Doubts about the Asteroid Impact
Contents Updated: Friday, April 28, 2000
Sunday, 2 March 2008
Doubts
Some scientists were not convinced that the meteorite impact could trigger mass extinctions any more than a large volcanic eruption could. Would the debris thrown up stay aloft long enough to cause any lasting damage? After all many genera and individual species did survive showing that, despite darkness, dust and poison gases, conditions could not have been bad for too long. A fairly short period of darkness could explain the excessive extinction of water dwelling species relative to land types because the food reserves of the plankton in the sea is only sufficient to last for between ten and a hundred days, but land plants can suffer the absence of light for longer.
A period of darkness of three years, as the Alverezes supposed, would have destroyed most genera, perhaps all higher ones. Many species were not seriously affected—and many others, we know, had been suffering decline before the hypothetical collision. According to E.G.Kauffman of Colorado University, 75 per cent of marine organisms were in decline at the end of the Cretaceous period. They had been on the wane for two to five million years and few species seem to have died off at exactly the same time.
Ammonites had fluctuated in population previously. Some ammonite genera declined and expanded periodically while others seemed relatively steady. When extinctions had occurred before, the steady species had tended to survive while the fluctuating species had died off. In the upturn the steady species had radiated into available niches including those suiting the fluctuating species and the cycle continued. At the end of the Cretaceous the ammonites were in such decline that there were no reliable steady species to bring them through. What was different? What had killed off the steady species of ammonites?
Dinosaurs were similarly on the wane and by the end of the Cretaceous survived in numbers only in the West of North America, having died out in South America and possibly Europe. Even in North America the decline was severe. Half of the 36 genera of dinosaurs alive about ten million years before the end of the Cretaceous had died out by the time the final million years was entered.
Robert Bakker is among those who do not support the catastrophe theories: he claims there is no doubt that dinosaurs did not die out in a geological instant but petered out over thousands if not millions of years. Plainly life was under stress. What was its cause?
Leigh Van Valen and Robert Sloane attributed the extinctions to climatic changes over the last five to ten million years of the Cretaceous. At the beginning of the period vegetation was prolific and typically tropical or sub-tropical: towards the end of the period the climate had become typically temperate with cool woodlands. Dinosaurs thrived in the warmer climate but in the cooler one mammals had the advantage. The proposed reason for the change in climate was that the ocean floor had lifted with renewed mid-ocean spreading and sea levels had risen. Shallow seas divided North America and also divided Europe from Asia. Ocean currents and wind patterns may therefore have altered.
On the other hand, Bakker argues a fall in sea level draining the shallow continental seas could have triggered the mass extinctions. This accounts for the loss of a lot of marine species, those preferring the continental margins and intra-continental seas obviously, but the loss of light warm water draining from them on to the ocean surfaces would also lead to the demise of many open sea species that could not adapt to the colder surface conditions. What though of the land vertebrates? Surely they would have had more lebensraum and should have multiplied. No. The linking of previously isolated continents by land bridges created conditions similar to those in the great Permian extinction—hypercompetition between species and the unchecked spread of disease and parasites in populations not adapted to be immune from them. Large active animals like the dinosaurs could migrate faster, therefore they experienced more competition and disease, and suffered most. Smaller creatures like the mammals could migrate, but more slowly, having more time to adapt and freshwater species, which seemed least affected, could not migrate, though they were affected to a lesser extent by pests or diseases carried in by the migrants.
Late in the Cretaceous some Asian genera of dinosaurs appeared in North America having crossed the Bering Straits (or whatever the dinosaurs might have called them) showing the two continents had linked. A more recent example was the land bridge which formed between North and South America, about 30 million years later than the dinosaurs, when many South American species failed to survive competition from invaders from the North.
Bakker puts particular emphasis on the spread of diseases unrestricted by hereditary immunity. Warm-blood is at the ideal temperature for bacteria and viruses to multiply. The warm-blooded dinosaurs would therefore be susceptible to the new pathogens being introduced to all the continents. He quotes examples like the spread of the Black Death and the carrying of rinderpest from India to Africa with devastating effects on the antelope herds. V.D. and smallpox devastated the Amerindians. Virulent strains of myxomitosis were deliberately developed by CSIRO in Australia and introduced into the wild to control rabbits. All have depended upon the intelligent mammal with its capacity for travel and his insensitivity towards other species.
Volcanic Activity?
Bakker contradicts his advocacy of the evolutionary resilience of the dinosaurs. The dinosaurs, he persuades us, are great competitors and the forming of land bridges, while leading to mass extinctions, also provides lots of empty niches for enterprising species to adapt to. If the theory is true at all, and it might be partly true, we need to know why the genetic variability of the dinosaurs had been reduced to such an extent that they could not cope with the new challenge as they had always done before.
Those doubtful of the Alverez’s theory thought the iridium anomaly could be explained by differential sedimentation rates or volcanic activity. Further careful testing of the iridium layer showed that the iridium concentration seemed to build up slowly during the last few thousand years prior to the supposed cataclysm. Volcanic activity occurring over an extended period, they argue, would be more likely to match such a pattern. India had broken from Africa and raced (in geological terms) across the Indian Ocean to hit Asia. The collision pushed up the Himalayas and created such friction that lava spewed forth for centuries to form the Deccan Peninsula. This was the extended vulcanism they sought.
Calder noted that 65 million years ago in Western North America plants were dusted with exotic elements. Besides iridium, volcanoes emit other metals present in the K-T sediments not commonly present in meteors, like arsenic and antimony. For Charles Officer and Charles Drake of Dartmouth College this proved the boundary layer and the extinctions were related to the break up of the old continents. The level of the seas fell to their lowest for 200 million years. The warmth which gave subtropical conditions to northerly climes subsided. Widespread volcanic activity over a long period (but short on a geological timescale) led to pollution, climatic changes and ecological damage which destroyed species.
You will, by now, have noticed that some experts have postulated high sea levels and others low sea levels as reasons for the Cretaceous extinctions. You may well ask: Don’t they know where the sea level was? Wasn’t it where it usually is? Sea level provides a riddle of its own. Geologists, notably those working for oil companies have built up a detailed knowledge of changes in sea level over the ages. In the last 200 million years it reached its highest consistent level from about 85 to 67 million years ago when about twice the area of continental shelf presently inundated was flooded. In the preceding 100 million years sea levels had steadily risen due to the activity of the mid-oceanic ridges and the sea floor spreading associated with continental drift. The welling up of magma under the mid-oceanic ridges displaces the water of the oceans causing higher sea levels.
Sudden unexplained falls in sea level occur periodically but are rarely linked with meteorite falls. The sudden onset of an ice age freezing large amounts of water in extensive ice sheets might explain some recent sea level changes but not those in the Cretaceous. They also do not normally coincide with mass exterminations, throwing doubt on an idea like Bakker’s. The sea level did fall rapidly about 67 million years ago, the event identifiable with Bakker’s theory, but there was a greater fall in sea level 95 million years ago which is associated with only a minor turnover of species compared with the extinctions terminating the Cretaceous. Why did this earlier event not have the impact of the later one if Bakker’s idea is correct?
Any convincing explanation of the extinctions has to account for all the genera that became extinct not just those that are representative of the dinosaurs. It also has to be sudden, at least on a geological timescale. The dinosaurs had shown that they were well able to adapt over 140 million years and were still evolving in the Cretaceous. A gradual change of conditions was unlikely therefore to overwhelm them—they would have adapted into the new conditions.
Though doubt is being cast on the asteroid impact, many of its anticipated effects like adverse climatic disturbance and pollution of the environment remain persuasive—but were these the shadow of an asteroid or did they come from closer to home? Today we see similar effects created by the intelligent mammal. Volcanoes and asteroid impacts do not have to be invoked to explain the environmental problems we are experiencing, or the mass extinction of species currently taking place. Similar things are happening today to events at the end of the Cretaceous.
Features of the K-T Breccia Layer
Is the breccia layer at the K-T boundary the residue of tsunamis produced by the K-T meteoric impact at Chicxulub in the Yucatan Peninsula of mexico, or is it , as critics of the impact theory say, erosion caused by a drop in sea level, a sequence boundary.
At a meeting of the national Geological Society of America in Denver there was a discussion of impact breccia layers. Examples were from the Upper Devonian of Nevada, the Tertiary Chesapeake Bay crater, and the K-T boundary megabreccia, so called because in the Yucatan it contains clasts up of ten or more metres in size. It is not a sequence boundary but a spectacular deposit of the coarsest-grained chaotic material .
The breccia layer is up to 100 feet thick, containing striated pebbles, cobbles, and boulders, in a matrix of finer material including sub-spherical clasts of green recrystallized glass. The striations are believed to have formed by a partially melted exterior scraping across country rock, or abrasion by smaller, high-speed projectiles. The striations include the nail-head variety, looking like the impressions of nails, with a short crosspiece on one end, which have been seen as diagnostic of glacial deposits. Some of the clasts are pitted by many 1 cm pits, apparently impacts from small projectiles. Samples were available for inspection.
There was no Late Cretaceous glaciation in the Yucatan. And clearly we now must re-evaluate certain supposed tillites from the Precambrian and other ages like Late Devonian, and review the paleoclimatic inferences based upon them.
The Yucatan K-T breccias also contain crystalline dolomitic lapilli of spherical, ovoid, and tear-drop shape up to about 3 cm in maximum diameter, some with peculiar bright green outer rims. These are new features that have not been found previously in volcanic deposits on the Caribbean region. There may be an alternative explanation besides near-by impact, but we will soon reach the limit of our ingenuity in finding ways to explain such features as the result of something other than an impact. As more evidence accumulates, we have seen that it continues to be consistent with a major impact in the Yucatan area at precisely the end of the Cretaceous, while alternative explanations for the same data are becoming ever more scattered, careless, and labored.
Age of the Manson Impact Crater
The Manson crater in Iowa with a diameter of 38 km is the largest known impact crater in the US. It is also one of the best known large impact craters on earth. Initial radiometric datings of the crater indicated an age corresponding with the extinction of the dinosaurs, raising speculation about the role of the impact in the K-T extinctions. But core drilling of the crater in 1991-92 resulted in a new dating from melt-precipitated feldspars reported in Science of about 74 million years ago.
The date of the impact now seems secure, placing it ten million years too early for the Cretaceous terminal event. Indeed it does not correspond to any major terrestrial extinction event, and there is little sign of extinction in any lineages of marine invertebrates at that time. In fact, it immediately precedes the peak diversity of North American and Asian dinosaurs. The age of Triceratops and Tyrannosaurus rex, the Maastrichtian, begins after the impact event. However, some mosasaurs, marine turtles, and fish did vanish from the Western Interior at a time that approximates the Manson impact event.
The Manson impact released about 2.2 x 1021 Joules of energy and launched about 1000 cubic kilometers of ejecta—15% into the troposphere, shrouding the earth. Manson ejecta is now clearly identified within the Pierre Shale of South Dakota and Nebraska. The effects of the shock wave produced by the impact have been tentatively scaled:
- all combustible material within 200 km would have been ignited
- all standing vegetation would have been devastated to 600 km
- most terrestrial animals would have been killed by the shock to 1000 km, as far as Montana
- large animals (eg dinosaurs) would have been knocked off their feet as far as 1300 km. Tsunami-like waves may have surged across the nearby Western Interior Seaway.
Considering the extent of the devastation, the terrestrial biota apparently quickly recovered, undoubtedly replaced by migrants from elsewhere on the continent.
There is a lesson here concerning the K-T extinctions. If the Manson impact was only large enough to cause local and easily recovered destruction of species leaving negligible geological traces, then only an immensely larger impact could devastate global biota to the extent of the Cretaceous terminal event. The Chicxulub crater in Yucatan is thought to have been the culprit but according to some, this too can be dated in the Campanian, leaving no known candidate as a likely impact crater.
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