Download Lecture 10

ASTR/GEOL-2040: Search for life
in the Universe: Lecture 10
• Sources/sinks of C
• Plate tectonics
• BIFs and GOE
Thanks for the flower!
• And the
wishes in
German!
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To determine the rock’s age
A. Determine its chemical composition
B. Identify its mineral structure
C. Measure ratios of different isotopes
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To determine the rock’s age
A. Determine its chemical composition
B. Identify its mineral structure
C. Measure ratios of different isotopes
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Typo on page 21 last time
not
0.102
t/t1/2=log2(1+[40Ar]/0.109 [40K])
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Starting values unknown
Divide the 2 equations by each other
In HW4, just use final equation from lecture
Derivation merely for completeness
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Today
• HW4 (due Thursday, Feb 26)
– Last before midterm exam (Mar 2)
• Carbon dioxide cycle
• Plate tectonics
• Great Oxidation Event (GOE)
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On Earth, CO2 is recycled
• Sources of CO2
– ….
– ….
• Sinks of CO2
–…
–…
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On Earth, CO2 is recycled
• Sources of CO2
– Animal life on Earth
– Oxidation of exhumed CH2O
– Other C oxidation (e.g. fire)
– Outgassing (volcanoes)
– CaCO3  CaO+CO2 or rather
• Silicate minerals + CaCO3 
new silicate minerals+CO2
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On Earth, CO2 is recycled
•
•
•
•
Sources of CO2
….
….
Decarbonation
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On Earth, CO2 is recycled
•
•
•
•
Sources of CO2
….
….
Decarbonation
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Sinks of CO2
• Photosynthetic life (of course)
• Acid rain: H2O+CO2 =H2CO3
– Contact with rock: weathering
• CaSiO3 +H2CO3 CaCO3 +SiO2
– Calcium carbonate
– solid deposit (sea bed)
– carbonate rock (limestone)
– Details in RGS p.51
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Sinks of CO2
• Acid rain: H2O+CO2 =H2CO3
– Contact with rock: weathering
• CaSiO3 +H2CO3 CaCO3 +SiO2
– Calcium carbonate (solid deposit)
– Details in RGS p.51
• On Earth: 170,000 times more CO2 in
carbonate rocks than in atmosphere
See BS p.139 for details!
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The CO2 thermostat
• Recycling rate sensitive to temperature
• CO2  warmer (greenhouse)
– More evaporation, more rainfall
• Pulling more CO2 out of atmosphere
– Weaker greenhouse effect
• Negative feedback
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Feedbacks
• Negative feedback
– Stable
• Positive feedback
– Unstable, runaway
• Examples?
– loudspeaker
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CO2 thermostat: other way around
• Less CO2  cooler
– less evaporation, less rainfall
• Less removal of CO2 out of atmosphere
– greenhouse effect becomes stronger
– and it gets warmer again
• Again: negative feedback
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The CO2 thermostat
CO2 high, warm, more rain
atmospheric CO2 reduced
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The CO2 thermostat
CO2 low, cool, less rain
atmospheric CO2 builds up
CO2 high, warm, more rain
atmospheric CO2 reduced
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Why so much CO2 on Venus?
• Venus: 200,000 times more CO2
than Earth
– Remember: Earth 170,000 times
more CO2 in carbonate rocks
• Venus: no return to carbonate rocks
• Why?
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Why no CO2 return into rocks
on Venus?
•…
• ……
• ………..
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into rocks
Why no CO2 return on Venus?
• No water, no rain, no feedback
• Yet, volcanic activity
Idunn Mons
 infrared
 topography
enlarged
0.25 Myr old
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Why did Venus lose its water?
• Same reason as for Mars
• UV light: H2O  H2 + ½ O2
 photolysis
• H2 lost through thermal escape
• Why?
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Thermal escape?
• Escape velocity?
– Apollo 8
(Borman, Lowell, Anders)
• ½ mve2=GMm/R
– ve=(2GM/R)1/2 = 11.2 km/s
– ½ mvH2=kBT
• H2 is so light
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Why not on Earth?
• Water vapor condenses to rain
before too much gets lost
• Venus: unable to protect itself
• Too hot: H2O also greenhouse gas
– Hotter  more vapor  hotter still
• Runaway greenhouse effect
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Runaway greenhouse effect
• Water
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Earth has far less atmospheric
CO2 than Venus because
A. Earth was born with less gas
B. CO2 was lost in giant impact
C. CO2 is locked up in carbonate rocks
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Earth has far less atmospheric
CO2 than Venus because
A. Earth was born with less gas
B. CO2 was lost in giant impact
C. CO2 is locked up in carbonate rocks
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If Earth had more greenhouse
gases in its atmosphere, it would
A. Heat up
B. Cool off
C. Accelerate plate tectonics
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If Earth had more greenhouse
gases in its atmosphere, it would
A. Heat up
B. Cool off
C. Accelerate plate tectonics
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If Earth had more greenhouse
gases in its atmosphere, it would
A. Heat up
B. Cool off
C. Accelerate plate tectonics
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Earth’s structure
• cc
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We know this from seismology
• Crust
• Mantle
– Upper
– Lower
• Core
– Inner
– Outer
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Asthenosphere: viscous & weak
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Mantle: 45% by radius
• cc
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Earth’s structure
• cc
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Earth’s structure
• cc
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Plate tectonics
• Upwellings (heat)
• Subduction
• shear
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Also deep in the pacific & atlantic
Rock locations
Acasta
gneiss
• Predictable
• Phoenix
observed
precipitation
• But evaporated
before reaching
ground
Isua
Barberton
Shark
Bay
Greenstone belt
Long, p.211
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Banded iron formations (BIFs)
• Best evidence
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Banded iron formations (BIFs)
• BIF deposits of >2.5Gyr
– Estimated mass 3x1016 kg
– 30% in hematite (Fe2O3)
• How much O?
– 2x56+ 3x16=160
– Amount of O: 0.3x3x1016 kg x 48/160 =
3x1015 kg
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Banded iron formations (BIFs)
• Large amounts of O into BIFs early on
– Interpretation: Fe minerals dissolved in
ocean (e.g., from hydrothermal vents)
• Hematite precipitated
– Deposited at ocean floor
– Atmospheric O2 much came later
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Snowball Earth events
• Geological record
– Dramatic climate changes
• 0.75 – 0.58 Gyr ago (up to 4)
– Global glaciation (snowball Earth)
– Mass extinction (CO2 sinks cease)
– Explains Fe-rich betw glacial deposits
• Similar events earlier in history!
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Oxidation of Earth atmosphere
• Evidence
– Oxidized sulfur compounds in rocks
– Also reduced sulfur (FeS, sulfide)
• First 1 Gyr, only trace amounts of O2
– Plausible source: photosynthesis
• Higher levels of O3
– Life under water (protected)
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Ice ages are different
• Temperature drops by a few degrees
– More snowfall, down to low latitudes
– Last one 10,000 yr ago
– Duration: 35 Myr
• One contributor: cyclic changes in
Earth rotation (tilt 22o-25o) & orbit
– Milankovich cycles
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The great oxidation event (GOE)
• 2.3 Gyr ago (weathering saturated)
• Diversity of minerals & later life
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Snowball Earth refers to
A. One in a series of deep ice ages that
occurred >0.5 Gyr ago
B. The idea that Earth would be frozen
without greenhouse effect
C. Any of the ice ages that occurred in
the past few million years
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Snowball Earth refers to
A. One in a series of deep ice ages that
occurred >0.5 Gyr ago
B. The idea that Earth would be frozen
without greenhouse effect
C. Any of the ice ages that occurred in
the past few million years
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Next week
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Evidence for early life on Earth
Oceans 4.4 Gyr ago
Significance of 13C isotope
Cambrian explosion of life
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