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Lecture 25
The Impact Theory, Pt. II

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 enhansed greenhouse effect, but the sulfuric acid would result in global cooling.
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 10ƒ (right). This massive heating effect would have lasted for hundreds to thousands of years, probably making the interior of continents virtually inhospitable. Eventially the CO2 would be absorbed by the oceans, plants and the wweathering process. IMPACT

Possible impact-generated CO2 effect on the atmosphere.

Some Other Factors in Mass Exinctions

While there is now a general concensus amoung the geological and paleontological community that a giant impact did occur, there is still debate about its effect on the Earth's biota.

This debate breaks down into two catagories.

1. Is the pattern of extinctions actually compatible with an impact.

2. Were there other things going on that might have contributed to the extinctions, or even been more important. First we will look at the biological pattern.

Signor-Lipps Effect

Deccan Volcanism

DECCAN

Possible CO2 effect on the atmosphere from Deccan Lavas. From Caldeira and Ramino (1990).

References

Caldeira, K. G. and Ramino, M. R., 1990, Deccan volcanism, greenhouse warming, and the Cretaceous/Tertiary boundary. Geological Society of America, Special Paper 247, p. 117-123.

O'Keefe, J. D., and Aherns, T. J., 1989, Impact production of CO2 by the bolide and the resultant heating of the Earth. Nature, v. 338, p. 247-249.

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