Transcript L5_Tect - Atmospheric and Oceanic Sciences

Lecture 5:
Plate Tectonics and Climate
Last 550 Myr
(chapter 4)
Atmospheric CO2 Evolution
Plate Tectonics
Alfred Wegener (1914, German Meteorologist) proposed continental drift
3.2g/cm3
2.7g/cm3
>3.7g/cm3
1-10 cm/yr ==>5000km/100myr
Soft acts as viscous fluid allows the tectonic plates above to move
Formation of the continents
Sea floor spreading
creation/destructoin of ocean crust
since 175 myr
Continental magnetic field
reconstruct continental positions (lat.)
since 550 myr
Paleomagnetic determination of past location of continents
the position of basalt rocks on (older than 175 myr)
Forming compass
Molten lavas cools extruding onto the earth’s
surface cools, the iron rich component aligns with
the earth’s magnetic field at that time
Fossil compass
Further cooling forms
basalts and fixes the
magnetic field
compass pointing vertical compasses, forming
fossil compass
compass pointing horizontal
Dating
Potassium is also locked
in basalts, its slow
decaying rate (half time
1.3 Byr) allows us to
identify the time
Paleomagnetic dating of ocean crust
Magnetic reversal (over land) and magnetic lineations (in ocean)
Moving Continents
Gondwana and S. P. Glaciation
100 Myr ago
Cretaceous
Pangaea and Supermonsoon
200 Myr ago
The Polar Position Hypothesis
for alternative icehouse and greenhouse climate
Glaciation
occurs near
polar position,
But
polar postion
does NOT
guarantee
glaciation
?
?
?
?
polar postion is
NOT the only
mechanism for
glaciation
What else?
Atmospheric CO2 Evolution
Why no ice sheet at S. Pole between 425-325Ma, 125-35 Ma?
?
CO2?
?
Tectonic Control of CO2 Removal:
Uplift-Weathering Hypothesis
Marueen 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
100
1
Why fresh rocks has a much higher weathering rate (over 100 times…)
1)Fresh rocks has more weathering materials – fresh, unweathered silicates
2)Fragmentation increases surface area
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
CO2 Change in the last 100 myr: Hypothesis 1
BLAG hypothesis:
sea floor spreading ==>volcanic activity==>CO2 increase
100 ma
now
Carbon cycle and stabilization of CO2
BLAG hypothesis:
sea floor spreading ==>volcanic activity==>CO2 increase
Further weathering effect stabilizes CO2 and climate
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
Extreme hotness
may prevent
glaciation
Evidence of strong
monsoon reversal:
red beds
Evidence of
dry continents
End of Chapter 5
Experiments in South America:
More weathering on the slopes of Andes