GEOL 102 Historical Geology
Spring Semester 2011
The Mesozoic Era VI: The K/Pg Extinction
Probably the most intensively studied of all mass extinctions in the rock record
- Many species of coccolithophorid: never recover diversity
- Many species of foram (although these do recover)
- All ammonoids
- All belemnoids
- All inoceramids
- All rudists
- All plesiosaurs
- All mosasaurs
- All pterosaurs
- All non-flying dinosaurs and several clades of birds
- Many mammal groups (only monotreme prototheres, multituberculate allotheres, the ancestors and closest relatives of the marsupials among
the metatheres, and the ancestors and closest relatives of the placentals among the eutheres)
- All bennettitalians
Of course, many survivors as well.
Although many untestable hypotheses suggested (hunting by aliens, supernova radiation,
etc.), three contributing factors have strong independent physical evidence:
The Maastrichtian Regression:
Draining of epeiric seas would alter terrestrial climate by:
- Changing Earth's albedo, and thus its solar budget, and thus its weather
- Producing more continental climates in interiors, changing regional ecosystems
- Change oceanographic conditions, by removing the once-vast epeiric sea systems and
by removing a major source of productivity
- Would operate over a 4 million year scale
Increased Maastrichtian volcanism, especially the Deccan Traps:
- Decrease insolation (incoming sunlight) by presence of fine particles in high
- Also change Earth's albedo, although not as dramatically
- Would operate on the scale of a few tens of thousands to hundreds of thousands of
The Chicxulub Impact
1980: Walter Alvarez was investigating a layer of clay in
Gubbio, Italy at the K/Pg 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/Pg boundary
- Calculated probable size need to add this much iridium: suggested a 10-15 km diameter object (Manhatten-sized).
- Calculated probable effects of impact of an asteroid this size:
- Short term:
- Release lots of energy near impact, form huge crater: 1.8 x 108 megatons!!
- Burst of light would vaporize material for kilometers around, just like thermonuclear weapons
- Blast wave would devaste nearby region; it would be felt around the world, but decrease with distance
- Shockwaves from impact would generate huge tsunamis ("tidal" waves)
- Newly recognized minutes-to-hours event: the "Easy Bake Oven Effect":
Material that was thrown up above atmosphere and reentered generates substantial
infrared radiation. This heat raises air temperature by only about 10C° (18F°), but would be fully absorbed by rock, leaf, flesh, and
any other opaque material. It is predicted that the increase in infrared radiation would be 8-10x that of high noon at the hottest spot of the Earth,
and persist for many minutes to hours. Living tissue would bake, unless underground 10 or more cm (heat wouldn't have time to make it that deep) or
underwater (upper few microns of water might boil off, but that would be it).
- Longer term:
- 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 1815 produced chilling effects
worldwide for more than a year 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): Impact Winter
- Collapse of foodwebs would require long term to recover, as many parts of each foodchain might be lost
- Paleoceanographic evidence suggests that seas were cooled considerably, and took 10s of kyr to reheat at depth
- "Greenhouse Summer": release of substantial carbon dioxide from Deccan Traps and vaporization of carbonates
at Chicxulub raised carbon dioxide levels from ~500 to ~2300 ppm, with long-term (100s of kyr) greenhouse warming
Modern analogue: fear of nuclear war during 1980s concerned with nuclear winter, the
likely consequence to a large-scale nuclear war first proposed shortly after (and suggested by) the Alvarez scenario
- Animals with larger total food requirements die more those with less
- In marine communities, foodwebs tied into photosynthesis (that is, direct from the phytoplankton) would be hit harder than bottom feeders
(which feed on the accumulated decayed remains of organisms)
- Additionally, taxa dependant on symbiotic algae would be devastated
- 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.
- 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/Pg 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/Pg sites
Tsunami ("tidal wave") and ejecta deposits:
- Thick units probably formed by
tsunami found at K/Pg in Carribbean, Gulf Coast of Texas,
Mexico, Central America, and South America
- Thinner but
widespread deposits of ejecta (material flung through the air) at K/Pg in Carribbean, Gulf Coast of Texas,
Mexico, Central America, and 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))
- Nearly all
geological lines of evidence (tektites, tsunami deposits, ejecta deposits, shocked quartz, etc.) were more abundant in Western Hemisphere,
and especially in the Gulf of Mexico, than the rest of the world: pointed to impact in that region!
- In Yucatan, Mexico: disrupted
layers at K/Pg boundary in buried rock
- Seismic and gravity scan suggested a
crater 180 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/Pg independent of extinction. Also, pattern consistent with proposed effects (although some versions of the
superacid rain, global fires, and global super tsunamis do not have good evidence and are probably "overkill" scenarios).
Suggestions that all these systems were in effect:
- Some suggestion of million-year scale decline in some groups, but not as strong as once thought
- Change in flora of western North American Interior, consistent with climate/ecosystem
changes due to Regression
- Many extinctions, however, seemingly instantaneous
- In marine realm, planktonic forms and creatures that eat them (and those that ate
them, and so on) suffered greater than benthic detritivores, consistent with shut down of
photosynthesis. Impact Winter seems to have been the primary killing agent.
- In terrestrial realm, basic pattern is that animals dependant on large food supply and/or metabolism: larger and/or fully terrestrial creatures
survive better than smaller and/or subaquatic forms. A combination of the "Easy Bake Oven" and "Impact Winter" seem to have
been the main killin agents, with the longer "Greenhouse Summer" picking up many of the survivors.
Extinction of non-avian dinosaurs paves the way for the rise of mammals as the dominant
group of terrestrial animals. Marine realm recovery represents survival of many groups,
but less change in structure.
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Last modified: 14 January 2011