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1/30/12
Why is Earth Habitable?
Sun has increased in strength by 25% to 30% since the earth was formed. If
true, why has Earth’s climate been relatively stable?
Tectonic Control of Climate
Tues – Graduates: Research questions due
Thurs – Outline and references due
Natural thermostat that allows its climate to warm up during greenhouse eras
Without boiling oceans and to cool off during icehouse eras without freezing
solid.
What controls atmospheric CO2?
1 – Volcanoes release carbon from
Rocks into the Atmosphere.
2 – Weathering of silicate minerals
Removes CO2 from the atmosphere
and sequesters it in the shells of
Carbonaceous marine organisms.
The process of chemical erosion that
consumes atmospheric CO2 is known
as hydrolysis.
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CO2 Balance
The slow weathering of granite and other silicate rocks, and the associated
consumption of atmospheric CO2, must be balanced by the rate of CO2
input by volcanoes.
This does not imply that both have been constant throughout time. In fact,
these two rates most certainly have varied in response to Earth’s tectonic
processes.
Given that the release of CO2 by volcanoes is controlled entirely by tectonic
processes that are not affected by changes in climate, we must conclude
that the rate of chemical weathering is the climatic thermostat that
regulates climate.
aka Spreading rate hypothesis after Berner,
Lasaga, and Garrels
CO2 is expelled into the atmosphere from two
sources
1 – The margins of divergent plates where magma carrying CO2
erupts into ocean water.
2 – The margins of convergent plates where portions of subducting
plates melt and rise to the surface where CO2 and other
gases are released.
The BLAG spreading rate hypothesis
Proposes that increased spreading
rates lead to increased release of CO2
by magma into the atmosphere.
BLAG
Spreading
Hypothesis
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The BLAG hypothesis
•  Globally averaged spreading
rates
•  Faster spreading = more CO2
and vice versa
•  Also factors in silicate
weathering as a negative
feedback
•  Timescale of 10s of Ma
•  Only reliable for last <170 Ma
because older ocean crust has
been destroyed
•  Sea level records (high when
spreading is fast) can also be
used to estimate spreading
Negative Feedback in BLAG
Upli7
Weathering
Hypothesis:
Scenario
1
Upli7
Weathering
Hypothesis:
Scenario
1
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Upli7
Weathering
Hypothesis:
Scenario
1
Upli7
Weathering
Hypothesis:
Scenario
2
Upli7
Weathering
Hypothesis
Nega>ve
Feedback
or…
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Posi>ve
Feedback?
Uplift Weathering Hypothesis: CO2 Removal
The Uplift Weathering Hypothesis
looks at chemical weathering as the
active driver of climate change,
rather than as a negative feedback
that moderates climate change.
As the rate of mountain building
increases over Earth’s surface, the
abundance of freshly exposed,
unweathered rock increases, which
in turn increases the total amount
of chemical erosion and CO2
withdrawal taking place.
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