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Lecture 23 - The Impact Theory of Mass Extinction
In the last lecture we looked at the range of dinosaurs that existed at the close of the Cretaceous. We can now turn our attention to the events around the great mass extinction event that involved the elimination of the non-avian dinosaurs as well as the associated extinctions.
I. GENERAL PATTERN OF EXTINCTIONS
Recall that in the 18th century Georges Cuvier worked on the stratal successions around Paris and believed that the history of life was marked by periods of creation, stasis, then obliteration. This catastrophic view of the history of life was countered by Charles Lyell's extreme uniformitarian view that very small changes added up over vast amounts of time were responsible for the seemingly great changes seen in the geological record. Lyell actually believed, at least early on, that there were no extinctions, just changes in abundances.
However, by the mid 19th century the basic shape of diversity of life was fairly well understood, and there did seem to be an overall pattern. In 1860, paleontologist John Phillips formalized what was known about diversity from the Cambrian Period on by recognizing (left) the Paleozoic Era (or ancient life), the Mesozoic Era (or middle life), and the Cenozoic Era (or recent life), with each era boundary marked by a period of lowered diversity - in other words by a period of accelerated extinction. The most abrupt of these reductions in diversity was the boundary between the Mesozoic and Cenozoic - that is between the Cretaceous and Tertiary Periods.
Darwin, who applied Lyellian Uniformitarianism to biology, thought the fossil record was a bit of an embarrassment to his theory of evolution, especially in the apparent lack of intermediates. Darwin felt that the lack of intermediates was due to the imperfection of the geological record, and that they must be there - and of course intermediates between major groups, like Archaeopteryx, were soon discovered.
The dual successes of Lyellian Uniformitarianism and Darwinian evolution lead to a view that persisted for the next century - that interpretations of the geological record in terms of discontinuities or catastrophes was to be more-or-less abhorred - and indeed there was precious little real evidence that this approach was not justified.
However, by the 1960's and 1970's paleontologists began to carefully compile literature records of taxa from strata of different ages. It became apparent that there really were times of very high extinction rate.
A more recent tabulation (right) of shelly marine invertebrates by Jack Sepkoski of the University of Chicago shows that there have been several times in the last 270 million years (the better-preserved part of the record) when there were very high levels of extinction. In this graph, "percent extinction" is the number of extinctions of genera in an interval of time divided by the number genera present (or at risk) in that interval, times 100 (to make it a percent). Note the large peaks in extinction at the end of the Permian, Triassic, and Cretaceous periods.
Particularly the Cretaceous-Tertiary (K-T) boundary stands out because there was considerable stratigraphic evidence that marine and continental extinctions looked like they might be synchronous. In specific the marine biota was hit very hard:
15% of all marine families (Sepkoski, 1982)
but 50 % at generic level
and maybe 80-90 % of all species.
The continental biota was also hit hard:
about 25 % at the family-level became extinct
but about 56 % at the genus
Let look now at some specifics, sort of a body count:
MARINE ORGANISMS (particularly important ones in bold)
planktonic foraminifera 83%
ostracodes -50 %
sponges -69 %
corals -65 %
sea urchins -54%
ammonites -100%
marine reptiles 93 %
CONTINENTAL ORGANISMS
reptiles in general -56%
but non-avian dinosaurs and pterosaurs -100%
But many things also made it through or were relatively unaffected:
higher plants -10 %
dinoflagellates -5 %
mammals +120 %
ON LAND NOTHING BIGGER THAN 25 KG SURVIVED
ALL SURVIVORS WERE SMALL - LIZARDS, SNAKES, CROCODILES, TURTLES, MAMMALS, FROGS, SALAMANDERS
These extinctions were clearly of great magnitude, but in the intellectual milieu of the time, there was much speculation, but little specific work on the nature of the K-T boundary.
Explanations included:
Evolutionary Senility
Could not eat the evolving flowing plants
It got too cold
It got too hot
Mammals ate all of the eggs
They all died from a disease
They all became female when the climate changed
The sea level changed too fast
There was massive volcanism covering everything with ash
There was a nearby supernova of a star
Earth was hit by an asteroid
II. THE ASTEROID IMPACT THEORY BECOMES TESTABLE
All this changed in 1980 when Nobel laureate Luis Alvarez, his geologist son Walter Alvarez, nuclear chemist Frank Asaro, and paleontologist Helen Michael (on left, from right to left), published on their discovery of high levels of the element Iridium in a clay layer separating marine sediments of Cretaceous and Tertiary age.Originally, Water Alvarez, a geologist at Berkeley, was looking for some way to quantify the rates of faunal change around the K-T boundary. To do this he needed a timekeeper. He and his father reasoned that the rain of dust from outer space should be coming down, on average, at a constant rate.
Let's say it came down at a rate of 0.0001 g/yr on 1 cm2 of the ocean floor. It would be diluted by clay and microfossils also from the ocean to make sediment.
So, if you had a 10 g sample of Cretaceous oceanic sediment with 1 g of space dust in it, it would have been deposited in 10,000 years [1 g / (0.0001 g/yr = 10,000 yr]. All you had to do was find a way of measuring space dust.
It turns out that is not as hard as you might think.
Remember, back in Lecture 10 that during the iron catastrophe most of the compliment of heavy elements that the Earth received during its accretion sank to its core. Iron was the main element, but along with it went most of the platinum and related elements called the platinum group. That way Platinum is a rare and expensive mineral on the Earth surface. Platinum group elements are thus much more common in your average space dust, than on the crust. These Platinum group elements would thus be a good signature for space dust in sediments. Better yet, Platinum group elements don't move around much once they are deposited. They are some of the so-called noble metals that tend to be unreactive and virtually inert. Of the Platinum group elements, Iridium (Ir) was relatively easy to measure, detectable in parts per billion. Thus Ir proved to be the element of choice to measure as a proxy for space dust.
The next thing to do was to find an outcrop of rock deposited in the deep ocean where the K-T boundary was well exposed and paleontologists were sure it was properly identified. Their choice turned out to be a highway cut in Gubbio, Umbria, Italy (on right). The boundary was clearly marked out by the disappearance of many kinds of Cretaceous microfossils, particularly most foraminiferans. At the boundary is a thin layer of brown and black clay. So the Alvarez group sampled carefully through this rock section and analyzed for Ir (below).
(adapted from Alvarez, et al., 1980)
They were astonished to find that there was very little change in Ir content through the section, except in the clay layer. They did their calculations only on the basis of leaching out all of the calcium carbonate from the samples, so they would only be measuring clay. Ir was up by more than an order of magnitude (factor of ten) in the clay bed, exactly where the extinctions occurred. They checked at two other sites, one in New Zealand and one at Stevns Klint in Denmark (right). What could have caused this? Two hypotheses were possible.
1) something shut off the production of clay, while the rain of Ir in space dust remained constant.
2) something boosted the amount of space dust (and hence Ir) by and order of magnitude.
They could not find a reason that it should be #1, so they opted for #2. But why should space dust go up? There could be two easy hypotheses to do this:
1) a nearby star could have gone supernova, showering the Earth with newly formed elements heavier than iron - among them Ir.
2) the Ir could have come from a mass of extraterrestrial matter arriving in one chunk - as a giant meteor or comet.
The key in science is not postulating causes, but rather testing your hypotheses. So they reasoned that if it was # 1, one of the other elements that should have been created would be Plutonium (Pu), specifically Pu244, a radioactive isotope ("bomb" Plutonium). Pu244 was created during the supernova that led to our solar system (see Lecture 10), but nearly all has decayed to lead by now. However, if the Ir in the K-T boundary layer had come from a supernova 65 million years ago, there would be quite measurable amounts left. So they looked. The result was no measurable levels of Pu244. Their argument against a supernova was bomb proof (pun intended). However, it was a definite possibility. Click here to see links to a recent supernova.
Thus, they argued that this Ir-enriched layer was caused by the impact of a giant asteroid (~10 km) that put enough dust into the upper atmosphere to darken and hence cool the Earth for several years. This was theorized to result in shutting off global photosynthesis, with the resulting collapse of the global food chain. As a result nothing larger than 25 kg survived the boundary. This concept was promptly co-opted as a plausible scenario for the events following a nuclear holocaust (Nuclear Winter) as well.
III. IMPACT THEORY CORRABORATED
There were two big holes in the theory, however. One was that there was no impact site known.
Cretaceous-Tertiary boundary section in Raton Basin, New Mexico (left); shocked quartz (right).
IV. IMPACT SITE FOUND!
The impact structure, discovered buried beneath the shore of the Yucatan Peninsula (Chicxulub), turns out to be from 150 - 300 km in diameter, possibly the largest impact known in the world and said by some to be the largest known in the solar system. At the present diverse lines of evidence have largely confirmed the asteroid impact theory of mass extinctions and identified Chicxulub as the "smoking gun". Debate still rages by paleontologists about whether the impact was directly the cause of the extinction itself, or merely the coup de grace.
Chicxulub crater, Yucatan Peninsula.
V. SOME PALEONTOLOGISTS STILL DOUBT
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