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

Chicxulub, Yucatan

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

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