it
correctly, the main mechanism is the same as in spreading or
rift
zones:
Pressure on the upper mantle is relieved as the minerals rise
with
reduced overburden, causing a phase transition which we see as
melting.
The causes of pressure release are somewhat different in the two
arrangements of a) impact related hotspot and b) rift zone.
In case b) of a rift zone one possible initial cause of reduced
pressure seems to be thinning of continental crust due to
erosion of a
stable craton over many 100Ma. The African rift valley is a case
in
point. In North America, at the beginning of the Jurassic era,
the
Atlantic ocean first began as a rift, with the margin seen today
e.g.
in the New York palisades
rock facade. At present, the New Madrid fault along the middle
Mississippi may exemplify the same phenomena at an early stage.
However, a rift may begin with a hotspot, as one other
interpretation
of the Great African Rift suggests! One hotspot is in the Afar
region.
In case a) of an impact-induced hotspot the initial step is
that
the impact events destroy the phase equilibrium of the upper
mantle in
a narrow region underlying the crater. This may be due
partly to
the shock wave of impact upsetting crystalline structures, or
partly
because surface rocks are excavated and removed by the impact
explosion. A massive
melt results in the form of flood basalts from the suddenly
relieved
pressure, and/or from impact shock wave induced phase
transition. The
effect is a
snowballing phase transition and melting.
Again, to avoid misconceptions, and because this does seem to
remain controversial in some circles, it should be emphasized
that:
The melt in the plume after
impact
is _NOT_ caused by the initial energy yield of impact, but
rather
by the reduced pressure which forces a phase transition to
take
place that ends up in a phase equilibrium at a lower Gibbs
energy.
(Other views [as in Renne et al.] present a picture of a
spontaneous rise of mantle, liquifying over a huge area, for
which no
account of origin can be given. This can be considered for
basalt
floods, but could not explain narrow-bounded hotspots.)
Once in operation, lower mantle material appears to be
resupplied
continually from the sides to the punch hole, which maintains a
pore
where the pressure remains lower than in the surrounding
mantle.
Thus the plume can
rise indefinitely, as we see happen today in Hawai'i.
Any computations of effect of impact on the mantle not modeling
phase equilibria and transitions should be treated with
suspicion.
Above description of mantle plumes, to make a disclaimer, is
conjectural, not substantiated by actual computation. My
limited
understanding of these things
is based on a study of stable computation of phase equilibria,
working
with a petroleum engineer, on computing "flash" crude oil
separation.
Although apparently still controversial, years ago already I
have
heard mention made by geologists of the connection of hotspots
and
impacts,
as in the example of the Yellowstone hotspot, now in Wyoming,
that has
travelled East along the Snake river plateau for > 10Ma, and
that is
implicated in the flood basalts in Western Idaho and probably
Washington
State (?).
(Unfortunately, I was unable to attend the February 9 RAS
conference on impacts, where Christian Koeberl was keynote
speaker. I
missed talks by Adrian Jones and Simon Kelly on impact, flood
basalt,
& hotspot related topics, that might have led me to improve
this
account).
Best regards,
Hermann G.W. Burchard
burchard@math.okstate.edu
See a response by Andrew Glikson, Item 12, CCNet
23 Feb 01.
He questions the P/T link.
IMPACT VOLCANISM CLARIFICATION
>From Mark Boslough
<mbboslo@sandia.gov> CCNet 1
Mar 01
A few inaccuracies crept into the March Scientific American
article.
1) Our seismic focusing calculations showed that the peak in
seismic energy
dissipation is in the asthenosphere both antipodal and directly
beneath
the
point of impact. We suggested that for a sufficiently large
impact the
increased melting in the asthenosphere would be a significant
contributor to
any impact-induced volcanism, but we did not speculate about
effects on
pre-existing plumes or extinctions (although these ideas are
worth
considering). Our idea was that a narrow column of hotter mantle
could
create an instability that *looks* like a plume (as opposed to a
classic
fluid plume that pushes its way up from the CM boundary).
2) I'm not sure where the "may not have been antipodal" phrase
came
from.
The impact antipode was clearly something like 30 degrees from
the
Deccan
Traps at the time of the K/T boundary. If the Deccan Traps
are
impact-induced it was not the Chicxulub impact (which came too
late
and in
the wrong place!) but an earlier impact either into the east
Pacific or
into
India.
3) We suggested that an impact might generate the same surface
manifestations normally associated with mantle plumes (i.e.
flood
basalts
and long-lived hotspots). We did not connect them to superplumes
which
is
what Dallas Abbott proposed. It was Jon Hagstrum of the USGS who
suggested
the connection to sea level, weathering, ocean chemistry,
sediments,
etc.
The ideas of Abbott and Hagstrum are also interesting worth
considering--but
they're not mine as the article implies.
Mark Boslough
Sandia National Laboratories
Can Impacts Induce Volcanic Eruptions?
Authors: Melosh, H. J.
Journal: International Conference on Catastrophic Events and
Mass
Extinctions: Impacts and Beyond,
9-12
July
2000, Vienna, Austria, abstract no.3144
Available
from
ADS Abstract service.
Conclusions: The bottom line of this discussion is that there
is
not a
single clear instance of volcanism induced by impacts, either
in
the
near vicinity of an impact or at the antipodes of the planet.
This
accords well with theoretical expectation from our current
understanding
of the impact cratering process. The possibility of
impact-induced
volcanism must thus be regarded with extreme skepticism.
The Shiva Hypothesis - Periodic Mass Extinctions![[from Jan/Feb 98 issue of Planetary
Report]](shiva.jpg)
The Jan/Feb 98 Issue of Planetary
Report
has an article by Michael Rampino "The Shiva Hypothesis". This
describes
a 30 million year cycle of mass extinctions over the past 540
million
years (see diagram). One hypothesis is that this corresponds the
the
solar system oscillating through the galactic plane as it orbits
the
Milky Way.
Rampino notes that the last crossing of the galactic plane
occurred a
few million years ago and it has been suggested that this led to
a
disturbance of comets in the Oort Cloud, some of which could
now be
approaching
the inner solar system.
This hypothesis was mentioned by Carl Sagan in the 1985 book
"Comet" and first raised by Rampino and Stothers in a 1984 issue
of
Nature. It
is still very controversial, but so was the Dinosaur/Impact
hypothesis
until the early 1990s.
See also:
Send suggestions for this page to Michael Paine.
Last update 16 Feb 2001
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