GEOL 104 Dinosaurs: A Natural History
Fall Semester 2006
The Cretaceous-Tertiary Extinction II: One REALLY Bad Day!
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
- 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-
- 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
- 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
- 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
- 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
- Animals with larger total food requirements die more those with less
- In marine communities, foodwebs tied into photosynthesis would be hit harder than
- Some geologic record other than just iridium might remain
- Effects would be global and essentially instantaneous: hours to days to months to a
Biotic prediction fits most of the predictions; search for geological signature was on.
- 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
- 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