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End Cretaceous Mass Extinction (Quantify Time for Clay Formation (Repeated…
End Cretaceous Mass Extinction
End-Cretaceous Mass Extinction
~66 Ma
Non-bird dinosaurs died out
Most recent of Big 5
At Mesozoic transition to Cenozoic
Therefore has
best fossil record
Severity
Less than End Permian
Worse than End Triassic
Late Cretaceous World
Dominated by dinosaurs
Many sizes
Many diets
Lived everywhere
Rich fertile world
Mammals living in shadows
Cause
Asteroid / comet impact
6 mile wide
asteroids = rocky
comets = icy
impact near equator
Impact Hypothesis
Walter Alvarez
1970 Findings
Thick limestone in gorge stretching Cretaceous into Paleogene
Fossils changed across boundary
Abrupt diversity loss
Size and shape change
across Cretaceous - Paleogene boundary
Extinction - sudden or gradual ?
Prevailing view at time=
Gradual / long drawn out
Alvarez troubled by this
Does clay represent long or short time period ?
Walter Alvarez Paleomagnetism
Reconstructed motion of Italy in Appenine Mtns
Alps created from collision of Africa and Europe
Magnetic minerals
orient to ambient magnetic field when rock cools/lithifies
Quantify Time for Clay Formation
Alvarez proposed test for trace metals in clay
i.e. iridium
With logic
lots of iridium reps compressed time
(cosmic iridium accumulation)
little iridium suggests long time
Iridium - slowly falls as cosmic dust
Iridium HIGHLY abundant
too much for compressed time
delivery so much so quickly required another explanation
Repeated studies globally corroborated
global iridium spike
fossil patterns
e.g. Jim Shmidt I msmt in Spanish rocks
Explanation
Comet / asteroid impact
Iridium abundant in asteroids
Rare in Earth's crust
Nomenclature
Cretaceous - Paleogene (K-Pg)
Cretaceous - Tertiary (K-T)
End-Cretaceous
Alvarez Findings
Fossil change
Cretaceous rocks held
abundant microfossils
below K-T boundary
diverse -mostly forams
Paleogene saw abrupt diversity loss
above K-T boundary
change size & shape - only small forams
Iridium spike
Sudden comet / asteroid impact
Lithology change
Thin (cms) clay layer
between units
Marks K-T boundary
Extinction - on what timescale?
Clay held key