The extinction at the end of the Cretaceous (65 Ma)

Two (not one) Questions:

• Was there a large impact at the time when dinosaurs became extinct? Answer (spring 2004): YES
• Did that impact (by itself) cause all extinctions? How did it cause the extinctions? Answer (spring 2004): we do not know

• Dinosaurs stupid: out-competed by mammals (mammals abundant by 10 million years after extinction)
• Dinosaur eggs eaten by mammals (mammals could have done that ever since the origin of dinosaurs and mammals way back in the Triassic)
• Dinosaurs poisoned by flowering plants, evolved in Cretaceous: flowering plants evolved much earlier in the Cretaceous
• Climate too hot: dinosaurs one sex (no evidence that it really warmed by much, no evidence that sex is linked to temperature of hatching in dinosaurs
• Climate too cold: dinosaurs cold-blooded: it cooled somewhat (short-term), no evidence for polar ice caps; dinosaurs may not have been cold-blooded

Foraminifera (microfossils) demonstrate better than dinosaurs that there was a true mass extinction: rare fossils never can demonstrate that. Also, one can look at evidence for impact in same sediments where we see effect of extinction on microfossils

• No major extinction (as at end-Permian): extinct forms mainly insect-pollinated
• Strong evidence for short-term ecological disturbance ('fern-spike': short time with floras dominated by 1 species of fern), especially severe in North America
• Strong evidence that land plants recovered within a geologically very short period (possibly < century. using modeling)

Which organisms hard hit?

• Large land-dwellers (> ~ 25 lbs)
• Large ocean dwellers (from mosasaurs to ammonites)
• Oceanic phytoplankton without resting spores (photosynthesize; have short life span, a few weeks at most)
• Organisms with photo-symbionts (corals, reefs)
• Filter feeders (feed on phytoplankton)
• Low latitude species

In short: species that would have been affected by cold and dark!

• Existence of Crater: Crater (diameter about 200 km, 65 million years) buried under sediments, Yucatan peninsula , Mexico (Chicxulub Crater). Impact directed from south-east towards northwest, hence severity of effects on land plants in northern America ('fern spike')
• Results of fall-out outside crater (over the whole world):
  • Pieces of asteroid found in sediment at time of extinction of micro-organisms
  • Vaporized material from Earth condenses into glassy lumps called tektites; found in sediments formed at time of extinction.
  • Material from the meteorite vaporid; metals such as Iridium common in mantle Earth, and in meteorites that are part of mantle of their ancestor planet. Iridium is enriched in sediments formed at time of extinction.
  • Earth crust subjected to very high pressure at impact; some minerals do not melt but develop 'cracks' in minerals structure; example: shocked quartz grains.
  • Evidence for occurrence of large fire storms: soot and pieces of charred vegetation.

• Fireball: minutes
• Thermal pulse, fires: hours
• Strong winds, giant waves: hours
• Dust veil (cold, dark): months
• Acid rain: (months, year)
• CO₂ greenhouse effects (impact in limestone): millennia
• Recovery biomass on land (biomass, not species diversity): within millennia; evolution of large animals: 5-10 million years
• Recovery biomass in oceans (biomass, not species diversity): several hundred thousands of years; evolution large animals; 5-10 million years

• (some) Iridium comes out of the flood-basalt type of volcanoes
• (some) shocked minerals come out of explosive volcanoes, but NOT out of flood basalt volcanoes
• NO volcano produces both: Deccan trap volcanism does not emit shocked minerals

So Deccan Traps volcanism does not explain all evidence for impact

Yet another problem:

There were many more episodes of flood basaltic eruptions and many more large impacts than there have been large mass extinctions!

• Maybe we see the largest mass extinctions only if the global ecosystem is already in an unstable state
• Climate change, flood basalts, impact

Compare to possible effects of human habitat destruction, transport of 'exotic' or 'invasive species' worldwide, global climate change