What Killed the Dinosaurs?

K/T Extinction Boundary 65 million years ago, earth experienced a global mass extinction event: dinosaurs disappeared that extinction defines the boundary between the Cretaceous and Tertiary (abbreviated K and T) geological periods and, a broader scale, the boundary between the Mesozoic and Cenozoic eras.

The cause of the K-T extinctions is one of the great mysteries in science, and scientists have proposed many kinds of theories to account for it. They range from asteroid or comet impacts, volcanism, sea level changes, supernova explosions, and on and on.

What Is a Mass Extinction? an episode in evolutionary history where more than 50% of all known species living at that time went extinct in a short period of time (less than 2 million years or so).

Permian-Triassic extinction Life on Earth nearly was wiped out -- an estimated 90% of all species living at that time were extinguished. Trilobites were the dominant marine organism; wiped out.

CSI: Dinosaur Demise The Fossil Record - It's not perfect, gaps Time Resolution: gets worse with increasing age; gradual decline of dinosaurs vs. a sudden cataclysm is almost intractable Falsifiability - Sad but true: many “hypotheses” about dinosaur extinction sound quite convincing and might even be correct, but, as you know, are not really science if they cannot be tested and falsified if untrue.

Wrong Hypotheses That Have Been Tested Dinosaurs got so darned big that they crushed themselves Mammals outcompeted the dinosaurs Mammals ate all of the dinosaurs eggs Cosmic rays killed the dinosaurs

Common Ground There was global climatic change; the environment changed from a warm, mild one in the Mesozoic to a cooler, more varied one in the Cenozoic Many organisms; both marine and terrestrial, vertebrate and invertebrate; went extinct At or near the K/T boundary, there was iridium, shocked quartz, tektites, and a soot layer was found in many areas (evidence for widespread forest fires), all consistent with a catastrophic event

Iridium spike at K/T boundary Iridium is an element that occurs in the Earths crust in only tiny proportions, but is much more common in chondrite meteorites. Deep volcanoes also a source

Shocked Quartz High pressures cause what is known as shock metamorphism, this deforms the structure of the quartz and creates parallel laminar ripples. It is found all over the world, not just locally. No other process on the planet creates this type of quartz. Meteors; volcanoes?

TEKTITES Tektites are naturally occurring silica glass formed during the impact of a meteor. Also formed from volcanic eruptions

Cosmic Catastrophes Tektites are formed when molten material is injected into the atmosphere and cools before touching the ground Volcanoes, meteors are a source Psychically they are used for extraterrestrial communication, astral travel, and lucid dreaming.

Dinosaurs on Fire There is a large soot layer associated with the K/T boundary. This layer is consistent with catastrophic fires that may have swept the surface of the earth at this time. Such a fire would have killed most large terrestrial animals.

Nuclear Winter Lingering airborne debris is believed to have triggered darkness and a decline in the global temperature

Intrinsic Gradualists: CSI Volcanoes

Deccan Traps Volcanism About 65 million years ago, the mantle plume that gave rise to the Reunion hotspot volcano burned its way through earth's crust, flooding western India and surrounding areas with the Deccan Traps flood basalts.

Deccan Traps Volcanism Rapid eruption of the vast Deccan Traps lava fields would have flooded earth's surface with CO2, overwhelming surficial systems and sinks, triggering rapid K-T transition greenhouse warming, chemical changes in the oceans (McLean, 1985a, b, c; 1988, 1995), and the K-T mass extinctions. This increased volcanism could have created enough dust and soot to block out sunlight; contributing to the climatic change. Source of Iridium; can explain iridium spike Source of tektites Shocked quartz; maybe a source

Plate Tectonics Major changes in the organization of the continental plates (continental drift) were occurring at the K-T boundary. The oceans (especially the Interior Seaway in North America) were experiencing a regression; they were receding from the land. A less mild climate would have been the result, and this would have taken a long time. Major plate activity is consistent with major volcanic activity

Extrinsic Catastrophists: CSI Asteroids

Extrinsic Catastrophists This side of the controversy holds that the ultimate cause of the K-T extinction was extrinsic, meaning of an extraterrestrial nature, and catastrophic, meaning fairly sudden and punctuated. The main hypothesis was proposed in 1980 by (among others) Luis and Walter Alvarez, geologists at the University of California at Berkeley.

The Alvarez Hypothesis A large extraterrestrial object collided with the Earth, its impact throwing up enough dust to cause the climatic change. The iridium layer is what prompted the Alvarez team to blame an asteroid impact for the extinction -- asteroids and similar extraterrestrial bodies are higher in iridium content than the Earth's crust, so they figured that the iridium layer must be composed of the dust from the vaporized meteor. No crater was found, but it was assumed that one existed that was about 65 million years old and 100 kilometers (about 65 miles) in diameter.

Where Is the Smoking Gun?

Subducted? Evidence removed!

CHICXULUB CRATER Survey work related to oil exploration in 1981 first identified concentric rings of gravitational anomalies around the northwest coast of Yucatan. Hildebrand (1991) located two concentric rings which suggested an outer rim of 180 km.

The Smoking Gun! Some 65 million years ago, a 10 to 12 km in size asteroid or comet crashed on the Yucatan platform and formed the > 200 km in diameter Chicxulub crater. If a 10km diameter object impacted at the point at which it struck it would have a velocity of roughly 100,000 km/h.

Magnitude of Impact The K/T impact, as it is now referred to, resulted in 100 million megatons of energy. Equivalent to the energy of 300 million nuclear weapons

Impact Forces Computer model of the Chicxulub impact showing the raising fireball and CO2 plumes and ejecta curtain material (from Alvarez, Claeys and Kieffer, Science 1995)

CONCLUSIONS There has been no settlement to the issue so far, and no clear one is foreseeable. Both sides claim to hold the majority of proponents in science; it seems that (greatly over- generalizing) many paleontologists lean towards the intrinsic side, while many astronomers and physicists favor the extrinsic side, and geologists are probably evenly split between the two.