Earth Sciences 240A
Lecture 10
The Cretaceous Period And The K-T Extinction
Event
Mesozoic
Dominated by Pangaea breakup
Abundant marine life similar to
modern
Unusually rapid seafloor spreading;
Large undersea volcanic ridges
Continental seaways
By Mid-Cretaceous
Seas high and warm (8oC above
today); No polar ice
Strongest greenhouse since
Precambrian
Most prolific terrestrial/marine
life ever
By Late Cretaceous
Global temperatures began to fall
End Cretaceous
65-75% all species died; Slightly more terrestrial than marine
Early gradualist view:
Took at least 1
million years
Early catastrophist view:
Critical event(s)
took place between 1 and 1000 years
Problems
Incomplete fossil
records
Poor preservation
of many fossils
Reworking of
sedimentary layers
The K-T Event Horizon
Extinctions
(Most) Dinosaurs; They had ruled for 160 m years
All large
reptilian swimmers
Most small plant
life (marine, terrestrial)
Bottom of the
food chain
Starvation for
dependents
K-T horizon:
Petrology/Chemistry
1980: Alvarez et al. publication
Extraordinary Ir (30 ppb) in clay
layer
Change in spore
pattern
Est. 10 km
diameter asteroid; no crater!!
1991: Hildebrand et al. publication
Defined fireball
layer
Tektites, shocked
minerals, tsunami debris
The Chicxulub Crater
Magnetic anomaly 1978: Pemex
Penfield & Camargo
1981: gravity
Hildebrand
Beneath 1 km
recent sediments
Approx. 180 km
diameter
Complex structure
Age of 66 mya (from melt glass)
Impact Event
10 (to 14) km asteroid NEO
Velocity est. 30 km/s
Released kinetic energy: 1025
joules
15 km deep crater
5x1012 tons rock
to atmosphere
Global ‘Night’ for
min. 6 months
10 kg per square
meter fall-back
Limestone: CO2 to
atmosphere
Firestorm and
nitric acid-rich rain
35-770 billion
tons S to atmosphere
Sulfuric acid-rich
rain
Tsunami
T-Rex and Death
Preliminary studies
Study 1: Dinosaurs
only: last 10 my of Cretaceous
A few ‘holes’ in
data
“No correlation
of deaths to impact event”
Study 2: All
vertebrates: 64% survived
“Gradual
extinction supported”
Study 3: Same data
as #2 but sorted
88% terrestrial:
died; 90% freshwater: survived
Nothing larger
than a dog survived
Extinction
Periodicity?
1973 Harold Urey
suggested periodicity; Suggested impact cause
1984 Raup
and Sepkoski; 26 my periodicity for last 250 my
Causes:
Nemesis
Planet X
Tilt of plane
thru debris ‘cloud’
Critics abound!
Next: Plate Tectonics
READ THE PREREQUISITE MATERIAL BEFORE LECTURE
(see instruct website)