The global nature of the K-T extinction would seem to favor some causal agent which could affect the whole planet. Cosmic (extraterrestrial) phenomena might be a good possibility.

1971: Suggestion by Dale Russell (dino paleontologist) and Wallace Tucker (astrophysicist): a supernova killed the dinosaurs.

Supernovae are exploding stars: put out TREMENDOUS amount of energy. If a star in a nearby solar system exploded, it would bombard surface of planet with radiation, bringing radiation sickness, cancer, etc.

Modern analogue: during 1950s through 1970s, greatest fear about nuclear war was radioactive fallout.

Predictions:

• Large animals would suffer worst; small animals and burrowers might survive
• Organisms on the surface of the water (or creatures that fed off of these) would be hit worse then bottom-dwellers
• Would affect whole planet simultaneously (essentially every part but the poles would get clobbered once per day until the supernova faded)

Fits prediction. However, problem because it is an untestable (and thus non- falsifiable) hypothesis:

• Cannot observe remnants of the star, because supernova would have dispersed (and besides, the Solar System and all other star systems have moved greatly since the Cretaceous)
• Would leave NO geologic signature other than the extinction itself.

So, remains as a potential but no reason should be supported. Was the leading candidate during the 1970s.

1980: Walter Alvarez was investigating a layer of clay in Gubbio, Italy at the K-T boundary. Wanted to determine length of time represented by the clay layer. Consulted dad (Nobel winning physicist Luis Alvarez) for possible solution. Suggestion:

1. Meteors impact the Earth's atmosphere all the time
2. Some chemical elements more common in meteors and such than on Earth's surface: these should be traceable in minute quantities in sediment
3. Find the average infalling rate of these elements today; use this rate and observed amount at the Gubbio clay layer to find out how much time

The element used: iridium (a platinum-like metal, common in metallic asteroids but very rare in Earth's crust).

When examined Gubbio clay, found a huge increase in iridium (iridium spike) at base of clay: clearly not an “average” of infall.

Hypothesized: an asteroid impacted Earth at the K-T boundary

• Calculated probable size need to add this much iridium: suggested a 10 km diameter object (Manhatten-sized).
• Calculated probable effects of impact of an asteroid this size:
  • Release lots of energy near impact, form huge crater
  • Material vaporized by impact kicked high up in atmosphere: reduced amount of incoming sunlight
    • Observations on Mars showed big temperature drops due to high-level particles
    • In human history, eruption of Tambora in Indonesia in 1814 produced chilling effects worldwide months later
  • Dust and ash would block out sunlight, reducing photosynthesis and killing off plants on land and surface algae in water; herbivores feeding on these would die; carnivores feeding on these would starve (after a brief feast)
  • Later suggestions by others:
    • Superacid rain
    • Global firestorms
    • Global tsunami

Modern analogue: fear of nuclear war during 1980s concerned with nuclear winter

Predictions:

• Animals with larger total food requirements die more those with less
• In marine communities, foodwebs tied into photosynthesis would be hit harder than bottom feeders
• Some geologic record other than just iridium might remain
• Effects would be global and essentially instantaneous: hours to days to months to a few years

Biotic prediction fits most of the predictions; search for geological signature was on.

Shocked Quartz:

• Quartz is one of the most common of all minerals
• When subjected to intense heat & pressure, forms shock planes
• Shocked quartz has been found in over 100 K-T boundary sites worldwide

Melt Glass (Tektites):

• Material thrown up by impact would melt during reentry, form glassy spheres
• These have been found at some K-T sites

Tsunami (“tidal wave”) deposits:

• Thick units probably formed by tsunami found at K-T in Caribbean, Mexico, Gulf Coast of Texas, and now South America

Crater:

• Chances were that the impact was in ocean basins, but most Cretaceous ocean basins have been recycled by plate tectonics
• Some early leads were in Siberia (too early); Manson, Iowa (too small and too early (within Late K)
• In Yucatan, Mexico: disrupted layers at K-T boundary in buried rock
• Seismic and gravity scan suggested a crater 50-125 km across: the right size!
  • Although not visible as a crater because buried under 300-1000 m of Cenozoic rock, it can be seen using sensitive satellite and other data
• Crater was named Chicxulub, after nearby town

So, great evidence for an impact at K-T independent of extinction.
Also, pattern consistent with proposed effects (although superacid rain, global fires, and global tsunami do not have good evidence and are probably “overkill” scenarios).

Question, though: was the extinction just from impact?

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Last modified: 14 July 2006