Smit smited
"Fluidized, squeezed, diapir-like slumped and folded? Méndez-Formation at Rancho Nuevo?
From Markus Harting*
In Smit's recent reply to Keller et al. he claims that the Upper Cretaceous Méndez shales at the Rancho Nuevo section are fluidised and squeezed up, diapir-like. He further claims that no additional spherule layers are present in the obviously bedded and undisturbed Mendez marls below the spherule layer of unit 1 at the base of the siliciclastic deposit (Fig. c of Smit). No data are presented in support of these claims.
We have conducted a detailed investigation of the Rancho Nuevo outcrop based on field examinations and laboratory studies, including sedimentology, geochemistry, mineralogy and microscopy. Our studies revealed no evidence of fluidised or squeezed Méndez Marls, or squeezing at the edges of the sandstone unit. The outcrop feature interpreted by Smit as 'diapir-like' appears to be the result of the unique view across, and as a cross section, of the channelised siliciclastic deposit.
Smit's claim that no additional spherule layers are present below the unit 1 spherule deposit is also unconfirmed. Our investigation revealed at least one more spherule horizon approximately 60-70 cm below the base of the spherule-rich deposit (SRD) of Unit 1, which supports the existence of multiple spherule horizons (Keller et al., 2002, 2003).
Our geochemical and mineralogical analyses of the spherule unit 1 (Smit's primary ejecta layer) revealed altered ejecta material and secondary authigenic minerals due to hydrothermal influence, in contrast to well preserved ejecta within the Méndez Marls. These investigations, as well as the stratigraphic position, indicate a reworked origin for the uppermost spherule layer, which is commonly labelled unit 1 of the siliciclastic deposit.
References
Keller G. et al. 2002. Multiple spherule layers in the late Maastrichtian of northeastern Mexico. Geological Society of America Special Paper 356: 145-161.
Keller G., Stinnesbeck W., Adatte T. and Stueben D. 2003a. Multiple impacts across the Cretaceous-Tertiary boundary. Earth-Science Reviews 62, 327-363.
*Institute of Mineralogy and Geochemistry, Karlsruhe University, Germany
Multiple impacts?
From Tommy Tyrberg* Received Monday 10 Nov
Sir, The problem with the data of Keller et al. is that they are all derived from the area in or around the Chicxulub crater where the sedimentation conditions were very disturbed and abnormal to put things mildly. More distant K-T boundary sites, of which there are now a considerable number, both marine and terrestrial invariably contain only a single impact layer.
In particular I would like to point to the sequence from the Denver basin described in Rocky Mountain Geology, Vol 38:1. This is a K-T boundary sequence that 'has everything', good magnetostratigraphy, good palynology, a fairly high and seemingly even sedimentation rate, several volcanic ash layers with good absolute dates and a boundary sequence with boundary clay, shocked quartz, iridium, spherules and a fern spike all in the usual order. The palynology indicates a large scale floral extinction at the boundary layer. There is even hadrosaur and ceratopsian fossils c. 4m below the boundary and and Protungulatum fossils about 12 metres above it showing that the turnover between Maastrichtian and Early Paleocene fauna must have happened quite close to the K-T boundary. There is no trace of multiple impacts.
Of course it is always possible that all impact layers except one have eroded completely away while the last one was left intact and that the sedimentation rate happened to vary in such a way as to hide this process but it does not seem very likely.
However there is one scenario that could result in multiple impact layers in the Caribbean and a single layer anywhere else. This is if there were multiple impacts quite closely spaced in time and space, say within a few hours and several hundred kilometres of each other. In that case the ejecta blankets emplaced by the ejecta curtains and/or base surges in the near zones would be superposed upon each other while further away where the ejecta arrived ballistically at the top of the atmosphere the impact signatures would merge. Such a pattern would be expected from a binary asteroid or a fragmented comet like Shoemaker-Levy.
*Norrkoping, Sweden.
Gerta Keller replies to Tyberg: (Rec'd 14.11.03)
Terrestrial record more complete?
Tyberg's claim that terrestrial sections are more complete than marine sequences comes as a complete surprise to us or any biostratigrapher, sedimentologist, palaeontologist or geochemist working with the temporal sedimentary record. It is well known that terrestrial environments are inherently incomplete and fragmented due to sporadic deposition in lakes, rivers or deltas where non-deposition and erosion predominate. This is particularly the case during the sea level regression of the Western Interior that characterizes deposition in the Denver Basin across the K/T transition. Although these sections are excellent terrestrial records, they are not temporally complete. The absence of impact evidence does not prove anything in areas of frequent erosion and non-deposition.
Tyberg states that 'there are several volcanic ash layers with good absolute dates and a boundary sequence with boundary clay, shocked quartz, iridium, spherules and a fern spike all in the usual order'. But what is the exact timing of these layers? Absolute dating of the ash layers varies from 65.03±0.26Ma to 65.74±0.43Ma (Raynolds and Johnson, 2001) with the error margin as large or larger than the age between the Chicxulub and K/T boundary impacts. To our knowledge there are no terrestrial sections where shocked quartz, iridium and spherules are found in precisely the same stratigraphic interval. In terrestrial sections it is quite difficult to evaluate the time of deposition of each of these layers, which are often amalgamated, reworked or simply eroded.
Mexico's sections disturbed?
Tyberg's claim that sites in and around Chicxulub are 'very disturbed and abnormal to put things mildly' is also curious. The northeastern Mexico sites are 2000km from Chicxulub and not disturbed. Hemipelagic marine sediments characterize these sequences and they are only interrupted by the sandstone-silt-shale unit below the K/T boundary and above the highest impact spherule layer. As noted in the position paper for this debate, this siliciclastic unit is also bioturbated at several discrete intervals, which indicates deposition of this unit occurred over an extended time interval during which marine burrowing organisms repeatedly colonized the ocean floor (Ekdale and Stinnesbeck, l998). The up to four Chicxulub impact spherule ejecta layers below this deposit therefore must predate the K/T boundary as also indicated by biostratigraphy.
K/T Ir anomaly & chicxulub ejecta not the same
Tyberg argues that since deep-sea sections contain only a single impact layer, this means that only one impact occurred. Most deep-sea sections contain a millimeter-thin red layer enriched in iridium at the K/T boundary and the mass extinction of tropical and subtropical planktic foraminifera characterizes and defines this interval. That same red layer and iridium enrichment is also present in the sections of northeastern Mexico, where it marks the K/T boundary and mass extinction - just like in the deep-sea sections. These records are therefore comparable and document the same event.
However, in the Mexican sections, the K/T boundary, red layer, Ir-enrichment and mass extinction of planktic foraminifera are stratigraphically and temporally well above the Chicxulub impact glass spherule ejecta layers, as well as above the siliciclastic unit. These spherule layers are interbedded in hemipelagic marls. As noted in our position paper, we consider the oldest and stratigraphically lowest spherule layer as the original ejecta depositional event and the other layers as reworked.
To date bona fide Chicxulub impact spherule ejecta has been documented from the Caribbean, Central America, Gulf of Mexico, southern USA to New Jersey, Blake Nose, and offshore Brazil. Why are they not present in the deep-sea record globally? There are a number of reasons.
- The Chicxulub impact with its crater size of about 120-150km is smaller than necessary for global ejecta dispersal.
- The search to date has concentrated on the K/T boundary and Ir anomaly based on the assumption that this is the Chicxulub impact signal.
- The older pre-K/T age of Chicxulub necessitates a search for impact ejecta in latest Maastrichitan sediments.
Tyberg's multiple impact scenario assumes that several impacts occurred over the same time and space. This is not the case. The stratigraphic evidence indicates three impact events distributed over about 500Ka:
- Chicxulub at 300Ka before the K/T boundary (evidence impact glass spherule ejecta).
- K/T boundary impact (global distribution of iridium enrichment). 3. early Danian P. eugubina zone impact (iridium enrichment).
We agree with Tom Van Flandern (this debate 5.11.03) that this pattern is not compatible with Jupiter's capture of Comet Shoemaker-Levy 9, but implies the explosion of a planet-sized parent body in the main asteroid belt.
A summary of the evidence for multiple impacts is presented in the article that kicked off the Chicxulub Debate. Detailed evidence and discussion is presented in Keller et al. (2003a,b) and many illustrations and brief summary are given on the website:
http://geoweb.princeton.edu/people/faculty/keller/chicxulub.html
References
Ekdale A.A. and Stinnesbeck W. l998. Ichnology of Cretaceous/Tertiary boundary beds in northeastern Mexico. Palaios 13: 593-602.
Keller, G., Stinnesbeck, W., Adatte, T. and Stueben, D., 2003. Multiple Impacts across the Cretaceous-Tertiary boundary. Earth Science Reviews, 62, 327-363.
Keller, G., Stinnesbeck, W., Adatte, T., Holland, B., Stueben, D., Harting, M., C. de Leon and J. de la Cruz, 2003. Spherule deposits in Cretaceous-Tertiary boundary sediments in Belize and Guatemala. J. Geol. London, 160, 1-13.
Raynolds, R.G. and Johnson, K. R., 2001. Synopsis of the stratigraphy and paleontology of the uppermost Cretaceous and lower Tertiary strata in the Denver Basin, Colorado. Rocky Mountain Geology, 38(l), 171-181.
Wolfgang Stinnesbeck replies to Tommy Tyberg (rec'd 14.3.11)
What does Tyberg mean by "disturbed and abnormal"? Originally, these sections around the Gulf of Mexico have been chosen by Smit, Bourgeois, Izett and others because they were so expanded and show a thickness of several metres of layered and correlatable sediments (e.g., clastic deposits), whereas distal sections are generally condensed to a few cm or less, and often present considerable hiatuses (e.g., Gubbio). Also, the above authors agreed upon the fact, that the sections expose a sequence of events (which they interpreted as deposited within a few hours) at a position where distal sections only present a single layer.
The difference between their interpretation of Gulf of Mexico sections and ours is essentially the timing of events, not much more. While they lumped these expanded sequences and several events into a scenario of one catastrophic impact, we were able to differentiate events which are clearly separated in time. It is essentially this biostratigraphic difference in interpretation with events in the late Maastrichtian, at the K/T boundary, and in the Danian that made us conclude that there is nothing abnormal about these sections and that disturbances are minor, and where present, clearly recognizable.
We suggest that authors go back to distal sections and also start to look in more detail at stratigraphic signals. Biostratigraphic timing of events is excellent around the Gulf of Mexico as a result of refined planktic foraminiferal zonation, which allows separation of events of less than 100Ka apart. There is no comparable time resolution at present in terrestrial sections such as the sequence in the Denver basin, and in consequence it is difficult to evaluate the question whether a complete section is present there, or whether erosion led to an incomplete record and hiatus.
ENDS (Tyberg + replies)
One for the palaeomagicians...
From Matt Terry* - Received 6.11.03
Sir,Clube and Napier have suggested that passage of the Sun through denser than average regions of the Galaxy, such as molecular clouds (the remnants of the one we currently inhabit is called Gould's Belt), and as is also presently the case, when nearest the extremes of its orbital parameters (height above/below the plane and crossing the plane, proximity to spiral arm, and perigalacticon) would serve to inundate the inner solar system with cometary bodies, some of which are likely to be of a proportion unseen by scientists, (except those of the keenest perception). This process takes time, on the order of one to three million years after maximum stress on the Oort cloud, and/or gravitational acquisition of interstellar cometary material, to reach the inner solar system, and a subsequent similar scale for the inevitable disintegration of these super-massive bodies into a huge swarm of smaller comets. The Kreutz Group of sungrazers is one such presently observed swarm, which fortunately intersects only our intellect, not our planet.
I note that two million years is approximately the time between the beginning of the Deccan traps volcanism and the K/T impactor, and so I would propose to the geologists this question: Speculating that the Deccan event could have been antipodal to a very large impactor, has anyone investigated the location which lay opposite India in the late Permian? Does that site even exist today, or has it been subducted or otherwise erased? If still extant, it would seem a likely plausible candidate region for a crater search.
I think it should here be pointed out that comets fragment much more readily than do asteroids, and so consideration of multiple impact scenarios should reflect the much greater likelihood of such bodies being not asteroids but comets, which ice balls are, especially if cosmogenic, very different from rocky and metallic (Metallica Rocks!) asteroids.
(As with everything else in astronomy, there is a continuum of creation bridging comets and asteroids, but the number of such directly related objects is relatively small, although those in the Taurid meteor stream, and other Apollo asteroids, could still hit hard enough to finish all but the hardiest of us off.)
*Tampa, FL
13th impact crater associated with K/T boundary
From Tom Van Flandern* Received 5.11.03
Last year, a British team of scientists announced the discovery of a multi-ringed crater (named Silverpit) with a central peak beneath the floor of the North Sea, believed to have been caused by an asteroid impact between 60 million and 65 million years ago [1-4]. The crater has 10 concentric rings from 2-20 km in diameter. The rough dating suggests that the asteroid that caused Silverpit might have been a chunk that broke off the larger asteroid that hit the Yucatan Peninsula 65 million years ago. Together with the Chicxulub and other craters, this discovery gives new support to the idea that killer objects from outer space may have sometimes arrived in pairs or even swarms. "It's so clear," said Dr. Gerta Keller, a geologist and paleontologist at Princeton, who studies the links between cosmic bombardments and life upheavals. "A tremendous amount of new data has been accumulated over the past few years that points in the direction of multiple impacts."
Actually, that brings the list of impact craters sometimes associated with the K/T boundary (65Ma) to 13. These are: Beyenchime-Salaatin (Siberia), Eagle Butte (Oregon), Upheaval Dome (Utah), Manson (Iowa), Kara (Western Siberia), Kamensk (Siberia), Gusev (W. Russia near Ukraine), Unnamed (Pacific Ocean), Chicxulub (Yucatan), Belize (south of Yucatan), Haiti (Caribbean), Alvaro Obregón (N. Mexico), Silverpit (North Sea) [5-10]. However, the global distribution of these impacts argues strongly against chunks off a parent asteroid, which would be expected to have much less than a hemispheric distribution, assuming Earth's atmosphere is the agent responsible for the break up. Capture and tidal break up by Earth, similar to Jupiter's capture of Comet Shoemaker-Levy 9, is extremely unlikely (by a factor of over 10 million) because Earth has a much smaller mass and is closer to the Sun. The clear implication of this global terrestrial cratering cluster is the explosion of a planet-sized parent body in the main asteroid belt, a hypothesis for which considerable astronomical evidence already exists [11-22].
*Meta Research
References
(2002), Nature 418, 520-523.
(2002/11/06), CCNet.
(2002)
http://www.nytimes.com/2002/11/05/science/life/05CRAT.html?ex=1037250000&en=c771927311f6e53f &ei=5040&partner=MOREOVER
(2002),
http://www.space.com/scienceastronomy/newfound_crater_020731.html
(1996), "Terrestrial impact crater list", http://www.meteorite.ch/crater.htm
(1991), Lunar&PlanetarySci. XXII, abstracts, 961-962.
(1993), Nature 363, 670-671.
(1993), Nature 363, 615-617.
(1980), Nature 288, 651-656.
(1998/03/12), "More evidence points to impact as dinosaur killer", JPL Press Release 98-42, NASA HQ & Pasadena, CA.
T. Van Flandern, Dark Matter, Missing Planets and New Comets, North Atlantic Books, Berkeley, chapter 11, (1993; 2nd edition 1999) - synthesis of exploded planet hypothesis (EPH) evidence.
Icarus 36, 51-74 (1978) - technical justification for the EPH.
http://metaresearch.org. 'Solar System' tab, 'EPH' sub-tab - recent updating and distilling of the most telling EPH evidence, and how its predictions have fared; to be published in 2002.
Mercury 11, 189-193 (1982) - the EPH as an alternative to the Oort cloud for the origin of comets.
Icarus 47, 480-486 (1981) - the EPH's 'satellite model' for comets as an alternative to the 'dirty snowball' model.
Science 203, 903-905 (1979) - asteroid satellite evidence, confirming an EPH prediction.
Science 211, 297-298 (1981) - technical comment on previous paper.
Asteroids, T. Gehrels, ed., U. of Ariz. Press, Tucson, 443-465 (1979) - theory and observations of asteroid satellites.
Dynamics of the Solar System, R.L. Duncombe, ed., Reidel, Dordrecht, 257-262 (1979) - short summary of selected EPH evidence.
Dynamics of Planets and Satellites and Theories of their Motion, V. Szebehely, ed., Reidel, Dordrecht, 89-99 (1978) -- short summary of selected EPH evidence.
Comets, Asteroids, Meteorites, A.H. Delsemme, ed., U. of Toledo, 475-481 (1977) - short summary of EPH evidence with technical critiques and author responses.
Science Digest 90, 78-82 + 94-95 (1982) - popular exposition of the EPH and its implications.
Multiple impacts not unusual
from Andrew Glikson* Received 5.11.03
Sir, The Archaean imapct records includes the following documented multiple impacts:
- At least 2 and possibly more 3.47Ga impacts in the Antractic Chert Member, Warrawoona Group, Pilbara (Glikson et al., in press) and probably a double 3.47Ga impact in the Hoogenoeg Group, Barberton, South Africa (Lowe et al., 2003).
- A double impact in the 3.24 Ga Mapepe Formation, Barberton, South Africa (S2 and S3) (Lowe et al., 2003).
- A double impact in the 2.56Ga Bee Gorge Member of the Wittenoom Formation, Hamersley Basin (SMB-1, SMB-2).
- A double impact in the 2.47-2.50Ga Dales Gorge Group, Brockman Iron Formation, Hamersley Basin, Western Australia (DG4 and DG7).
Major impact clusters are recorded in the late Devonian (Woodleigh, Siljan, Alamo Breccia, Charlevoix), late Jurassic (Gosses Bluff, Morokweng, Mjolnir), Late Eocene (Popigai, Chesapeake Bay). So far as the K-T boundary is concerned, already two impact structures are known (Chicxulub, Boltysh), as well as the fallout units recorded by Keller. The evidence suggests that double impacts and impact clusters are commonly recorded in the terrestrial impact record.
*Research School of Earth Science, Australian National University Canberra, ACT 0200
Smoking gun proponents "must defend themselves"
From Andrei Ol'khovatov, PhD Received 5.11.03
Sir, It is nice to read such interesting and open debates! This is the way Science should work! All participants can present their arguments (flawed arguments are quickly dismissed by opponents), and a scientific audience can compare the logic and weight of arguments of both sides.
My impressions, which are those of a person who is interested in Chicxulub but was never deeply involved in this research, are as follows. It seems that on the basis of "proved science", the arguments of G. Keller sound more logical, so her opponents have to defend themselves. It seems that their best argument is that our understanding of the processes discussed is not good enough for any solid conclusion to be drawn. This argument is hard to overcome, as indeed, here we are in terra incognita. For example, our understanding of all large impact phenomena is based on our knowledge of relatively small meteorite impacts extrapolated (with help of numerous calculations) to impacts many orders of magnitude larger.
Numerical calculations depend on a physical model of the event under consideration. In the case of smaller events, results could be verified just by comparing the outcome of one's calculations with reality. It is very much more complicated in the case of large impacts. So, all models of large impacts are "unverified" at the moment.
To sum up. It seems that nowadays a "complete victory" is not possible due to "being in the field of unknown". My impression is that more research should be conducted. And currently we should to recognise (at least to ourselves) that we really don't know why dinos disappeared!
*Moscow, Russia
www.geocities.com/olkhov
The timing's right
From Michael Paine* Received 4.11.03
Sir, The debate over the timing of the Chicxulub impact is fascinating and indicates a healthy state of scientific research on this topic. Irrespective of the timimg of the event, however, the wide range of research that has been conducted since Chicxulub's discovery has shown that the average frequency of such impacts is around 50 to 100 million years and the multiple environmental consequences of impacts the size of Chicxulub can easily account for the observed mass extinctions. In other words, the Chicxulub impact is an entirely plausible explanation for the KT extinction event.
Maybe there were multiple impacts and traces of the others have been lost to subduction and erosion or maybe there are other causes. My money is still on Chicxulub (this has nothing to do with it being Melbourne Cup horse race day in Australia).
For more on environmental effects of impacts see
www4.tpg.com.au/users/horsts/climate.htm
*The Planetary Society Australian Volunteers
More pluribus than unum
From Matt Terry*. Received Sunday 2 November
Sir, This vision of multiple large impacts fits better with the palaeontological protests that the dinos' decline was more stretched out than that postulated by a single global "Cosmic Winter" disaster. As Clube and Napier have made clear, the injection of one or more giant, likely cosmogenic, comets into the inner solar system necessarily implies its subsequent fragmentation into a swarm of hazards, as the Kreutz group gives current evidence for.
Three hundred thousand years is not so long a time, on deep-space timescales, and it is positable that the Chicxulub crater is one among many impactors from that time, again, in resonance with the slow decline of the Triassic hyper-fauna. Statistically, given the limits of our search and the oceanic plenitude, we should expect that Chicxulub is certainly not the only, or even the largest, impactor of its time.
*Tampa, FL, USA