The Biogeochemical Carbon Cycle: CO2,the greenhouse effect,

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The Biogeochemical Carbon Cycle CO2,the
greenhouse effect, climate feedbacks
Assigned Reading Kump et al. (1999) The Earth
System, Chap. 7.
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Overhead Transparencies
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Faint Young Sun Paradox
41H?4He Incr.density Incr.luninosity
Liquid H2O existed gt3.5 Ga (sed. Rocks, life,
zircon ?18O)
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Simple Planetary Energy Balance
Likely solution to FYSP requires understanding
of Earths energy balance ( C cycle)
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Energy Absorbed
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Neither Albedo or Geothermal Heat Flux Changes
Can Keep the Earth from freezing w/ 30 lower
S
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Lower S compensated by larger greenhouse
effect
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The Electromagnetic Spectrum
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Incoming UV, Outgoing IR
Greenhouse Gases absorb IR radiation
efficiently
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Molecules Acquire Energy when they Absorb Photons

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1.CO2 Feedbacks Geochemical Carbon cycle
?Transfer of C between rocks and
ocean/atmosphere (gt 106-yr) can perturb CO2
greenhouse effect ?Ocean/atmosphere C
reservoir small w.r.t. rock reservoir and
the transfer rates between them
Carbon Cycle Strong driver of climate
on Geologic timescales
2.Evidence for Long-Term CO2-Climate Link
3.Case Studies
Permo-Carboniferous Glaciations Warm Mesozoic
Period Late Cenozoic Cooling
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The Geochemical Carbon Cycle
The Bio- Geochemical carbon Cycle
1. Organic Carbon Burial and Weathering
2. Tectonics Seafloof spreading Rate
Mantle CO2 from Mid-Ocean Ridges
3. Carbonate-Silicate Geochemical Cycle
Chemical Weathering Consumes CO2 Carbonate
Metamorphism Produces CO2
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Chemical Weathering chemical attack of rocks by
dilute acid
Geochemical Carbon Cycle 2

1. Carbonate Weathering

2. Silicate Weathering
consumption for silicates
Carbonates weather faster than silicates
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Carbonate Rocks Weather faster
than Silicate rocks!
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Net Reaction of Rock Weathering
Carbonate and Silica Precipitation in Ocean
Net reaction of geochemical carbon
cycle (Urey Reaction)
consumed
Would deplete atmospheric
Plate tectonics returns
via Volcanism
and Metamorphism
Carbonate Metamorphism
produced from subducted marine
sediments
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Geologic record indicates climate has rarely
reached or maintained extreme Greenhouse or
Icehouse conditions....Negative feedbacks
between climate andGeochemical Carbon Cycle must
existThus far, only identified for
Carbonate-Silicate Geochemical Cycle
Temp., rainfall enhance weathering rates
(Walker et al, 1981) (I.e., no
obvious climate dependence of tectonics or
organic carbon geochemical cycle.)
How are CO2 levels Kept in Balance? Feedbacks
Adapted from Kump et al. (1999)
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A Closer Look at the Biogeochemical Carbon
the Organic Carbon Sub-Cycle
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BIOGEOCHEMICAL CARBON CYCLE
ATMOSPHERE CO2
Exhalation CO2
rock weathering sink
dissolution sink
fixation
OCEANS
CONTINENTAL EROSION
IGNEOUS ROCKS
Uplift
SEDIMENTS
BIOSPHERE
METAMORPHIC ROCKS
lithification
SEDIMENTARY ROCKS
Organic matter
Corg
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Earth's Carbon Budget
Biosphere, Oceans and Atmosphere
Crust
Mantle
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Steady State Residence Time
Steady State Inflows Outflows Any imbalance in
I or O leads to changes in reservoir size
Inflow
Outflow
Atmospheric
Respiration
Photosynthesis
The Residence time of a molecule is the average
amount of time it is expected to remain in a
given reservoir.
of atmospheric
Example
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Carbon Reservoirs, Fluxes and Residence Times
Species
Amount
Residence Time
Sedimentary carbonate-C
Sedimentary organic-C
Oceanic inorganic-C
Necrotic-C
Atmospheric-CO2
Living terrestrial biomass
Living marine biomass
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Carbonate-Silicate Geochemical Cycle
CO2 released from volcanism dissolves in H2O,
forming carbonic acid H2CO3 CA dissolves
rocks Weathering products transported to ocean
by rivers CaCO3 precipitation in shallow
deep water Cycle closed when CaCO3
metamorphosed in
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Simple Carbon Cycle Modeling
Total C entering atm. oceans Total C buried
in sediments
closed system
13C into atm. oceans 13C being buried in
sediments
conservation of isotopes
5 the avg. value for crustal C
isotopic fifference between inorganic and organic
carbon
Hayes et al., Chem Geol. 161.37.1999 Des Mariais
et al., Nature 359.605.1992
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Carbon Isotopic Excursions 808-500Ma
More complete sediment record Improved
chronology More detailed picture showing Abrupt
and extreme C-isotopic shifts A global composite
of ?13C data shows 4 excursions Plus one at the
pC-C boundary
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Carbon Isotopic Excursions 808-500Ma
More complete sediment record
Improved chronology More detailed
picture showing Abrupt and extreme C-isotopic
shifts
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Modeling the Proterozoic Carbon Cycle

• ? ? carb and ? org through time 2500-550 Ma in
  100Ma increments
• ? note the constancy of ? carbwhile ? org
  decreased. Why? biochemistry
• or pCO2
• forg increased through this time, was episodic
  and was linked to
• periods of rifting and orogeny
• ? also associated with extreme glaciations
• Increase in the crustal inventory of C requires
  increases in the
• inventories of crustal Fe3 , crustal and
  marine sulfate, nitrate
• and atmospheric oxygen.
• ? SO4 , NO3- and O2 increases changed patterns of
  respiration
• ? NO3- would have forced productivity changes