The Age of Dinosaurs and Major Crises in the History of Life.

Today's main points:

  1. Delta 13C curve in sediments gives us signal of organic matter burial.
  2. Curve as well as obvious coals shows massive Carbon burial during Carboniferous and Early Permian.
  3. Could the loss of carbon from terrestrial systems be due to a lack of herbivory?
  4. Burial of this carbon associated with massive Ice House conditions of Late Carboniferous - Early Permian glaciations.
  5. In Permian herbivory becomes common in insects and tetrapods.
  6. Burial of Carbon decreases - Hot House conditions prevail.
  7. End of Permian marked by mass extinction - largest of them all (~99 percent at species level).
  8. Cause poorly understood, but at least part of the extinction, at low levels of temporal resolution, could have been the out flow of the Siberian Traps, and associated CO2 rise.
  9. Recover slow, taking for marine invertebrates 100 my.
  10. During early Mesozoic (Triassic) we see evolution of first super-herbivores - the social insects and dinosaurs.
  11. Sociality and large size are key innovations for respiring cellulose via commesal gut floras.
  12. Biota becoming modern looking. For microbes it mostly was.
  13. End Triassic marked by another mass extinction.
  14. Dinosaurs, pterosaurs, crocodilians, mammals, lizards, turtles, modern amphibians survive.
  15. This time there is evidence for CO2 rise, asteroid impact, and lava flows (CAMP) over very short interval.
  16. Jurassic was still hot house time, giant dinosaurs rule, birds evolve from dinosaurs, social insects become very prominent.
  17. Cretaceous, still hot house time, but slow cooling begins.
  18. Dinosaurs still dominant, fantastic range of bird/dinosaurs, mammals diversifying, but none bigger than house cat.
  19. Angiosperms (flowering plants) evolve. Key innovation is fast reproduction. with doubly protected seed, and extra food for embryo. Angiosperms get very common quickly and dinosaurs adapt for low browse.
  20. As angiosperms become dominant, ecosystem efficiency decreases, chemical weather increases, climate cools.
  21. End of Cretaceous marked by mass extinction - this time definitely associated with giant asteroid impact and massive lava flows of the Deccan Traps.
  22. Massive but short-lived CO2 increase, part of cause of mass extinction.
  23. Non-flying dinosaurs wiped out. World open to mammals, but birds still more diverse in number of species.

I. Delta 13C curve: a signal of organic matter burial.

Perhaps one of the most useful measures of ancient ecosystem function is the measurement of the relative abundances of the two stable isotopes of carbon 12C and 13C.

Recall from introductory chemistry that isotopes are varieties of an element that differ in the number of neutrons (and therefore mass) but not in their overall chemical behavior.

12C and 13C are both present in the carbon in CO2 in the atmosphere and in the dissolved forms of carbon in water. In photosynthesis the transfer of CO2 from outside the cell to the production of cellular products (such as carbohydrates) favors the lighter isotope 12C. Therefore, the cell itself ends up slightly enriched in 12C and the water (or air) ends up slightly enriched in 13C. Thus, as photosynthetic organisms die and get incorporated into the sediment, the water in which they lived becomes progressively enriched in 13C.

The "Dell" Notation for "delta 13C" (δ13C)

Rather than try to measure the precise amounts of 12C or 13C present in samples and look at that change, a far more useful measure is the ratio of 13C to 12C compared to some standard. The "dell" stand for greek lower case delta (δ), and just to make sure it is really top notch jargon, δ13C is read, "delta C thirteen"!

(Note: An excellent discussion of the uses of δ13C are given at: http://epswww.unm.edu/facstaff/zsharp/505/carbonates2.htm)

For this the so called dell notation is really very similar to a percent difference, but it is per mille, that is per thousand, not percent, that is per hundred. Delta means change, and can be literally read as per mille difference from standard. The delta 13C measure is derived as follows:

The is where the subscript "samp" refers to the ratio of the isotopes in the sample and "stand" refers to the ratio of the isotopes in the standard.

It is nearly the same as asking, what is the percent difference between my bank account now and average for last year. Here the standard is last year's average value and the sample is the present value.

Obviously, if the present value is more than last year's value, percent change is positive, and if the present value is less than last year's average value, the per cent change will be negative.

You could graph, for example, 1999's month by month percent change from 1998's average value. Lets say your average account balance was $1000.00. This graph might look like this:

Note that I have indicated certain events that correlate with particularly dramatic changes in the percent change. Obviously this graph reflects very well the dynamics of your account.

In the same way, the graph of δ13C through time is an indication of the relative change in the enrichment of the water column in 13C relative to 12C as reflected in the composition of either carbon in organic matter or carbon in carbonate (CaCO3).

Note that up to the most recent times, ICE HOUSE periods occurred during times of positive δ13C excursions.

II. The Cretaceous Tertiary Mass Extinctions

The famous and brilliant 18th century comparative anatomist Baron 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 geologist Sir 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 (see graph at right: past is to right, present is to 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.

graph of extinctions from Sepkoski 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 much more recent tabulation (right) of shelly marine invertebrates by the late 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, "% generic 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. 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:

a Cretaceous foraminifer

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 level.


Let's look now at some specifics, sort of a body count:

MARINE ORGANISMS
(particularly important ones in bold)
CONTINENTAL ORGANISMS
planktonic foraminifera -83%
ostracodes -50 %
sponges -69%
corals -65%
sea urchins -54%
ammonites -100%
marine reptiles -93%
reptiles in general -56%
but non-avian dinosaurs and pterosaurs -100%

But many things also made it through or were relatively unaffected:

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:

THE ASTEROID IMPACT THEORY BECOMES TESTABLE

Alvarez, Alvarez, Asaro, and Mitchell 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 (Ir) 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.

K-T boundary, Gubbio, ItalyRemember, back in Lecture 4 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.

Iridum spike from Gubbio

(adapted from Alvarez, et al., 1980)

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 left). 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).

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.


STEVEN KLINT, DENMARKThey checked at two other sites, one in New Zealand and one at Stevens 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.

Supernove 1987A in 1994The key in science is not postulating causes, but rather testing 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, 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.

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.


IMPACT THEORY CORROBORATED

There were two big holes in the theory, however: One was that there was no impact site known, the other was that it seemed possible that the Iridium could have come from a volcanic source.

The solution to the second problem, came first, within a few years of the Alvarez et al. paper of 1980.

K-T boundary in Raton Basin.A search for the Iridium anomaly in deposits formed on the continents quickly identified the layer exactly where paleontologists saw a transition from Cretaceous-type floras to Tertiary-type floras. In addition, Dinosaurs were found below but (apart from a few reworked scraps) not above.

At left, the Ir-enriched layer is the thin yellow layer just below the much thicker coal (black) layer. (Cretaceous-Tertiary boundary section in Raton Basin, New Mexico.)

When the mineralogy of the yellow layer was examined by the Bruce Bohor, he found grains of quartz with a unique pattern of planer features criss-crossing the grains. Bohor had found so called "shocked quartz". Shocked quartz had previously only been produced in high-pressure experiments or found at nuclear explosion test sites and known asteroid impact sites.shocked quartz

Shocked quartz has never been found in the deposits of even the most explosive volcanic eruptions, so this was very good evidence that the Ir layer was produced by a giant impact.

Subsequently, shocked quartz was found at many other sites, always associated with an Ir spike, including Gubbio and Stevens Klint.

At right, shocked quartz seen in thin sections with a polarizing microscope.

Close examination of the Ir-shocked quartz layer in the western US by paleobotanists showed that the layer directly above it had virtually only fossil spores of ferns. This, "fern spike" indicated massive ecological disruption, during which time only ferns could colonize the area.

IMPACT SITE FOUND!

During the early 1990's it was realized that the impact site of the giant comet or asteroid was on the northern side of the Yucatan peninsula, in Mexico. The crater itself, called Chicxulub, is completely buried by younger deposits and water. It was actually discovered some years earlier by geologists looking for oil using geophysical methods and drilling, but even though one geologist suggested that it was in fact the K-T impact site, he was largely ignored.

At right, Chicxulub crater, Yucatan Peninsula.

Virgil Sharpton (Univ. Alaska, Fairbanks) and Alan Hildebrand (Canadian Geological Survey) concluded it was the site and they and others subsequently found deposits along the shores of the Gulf of Mexico consisting of vastly thicker impact debris and tsunami beds.

The impact structure 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.

Some paleontologists still doubt.

PROPOSED IMPACT KILLING MECHANISM

In the original Alvarez et. al. model, a 10 km bolide would have struck the earth sending a large amount of dust into the atmosphere. This would have blocked sunlight for a some period of time, resulting in massive cooling, globally, and a collapse of the food chain via. a cessation of photosynthesis.

Since that time (1980) it has been realized that an oceanic impact, like that at Chicxulub could have much, dire effects.

In specific, the Chicxulub bolide struck a thick deposit of marine limestone (CaCO3) and underlying marine calcium sulphate (CaSO4 = Gypsum). This probably put large amounts of CO2 and sulfuric acid into the atmosphere within minutes. The CO2 would have produced an enhanced greenhouse effect, but the sulfuric acid would result in global cooling.impact generated co2

First, we will look at the greenhouse effect.

In 1989 O'Keefe and Aherns estimated that the impact of a 12 km diameter comet or a 14 km asteroid would have raised the average temperature of the Earth by over 10° (right). This massive heating effect would have lasted for hundreds to thousands of years, probably making the interior of continents virtually inhospitable. Eventually the CO2 would be absorbed by the oceans, plants and the weathering process.

At right, possible impact-generated CO2 effect on the atmosphere.


REFERENCES

Alvarez, L. W, Alvarez, W., Asaro, F., and Michel, H. V., 1980, Extraterrestrial cause for the Cretaceous Tertiary extinction. Science. v. 208, p. 1095-1108.

Bohor, B. F., Foord, E. E., Modreski, P. J., and Triplehorn, D. M., 1984, Mineralogic evidence for an impact event at the Cretaceous-Tertiary boundary. Science v. 224, p. 867-869.

Tschudy, R. H., Tschudy, B. D., 1986, Extinction and survival of plant life following the Cretaceous/ Tertiary boundary event, Western Interior, North America. Geology v. 14, p. 667-670.

Updated March 18, 2005
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