EXTINCTION OF THE DINOSAURS

Dr. Paul F. Ciesielski
Department of Geological Sciences
University of Florida

I. LEARNING OBJECTIVES

Objectives of this lesson are to:

examine the role of mass extinction in the evolution of life.
examine the origin and frequency of bollide impacts with the Earth.
assess the magnitude of the terminal Cretaceous extinctions that caused the disappearance of dinosaurs.
discuss the cause of the terminal Cretaceous extinction and the conditions that caused it.

After you complete this lesson, you should be able to:

recognize that the history of the our planet has been punctuated by mass extinctions events and that one of these ended the reign of dinosaurs
explain the theory for the terminal Cretaceous extinction.

II. INTRODUCTION

A. Extinctions.

    The extinction of a species occurs when the last representative of a species is no longer capable of reproducing and dies, leaving only the fossil legacy of its time on Earth. Some species live for many millions of years while others have left only the briefest of records. While the longevity of individual species is highly variable, a rough estimate of the average duration of species is about 2 million years.
    The diversity of life on Earth today is remarkable and is the evolutionary product of some 3 billions of years. During the course of this three billion period, the number of species to evolve and become extinct exceeds 100 times those still living (extant). No wonder the fossil record is so rich and varied.

    Examination of the fossil record reveals the following generalizations about extinction:

the rate of extinction is not a uniform rate
extinctions do not always involve all forms of life (invertebrates, vertebrates, plants, etc.) at similar rates
some extinction episodes occur on a regional basis while others are global in scope
most extinctions were caused by changes in the natural environment: e.g. climate change or instability, reduction in ecospace and niche habitat, major sea level changes, major changes in oceanography, etc., Many of these changes were set in motion by plate tectonic changes.
some episodes of extinction may be related to catastrophic events or episodes (e.g. effusive volcanic episodes or even the collision of bolides (comets or asteroids).

The Phanerozoic Eon encompasses approximately the last 540 million years of Earth history. It is during this interval that there has been abundant life with fossil hard parts. Over the course of the lengthy interval of time there have been five mass extinctions. Perhaps the greatest of these preceded the dinosaurs at the end of the Permian Period (~245 Ma). Another occurred at the end of the Triassic when the dinosaurs were just beginning their reign. The most famous of all extinctions occurred at the end of the Cretaceous (~66 Ma) and marks the boundary between the Mesozoic and Tertiary Eras. The later extinction, know as the Cretaceous/Tertiary boundary (or C/T boundary), captivates the imagination of so many because it represents the extinction of dinosaurs and may have been caused by the impact of a bolide from outer space.

B. Visitors from space

Before we examine the evidence for the causes of the C/T extinctions, we first review the nature and origin of potential Earth impacting objects.

1. What kind of visitors?

Asteroids:

Most asteroids are composed of rock, many made of dense metals such as iron and nickel (heavy projectiles).

Diameters range from less than one mile to almost 500 miles across.

When entering the atmosphere, these big rocks are made incandescent by friction with the Earth's atmosphere, burning a luminous trail into the sky called a meteor. Exceptionally bright meteors are also sometimes called bolides, or fireballs. The core of the object falling towards Earth is considered a meteoroid. When a meteoroid survives the descent and strikes Earth, it is renamed a meteorite. Larger meteorites have left a number of craters around the world both on land and on the ocean floor.

Comets:

Comets are "dusty iceballs" of rock, frozen water and organic compounds.

Comets have a head, or coma, and a gaseous tail potentially thousands of miles long.

The comet head consists of a solid nucleus surrounded by a nebulous coma up to 1.5 million miles in diameter.

The tail is mainly ammonia, methane, carbon dioxide, and water.

Orbits of many comets have periods ranging from hundreds of years to tens of millions of years, indicating far flung orbits outside the orbits of Neptune and Pluto.

Meteorites
A meteoroid is a piece of debris (asteroid or comet debris) traveling through space towards our planet. A meteorite is a meteor which is not completely destroyed in the upper atmosphere, but survives to the ground. Around 2000 meteorites have been recovered, the largest is the 60
ton Hoba iron meteorite.

2. Where do they come from?

The vast majority of asteroids in our solar system are found in the Asteroid Belt between Mars and Jupiter. Those that are in potential Earth crossing orbits have had their orbits changed due to collisions with other asteroids, etc.

Comets appear to originate either from the Oort Cloud , a large, spherical cloud of comets surrounding the solar system; or the Kuiper Belt beyond the orbit of Neptune, between 4.5 to 7.5 thousand million km from the Sun.

3. How often do they visit and how big are they?

Stony asteroids of less than about 100 meters diameter rarely reach the Earth's surface. These smaller objects usually explode several kilometers above the surface. The most recent airburst of an object of this size was in 1908, the "Tunguska" Siberian event, resulting in an area of destruction of about 2,000 sq. km or a circle of radius 25 km.
A global catastrophe, where the "damage" would go well beyond the area of direct devastation, would occur with the impact of a 2 km diameter stony asteroid. An estimated one quarter of the world's population could die from starvation and other indirect effects due to such an impact, which may occur at an average interval of 1 million years (Morrison and Chapman 1995).
see Table 1 (below) for estimated frequencies of various sized impacts along with their kinetic energy and area of devastation.

Table 1 - Risk of direct impact for a given location. Data from AUSTRALIAN SPACEGUARD SURVEY, Tsunami from Asteroid/Comet Impacts by Michael Paine

DIAMETER (meters or kms)	KINETIC ENERGY (mega tons of TNT)	AREA OF DEVASTATION (sq. mi.)	AVG. INTERVAL IN YRS.
50	10	1,900	100 yr.
100	75	7,200	1,000 yr.
200	600	29,000	5,000 yr.
500	10,000	70,000	40,000 yr.
1 km	75,000	200,000	100,000 yr.
2 km	1 million MT		1,000,000 yr.
All			90 yr.

4. More information of impacts sites, etc.

A map of global impact sites with a listing of their locations

Maps of impact craters by continent showing the approximately 150 know impact sites with craters of 1 km or more

A close-up image of the asteroid Eros. NASA landed on the Eros asteroid in Feb. 2001, the first ever such landing.

Earth Impact Database

Barringer Meteor Crater, Arizona

crater is 0.737 mile-diameter (1.186 km), only 49,000 years old

Manicouagan Crater, Quebec, Canada

A forty mile diameter lake forms a ring around this crater. The entire crater extends outward to a diameter of 94 miles (151 km), estimated to have formed 214 million years ago.

Gosses Bluff, Northern Territory, Australi a

A 142 million year old crater with a diameter of almost fourteen miles (twenty two km).

III. THE IMPACT HYPOTHESIS:

In 1980, a hypothesis was published that suggested that the last major extinction of Earth history was caused by the impact of an asteroid (10 km diameter). This event was to have caused the extinction of many organisms, including the dinosaurs, and marked the end of the Cretaceous Period some 66 million years ago.

JOIN ME IN CAUSING AN IMPACT AND EXTINCTION:

The site below allows you to impact comets, rocky meteorites, and iron meteorites into objects of the Solar System
Objects you can impact include all the planets and the moon
Choose your projectile diameter and projectile velocity ad press "KABOOM"
If you selected the Earth, you will be informed of the energy released, the magnitude of seismicity, the size of the impact crater, and an image of a real similar sized crater on Earth.
Note: The velocity of the bolide upon impact depends on the gravitational attraction of the object being impacted. If you input the wrong velocity you will be instructed of the appropriate velocity.
Link: Solar System Collisions

A. Impact consequences

1. dust cloud 1000 times greater than Krakatoa (note the Toba eruption was 400 times that of Krakatoa) into the stratosphere causing cold and darkness
2. global cooling for up to 50 years
3. little photosynthesis, plants die
4. collapse of the food pyramid
5. impact crater 150 km wide
6. enormous forest fires
7. tsunamis

B. What organisms suffered extinction

1. all dinosaurs
2. some plants but otherwise little effect on other terrestrial organisms
3. many tropical marine organisms
4. some major corals
5. ammonite cephalopods (all)
6. all rudists (reef building clams)
7. marine reptiles (mosasaurs and plesiosaurs, ichthyosaurs already extinct)
8. some fish
9. many oceanic plankton (animals and plants)
10. many organisms which feed on plankton: mollusks, echinoids
11. birds and marsupials particularly hard hit

C. Some organisms that did not suffer extinction as acutely

1. fish, turtles, crocodilians, lizards, snakes, mammals and amphibians (40% or fewer suffered extinction)
2. mammals only reflect change in dominance from marsupials to placentals

D. The Evidence For The Impact

1. A high concentration ( 5-10 ppb) of the element iridium (rare in crustal rocks but more abundant in meteorites) at the boundary in more than 100 locations world-wide. This concentration is some 30 times normal background levels. The global iridium concentration was inferred to be caused by a bolide impact as there was no known terrestrial means for producing it in this quantity. The iridium concentration was used to calculate a 10 km diameter asteroid which would have impacted at 20-25km/sec. The energy released by the impact would have equaled billions of times more energy than released by the Hiroshima nuclear bomb. The predicted size of the impact crater was 100 km. The search was on for the impact site, an elusive search only ending in 1990.
2. A possible impact structure on the Yucatan Peninsula of Mexico

the impact site- shows the impact site on a global map showing a plate tectonic reconstruction of the Earth at the time of impact (66 m.y.a.)

3. Boundary boulder debris (Cuba) and possible tsunami deposits (Texas) at the boundary.
4. A form of quartz, called shocked quartz, in a few boundary sections. Caused by high velocity-high energy impact giving them their characteristic laminar deformations. Found at nuclear test sites and impact craters.
5. "impact" spherules (melted basalt crust thrown high into the air?)

Scanning electron micrographs of spherules- recovered from the K-T boundary section.
Photograph of spherules and spherule impressions recovered from the K-T boundary section.

III. THE VOLCANIC HYPOTHESIS:

Proposes that the late Cretaceous extinction was instead caused by a major episode of volcanism.
Extinctions was cause by volcanic induced chemical changes to the atmosphere and hydrosphere.
Carbon dioxide emissions caused global warming (greenhouse effect)

see Brazos River, Texas section

A. Major known volcanism during the terminal Cretaceous Period

1. Deccan traps of India and Pakistan- basalts covering 500,000 sq. km and up to 2 km thick
2. huge volumes of flood basalts in southern Brazil
3. western U.S.

B. Could partially explain "impact" evidence

1. shocked quartz found in Toba debris, therefore possible volcanic source
2. iridium has recently been found to be greatly enhanced in gases from Kilauea (Hawaii), Reunion Island, Kamchatka (Russia), and in Antarctica dust bands in ice.
3. sulfur dioxide, chlorine and increased carbon dioxide might have caused greenhouse effect and depleted ozone.
4. deep seated mantle volcanism could have produced basaltic spherules
5. fits more closely the true and total boundary distribution of iridium
6. Arsenic and antimony abundance more compatible with mantle source volcanism

IV. OTHER INTERESTING WEB SITES ON THIS SUBJECT

Cosmic Earth Impact Links- by NASA

Ask the Scientist- Solar System - Comets, Asteroids, Meteors

A humorous look at dinosaur news