CCNet SPECIAL: K/T MASS EXTINCTION DEBATE
KICKED WIDE OPEN AGAIN
The new evidence from Yaxcopoil-1, combined
with the spherule
ejecta evidence from NE Mexico, indicates that the Chicxulub
impact predated the KT boundary by about 300k y. Chicxulub
therefore was not the cause for the KT mass extinction.
The KT impact crater still remains to be found.
--Gerta Keller, CCNet, 1 November 2003
On Oct. 9 in CCNET (#7) CHICXULUB IMPACT CRATER
AND THE K/T MASS
EXTINCTION, REVISITED, Jan Smit questioned Keller and her collaborators'
evidence that Chicxulub predates the KT boundary. Here we reply and
discuss the evidence that indicates that Chicxulub is not the KT impact
crater and predates the KT boundary by about 300 kyr.
Thank you for running CCNET - it is an
important forum for dealing
with controversies and letting both sides have their say.
Department of Geosciences
Princeton, NJ, 08544, USA
phone: 609 258 4117
fax: 609 258 1671
CHICXULUB - THE NON-SMOKING GUN
Gerta Keller, Department of Geosciences,
CHICXULUB PREDATES THE KT BOUNDARY AND IS NOT
THE CAUSE FOR THE
END-CRETACEOUS MASS EXTINCTION. This conclusion was announced by Keller,
Stinnesbeck and Adatte at the April (2003) EGU-AUG meeting in Nice,
France, based on over 10 years of KT research (1) culminating with the
new drill core Yaxcopoil-1 in the Chicxulub crater. This evidence has
triggered a renewed debate over the cause and impact location of the KT
mass extinction and the role of Chicxulub. A public debate is sponsored
by the Geological Society of London beginning with its November 1 (2003)
issue of Geoscientist. (Log on to www.geolsoc.org.uk, or email the
Editor c/o firstname.lastname@example.org. To participate.)
Jan Smit questioned this evidence in CCNET of
Oct. 9, 2003. He claimed
that the varied evidence in Mexico presented by Keller and her
collaborators can all be explained by the mechanisms of "tsunamis, mass
wasting, slumping and earthquakes triggered by the impact" which make
explanation of the KT sedimentary deposits "very complicated indeed."
Here we address the major issues and demonstrate that the complicated
explanations by Jan Smit and his collaborators are born of the belief
that the "Chicxulub is the KT impact" theory is a proven fact, and that
therefore any contrary evidence must somehow be explained to fit into
this theory. However, this theory can no longer be supported by
empirical evidence - Chicxulub is not the smoking gun that caused the
end-Cretaceous mass extinction.
EVIDENCE LINKING CHICXULUB AND KT: Ever since
the discovery of the
Chicxulub subsurface crater in the early l990's many scientists have
assumed that this is the crater of doom that caused the demise of the
dinosaurs and many other animal groups at the end of the Cretaceous.
This very attractive theory was supported by: (a) 39Ar/40Ar ages of
about 65 ± 0.2 Ma of melt glass in the Chicxulub breccia and impact
ejecta in the form of glass spherules (microtektites) in Haiti and NE
Mexico, (b) the geochemical similarity of microtektites with melt rock
from Chicxulub, and (c) the stratigraphic proximity to the K-T boundary
in localities throughout Mexico, Guatemala, Belize and Haiti.
TSUNAMI & BURROWS? But
increasingly, detailed stratigraphic,
geochemical and paleontological analyses failed to support the central
thesis that Chicxulub is of KT age. The first complicating factor to a
KT age of the Chicxulub ejecta surfaced early in NE Mexico where a thick
siliciclastic unit separates the spherule ejecta layer(s) from the
overlying KT boundary and Ir anomaly. To reconcile the ejecta with the
KT irdium anomaly as a single impact origin, Smit et al. (2) interpreted
the siliciclastic unit as impact-generated tsunami deposits. In this
scenario the glass spherules settled out first, followed by a
megatsunami depositing the siliciclastic unit and finally settling of
fines depositing the Ir anomaly.
This interpretation was proven wrong by the
discovery of multiple
horizons of burrows within the siliciclastic unit that indicates the
repeated colonization of the ocean floor during deposition (3, 4). This
meant not only that deposition of this unit occurred over an extended
time period, which far exceeded a tsunami event, but also that rapid
deposition (gravity slumps) alternated with normal sedimentation. The
spherule ejecta below this unit could therefore not be of the same
origin and age as the Ir anomaly above the siliciclastic unit.
LIMESTONE LAYER WITHIN EJECTA DEPOSIT: Another
problem that surfaced
early on was the presence of a 15-20 cm thick sandy limestone layer
separating the impact spherule layer below the siliciclastic unit in
outcrops spanning a region of more than 300 km (5, 3). The top of this
sandy limestone layer was subsequently found to contain J-shaped burrows
infilled with spherules and terminated by an erosional upper surface,
followed by another spherule layer. Similar J-shaped burrows were also
observed in the sandstone of the siliciclastic unit above (4). The
presence of the limestone layer sandwiched between two spherule beds
indicates that spherule deposition occurred in two phases separated by
normal limestone deposition and burrowing colonies on the ocean floor.
These two spherule ejecta layers could therefore not represent
deposition during a single event - as assumed by Smit et al. (7-8). The
abundance of shallow water debris and benthic foraminifera indicated
that these spherule layers were reworked and re-deposited from shallow
MORE SPHERULES EJECTA LAYERS IN LATE
MAASTRICHTIAN: More evidence of
multiple spherule ejecta layers was discovered in the late l990's by
five masters students from the Universities of Neuchatel and Karlsruhe
and under the supervision of my collaborators Thierry Adatte and
Wolfgang Stinnesbeck. These students mapped and analyzed the KT
boundary, siliciclastic units, spherule ejecta deposits, and underlying
late Maastrichtian Mendez marls over an area spanning about 60km2. This
first detailed investigation of the late Maastrichtian Mendez marls
revealed the presence of three additional spherule deposits interbedded
in 10-12 m of pelagic marls of the Mendez Formation (8, 9). Only some
small local slumps spanning a few meters were observed. Impact triggered
slumps, mass wasting, or earthquakes cannot account for these normally
stratified Mendez marls (10, 11). To date, the spherule layers can be
correlated over 100 km.
PRE-KT AGE OF SPHERULE LAYERS: In over three
dozen sections examined,
these multiple spherule layers are within planktic foraminiferal zone
CF1, which spans the last 300 ky of the Maastrichtian (1). The
stratigraphically oldest spherule layer is near the base of this zone
and we consider it to represent the original ejecta layer because it
consists of almost pure spherule debris with only very rare clasts or
foraminifera. All subsequent layers contain clasts of marls or spherules
and reworked foraminifera, suggesting that these layers are reworked
from the original ejecta deposit.
It is all of this evidence, - the KT and Ir
anomaly above the
siliciclastic unit, the bioturbation within this unit that indicates
deposition over an extended time period, the two spherule layers
separated by a burrowed limestone layer below the siliciclastic unit
that indicate deposition occurred during two separate events, and the up
to three additional spherule layers below it, - that Smit refers to as
making "the deposits very complicated indeed" to interprete. In fact,
this evidence not only makes it very complicated, it makes it impossible
to reconcile with the KT impact hypothesis.
CHICXULUB A PRE-KT CRATER: And more evidence
against Chicxulub as the KT
impact event was discovered with the new drilling of the Chicxulub
crater. The new core, Yaxcopoil-1 (Yax-1), was drilled within the
Chicxulub crater and was expected to provide unequivocal evidence that
Chicxulub is the KT impact crater that caused the mass extinction.
Instead, the evidence supports a pre-KT age based on stratigraphy,
sedimentology, geochemistry, paleomagnetism and paleontology, consistent
with the evidence in NE Mexico.
The critical evidence is within a 50 cm thick
dolomitized micritic limestone that unconformably overlies the suevite
breccia and underlies the KT boundary. This interval contains diverse
planktic foraminiferal assemblages of zone CF1, similar to NE Mexico,
typical Maastrichitan carbon isotopes values, and paleomagnetic chron
29R, all of which support an age within the last 300 kyr of the
Maastrichitan. Sediment deposition occurred in variable, but low-energy
pelagic environments (see below).
BACKWASH & CRATER INFILL? In order to
have a common origin for the
suevite breccia and the KT boundary, this 50 cm layer must be
interpreted as part of the impact event, such as backwash and crater
infill, as argued by Smit (12). In support of this interpretation he
claims the presence of cross bedding and grain size grading. Sediment
analysis, however, reveals neither cross bedding nor grain size grading.
The larger grains, and " coated sand grains", that Smit refers to are
diagenetic dolomite crystals; no grains are found in insoluble residues,
except for glauconite or glauconite coated grains in five green layers.
The cross-bedding Smit refers to are three <1cm thick layers of oblique
bedding that suggest temporarily slightly more agitated bottom waters.
The absence of grain size grading in these minor oblique bedding layers
indicate that they are not cross beds.
GLAUCONITE RULES OUT BACKWASH:
Sedimentologically, the critical 50 cm
interval between the impact breccia and KT boundary consists of
laminated mciritic limestone with five thin green burrowed glauconitic
intervals. XRD and ESEM analyses indicate that no altered impact glass
(e.g. Cheto smectite) is present in the green layers, contrary to Smit's
earlier claim (13). Since glauconite forms at the sediment-water
interface in environments with very slow detritus accumulation, these
layers indicate long pauses in the overall quiet depositional
environment, the formation of glauconite, sediment winnowing, clast
generation and small-scale transport by minor currents. Thus, far from
backwash and crater infill by reworking over a short time period, the
sediments reveal normal low energy pelagic deposition over an extended
time period following the Chicxulub impact and preceding the KT
MICROFOSSILS OR CRYSTALS? Smit reports
that he and Arz (Zaragoza group)
could not distinguish the foraminifera from "dolomite crystal
overgrowths of sand grains". This is not surprising, since they
apparently searched for foraminifera in the dolomitic intervals where it
is well known that the large dolomite crystals absorb any evidence of
the original fossils. The recrystallized foraminifera are preserved in
the micritic limestones where the overall morphology of species is
preserved. Although thin section analysis of microfossils takes some
experience and can be time consuming, particularly in micritic
limestones, I am confident that with diligent search of the micritic
limestone layers they will find them throughout the section. They have
now been documented from all micritic limestone layers.
REWORKING? Smit claims that even if they missed
the foraminifera, their
presence should be attributed to reworking in the backwash and crater
infill. However, there is no evidence that this zone CF1 late
Maastrichtian assemblage is reworked, as there are no fossil species
from diverse age intervals as would be expected in any reworked
assemblage, no reworked clasts of the breccia or fossils from the
underlying lithologies, and no evidence for high energy deposition (see
above). Moreover, the Yucatan platform prior to the impact event was too
shallow to support planktic foraminifera. This means that they would
have had to be transported by high-energy currents over long distances
from the Gulf of Mexico; this would also have involved reworked species
from different age intervals. There is no evidence for reworking and
transport of the zone CF1 foraminiferal assemblage.
CONCLUSION: The evidence from
sedimentology and microfossils of
Yaxcopoil-1 indicates that the critical 50 cm interval between the
breccia and KT boundary was deposited under normal pelagic condition
during the last 300 ky of the Maastrichtian. Yaxcopoil-1 is not alone.
Limestones containing late Maastrichtian planktic foraminifera have been
reported from sediments overlying the impact breccia in wells T1, Y6 and
C1 (14) - a fact also supported by e-log correlations (15). The new
evidence from Yaxcopoil-1, combined with the spherule ejecta evidence
from NE Mexico, indicates that the Chicxulub impact predated the KT
boundary by about 300k y. Chicxulub therefore was not the cause for the
KT mass extinction. The KT impact crater still remains to be found.
1. Keller et al., Earth Science Reviews 62, 327-363 (2003).
2. Smit, J., et al.. Geology 20, 99-104 (l992).
3. Keller, G. et al. GSA Bull. 109, 410-428 (l997).
4. Ekdale, A.A. and Stinnesbeck, W., Palaios 13, 593-602 (l998).
5. Keller et al., Field Guide, LPI Contr. No. 827, 110p. (l994).
6. Smit et al., GSA Special Paper 307, 151-182 (l996).
7. Smit, J., Ann. Rev. Earth Planet. Sci. 27, 75-113 (l999).
8. Stinnesbeck, W., et al., Canadian J. of Earth Sciences, 38, 229-238
9. Keller, G., et al., Geol. Soc. America, Special Paper 356, 145-161
10. Soria, A.R., et al., Geology, 29, 231-234 (2001).
11. Keller, G. et al., Geology, 30, 382-383 (2002).
12. Smit, J., et al. Geophys. Res. Abstracts 5, 06498 (2003).
13. Lopez Ramos, E. Vol. 3, New York, Plenum Press, 257-282 (1975).
14. Ward, W. et al., Geology, 23, 873-876, (l995).
CHICXULUB - YOUR CHANCE TO HAVE A SAY
The Geological Society, 1 November 2003
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