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Lecture 5:
Plate Tectonics and Climate
Last 550 Myr
(chapter 4)
Atmospheric CO2 Evolution
Plate TectonicsAlfred Wegener (1914, German Meteorologist) proposed continental drift
1-10 cm/yr ==>5000km/100myr
2.7g/cm3
3.2g/cm3
>3.7g/cm3
Soft acts as viscous fluid allows the tectonic plates above to move
Sea floor spreading
creation/destructoin of ocean crust
since 175 myr
Continental magnetic field
reconstruct continental positions (lat.)
since 550 myr
Formation of the continents
Paleomagnetic determination of past location of continents the position of basalt rocks on (older than 175 myr)
compass pointing horizontal
compass pointing vertical
Forming compassMolten lavas cools extruding onto the earth’s surface cools, the iron rich component aligns with the earth’s magnetic field at that time
Fossil compassFurther cooling forms basalts and fixes the magnetic field compasses, forming fossil compass
DatingPotassium is also locked in basalts, its slow decaying rate (half time 1.3 Byr) allows us to identify the time
Magnetic reversal (over land) and magnetic lineations (in ocean)
Paleomagnetic dating of ocean crust
Moving Continents
200 Myr ago
100 Myr ago
Cretaceous
Pangaea and Supermonsoon
Gondwana and S. P. Glaciation
The Polar Position Hypothesis
for alternative icehouse and greenhouse climate
Glaciation occurs near polar position,
But
polar postion does NOT guarantee glaciation
What else?
Atmospheric CO2 Evolution
? ?CO2?
Why no ice sheet at S. Pole between 425-325Ma, 125-35 Ma?
Tectonic Control of CO2 Removal:
Uplift-Weathering HypothesisMarueen Raymo et al., 1980s
Uplifting ==> create fresh rock and mineral surfaces
==>stronger weathering ==> reduce CO2
The magic of “breaking up”
Quantifying weathering rate with time
Why fresh rocks has a much higher weathering rate (over 100 times…)
1)Fresh rocks has more weathering materials – fresh, unweathered silicates2)Fragmentation increases surface area
1
100
Uplift and chemical weathering
Atmospheric CO2 Evolution
? ?
Tectonic control of CO2:
BLAG spreading rate hypothesis
(Robert Berner, Antonio Lasaga, Robert Garrels, 1983)
sea floor spreading ==>volcanic activity==>CO2 input
Age of sea floor
100 ma
now
BLAG hypothesis:
sea floor spreading ==>volcanic activity==>CO2 increase
CO2 Change in the last 100 myr: Hypothesis 1
BLAG hypothesis:
sea floor spreading ==>volcanic activity==>CO2 increase
Further weathering effect stabilizes CO2 and climate
Carbon cycle and stabilization of CO2
? ?
Atmospheric CO2 Evolution
and Tectonic control of CO2 hypothesis
References for reading
Indonesian seaway, East Africa climate, and human evolution
Cane M. and P. Molnar, 2001: Closing of Indonesian seaway as a precursor to east African aridification around 3-4 million years ago. Nature, 411, 157- 162
Supercontinent Pangaea and Supermonsoon
Evidence of strong monsoon reversal:
red beds
Extreme hotness may prevent glaciation
Evidence of dry continents
End of Chapter 5
Experiments in South America:
More weathering on the slopes of Andes