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Plate Tectonics and Climate

陳奕穎

Chapter 5 of EARTH’S CLIMATE

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Plate tectonic process

Glaciation alternation

Climate modelChanges CO2

IcehouseGreenhouse

Ex: 200Myr ago

Two hypotheses

Polar Position Hypothesis

BLAG Spreading Rate Uplift Weathering

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Plate tectonics: The scientific theory which describe Earth is called plate

tectonics.

Wegener[1914] find the margins of eastern South America and western Africa, could fit together.

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Structure and Composition of Tectonic Plates Earth Outer Layers

chemical composition Continental crust (30-70 km)

Granites (花崗岩 ) 2.7 g/cm3

Ocean crust (5-10 km)Basalts (玄武岩 ) 3.2 g/cm3

Mantle (Fe, Mg) to 2890 km >3.6 g/cm3

Physical behavior Lithosphere (100 km)

hard, rigid unit that forms the tectonic plates

Asthenosphere (100-250 km)softer unit capable of flowing

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Tectonic plates

The outer rigid layer of Earth is broken into about dozen major segments called plates.

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Plate boundaries

Divergent margins Convergent margins Transform fault margins

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Evidence of past plate motions

Earth magnetic field evidence of Plate tectonics rearranging Earth’s geography

Energy from the magnetic field which result from molten fluids circulating in Earth’s liquid iron core.

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Magnetic lineations

Molten fluid record magnetic field.

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Paleomagnetic determination of past location of continents

Basalt is the best rocks to use (rich in highly magnetic iron).

No ocean crust older than 175 Myrs. For earlier interval, it must focus on basalts on the contin

ent. 500Myrs less reliable because of increasing likelihood t

hat their magnetic signatures have been rest to the magnetic field of a later time.

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Short Summery We can reconstruct the position of the continents wit

h good accuracy back to 300 Myrs ago. To measure rates of the seafloor spreading in ocean b

asin. Even we can compile spreading rates over enough of

the world’s ocean to estimate the global mean rate of creation and destruction of ocean crust.

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The Polar Position Hypothesis :

ice sheets should appear on continents when they located at polar or near-polar latitude,

but no ice should appear anywhere on Earth if no continent exist anywhere near poles.

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Moving continents

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Laurasia

Gondwana

Pengaea Laurasia: North-central Asia, Europe, North American Gondwana: Africa, Arabia, Antarctica, Australia, Sout

h America, and India.

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Gondwana and South Pole

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Glaciations and Continental Positions since 5

00 Myr Ago

Why ?

CO2

Icehouse Greenhouse

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Modeling Climate on the Supercontinent Pangaea

Climate scientist use general circulation models (GCMs) to evaluate the impact of geography as well as several other factors.

Questions:

What level of atmospheric CO2 ?

Dose it match geologic record ?

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Input to the Model Simulation of Pangaean Climate

Boundary condiction:Distribution of land and sea

Global sea level

Using simplified symmetrical

Comparable to today’s

200 Myr ago

Topography

1000 m

1.

2.

3.

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Input to the Model Simulation of Pangaean Climate cont.

4.Climate modelers constrain the likely CO2

level in atmosphere.

5.Astrophysical modelers indicate Sun’s energy weaker 1% than today’s.

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Output from the Model Simulation of Pangaean Climate

Dry continental climate

1.the great expanses of land at subtropical latitudes beneath the dry.

2.trade wind lose most of their water vapor by the time they reached the continental interior

Uplift

Downward

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Output from the Model Simulation of Pangaean Climate cont.

Monsoon circulations

Different rates of response of the land and sea to heating in summer and radiative heat loss in winter

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Tectonic Control of CO2 Input

BLAG[1983] (the geochemists Robert Berner, Antonio Lasaga, Robert Garrels) Climate changes during the last several hundred million years have been driven mainly by changes in the rate of CO2 into the atmosphere by plate tectonic process.(spreading rate hypothesis)

Spreading rates Climate changeCO2

Change

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Age of the seafloor

Spreading rates are as much as ten times faster in the Pacific than in Atlantic.

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Earth’s Negative Feedback

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Tectonic-scale Carbon cycle

2323 SiOCaCOCOCaSiO

landonweathreingChemical

Silicate rock Atmosphere Plankton Plankton

2323 COSiCOSiOCaSiO

zonesubductionintiontransformaandMeltimg

Ocean sediment Silicate rock Atmosphere

Imbalance CO2

Climate changes

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A Warmer Earth 100Myr Ago

The global mean spreading rate was as much as 50% faster 100Myr ago than it is at present, so the rate of input of CO2 from the rocks to atmosphere must be higher than today.

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The Uplift Weathering Hypothesis

Chemical weathering

Rock exposure

Fresh rock

Exposure time

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Fragmentation of Rock

Weathering and Exposure Time

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Three Hypotheses:

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Conclusions:

Plate tectonic process largely explains alternations between icehouse intervals.

Atmospheric CO2 changes in tectonic-scale in the last hundred million years needed to explain the climate variability.

Both spreading rate & uplift hypotheses attempt to link the changes in CO2 and in plate tectonic.