Earth Sciences 240A: Lecture
9: A trip through time
Proterozoic Life
Until ~2.0 bya
all life anerobic
First photosynthetic organisms; Huge extinction
Development of eukaryotes at 1.8 bya
First atmospheric ozone
~700 mya: first metazoa
Proterozoic Tectonics
Rodinia: first supercontinent; Vendian
time (before Cambrian)
Snowball Earth: huge extinction
Gradual breakup as Cambrian began
Many new niches for life (explosion
of life)
Paleozoic Era
Tectonics
Rodinia fragmentation throughout
Cambrian; ended by Mid – Ordovician
Pangaea assembly
First hints of
assembly: Late Ordovician
Plate collision:
mountain belt construction
Laurasia; Gondwana;
(
Permian: fully
assembled; redbeds
Paleozoic Era Life
Early stages: Atmosphere: more O2;
less CO2; Ozone shield
First land plants: 470-440 mya
First land animals: 440 mya
Ordovician-Silurian
Event: 439 mya
Large post-PC extinctions (est.
70%)
No unusual tectonic activity, but
terrible rock records
Apparent 2-stages of Ir enrichment
No tektites, shocked minerals,
soot, etc.
Strong glaciation
evidence??
Devonian Event: 367
mya
Multiple weak Ir peaks; Micro-tektites
6 craters correct age; all too
small?
Abrupt fossil changes
Debris flows/chaotic deposits
(tsunami?)
Same isotopic changes as at K-T
Conclusion: Significant
extraterrestrial event
Possible weak glaciation??
Carboniferous to
Permian Life
Amphibians became abundant; Reptiles
(amniotic egg)
Initial inland seas rich in life
and Initial luxurious forests
Late Permian:
Interiors dried up;
Abundant redbeds (shallow
inland seas); Interior deserts
Background
extinction high
End-Permian Event:
245 mya [The Mother of all
Extinctions]
96% loss of marine/terrestrial
species; Global extent
Gradualists:
high
stress over long time
Siberian Traps (huge volcanic
eruptions)
Catastrophists:
C, Ir, O isotopes consistent with
impact
Some tektites
Support:
No obvious crater large enough
Mesozoic Era
Pangaea fragmentation begins
Age of reptiles; Dinosaurs
dominated every environment
Mammals started; tough life
Triassic-Jurassic
Event; 208 mya
No abnormal tectonics
At least 3 weak Ir-rich boundaries
Shocked quartz
C and O isotopes look same as Late
Devonian event
Likely multiple impacts, but not
well studied
Next
The K-T Event
Remember:
13 October: Thanksgiving
15 October: Mid-Term Test (001 students only)