Lecture 5: Plate Tectonics and Climate Last 550 Myr (chapter 4)

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