Carbon Cycle
Fluxes and reservoirs
Organic carbon cycle
Inorganic carbon cycle
Residence times
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2
Discuss carbon using
-- Box diagrams
-- Reservoirs
-- Flow of matter
=
Reservoir
=
Flux of material
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A Bathtub
an example of a reservoir
(the amount of water is the Burden of the reservoir)
Input (Source)
(flow of water into
the tub)
Output (Sink)
(flow of water
out of the tub)
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When the flow of water into the tub
equals the flow out of the tub, the water
level does not change.
Steady state conditions:
Input = Output
Source = Sink
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Residence Time
The average length of time matter
spends in a reservoir
Residence time = Burden / Sink
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A Bathtub
tub holds 100 liters (Burden)
Source = Sink = 5 liters/minute
Residence time = 100 liters
5 liters/minute
= 20 minutes
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Organic and Inorganic Carbon
C is cycled between reduced and oxidized forms
by natural processes
Organic carbon
(reduced)
Inorganic carbon
-combined with C, H
-combined with oxygen
(oxidized)
‘CH2O’
CO2
carbon dioxide
H2CO3 carbonic acid
HCO3− bicarbonate ion
Example:
Glucose -- C6H12O6
CO3= carbonate ion
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Coal
Oil
JENNY HAGER/ THE IMAGE WORKS
http://www.nationalfuelgas.com
Organic
carbon
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http://www.upl.cs.wisc.edu/~stroker/jungle.jpg
Inorganic
carbon
Seashells
http://www.cmas-md.org/Images/Sanjay/UnivTop4.jpg
Coral
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http://www.summerclouds.com/Vero/Sea%20Shells.jpg
http://educate.si.edu/lessons/currkits/ocean/
CO2 in the Atmosphere
“the Keeling Curve”
Photo (spring /summer)
Photo
Decay (fall / winter)
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Primary Productivity – measure of
photosynthetic activity
http://www.gsfc.nasa.gov/gsfc/earth/pictures/2003/0530earthgreen/whole_flat_world.mpeg
Primary Productivity – Rotating earth
http://earthobservatory.nasa.gov/Newsroom/NasaNews/ReleaseImages/20030917/image6-seawifs_npp_linear_sm.mov
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On a global scale, we measure quantities of carbon
in gigatons (Gt)
1 Gt = 1 billion metric tons
1 metric ton = 1,000 kilograms
Typically, we only count the weight of the carbon
itself, i.e., for CH2O we neglect the weight of the
H2O. So, we write these units as Gt(C).
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Output
Atm. CO2
Photosynthesis
60 Gt(C)/yr
Input
Respiration & decay
60 Gt(C)/yr
CO2 reservoir size: 760 Gt carbon
What is the residence time?
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Output
Atm. CO2
Input
Photosynthesis
60 Gt(C)/yr
Respiration & decay
60 Gt(C)/yr
CO2 reservoir size: 760 Gt carbon
Residence time:
760 Gt(C)
60 Gt(C)/yr
=
12.7 yr
“I thought CO2 stays in atmosphere for much longer
time periods” --> Lifetime of anthro CO2
is different than residence time
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The Terrestrial Organic Carbon Cycle
Atm. CO2
Photosynthesis
Plants
Respiration
Consumers
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The Terrestrial Organic Carbon Cycle
Atm. CO2
760 Gt
Photosynthesis
Respiration
60
Plants
30
Consumers
600 Gt
∼0
Red numbers = Gt(C)/year
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The Terrestrial Organic Carbon Cycle
Atm. CO2
760 Gt
Photosynthesis
Respiration
60
Plants
30
Consumers
600 Gt
∼0
decay
death
30
30
death
∼0
Soils
1,600 Gt
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Long term Carbon Cycle
A small flux of organic carbon (0.05 Gt/yr) is buried in
sedimentary rocks, mostly on continental shelves.
Over time, this small flux has accumulated to create a HUGE
reservoir: 10,000,000 (or 1 x 107) Gton C.
Concentrations of this buried organic carbon include coal, oil
and gas--but most carbon is not concentrated.
Organic carbon in sedimentary rocks is ultimately returned as
CO2 resulting from oxidation by exposure to O2. This process
is called weathering.
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The Terrestrial Organic Carbon Cycle
Atm. CO2
760 Gt
Photosynthesis
Respiration
60
Plants
30
Consumers
600 Gt
∼0
decay
30
death
death
30
∼0
Soils and sediments
1,600 Gt
0.05
weathering
0.05
burial
Sedimentary Rocks
10,000,000 Gt
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The Inorganic Carbon Cycle
Carbon Uptake by the Oceans:
1. The biological pump
2. Air-sea gas exchange
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Atm. CO2
Air-sea exchange
~100 m
Surface Ocean
DIC
Biological pump
Deep Ocean
DIC
~4 km
DIC = Dissolved
inorganic carbon
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The Biological Pump
transfer of CO2 to the deep ocean:
Photosynthesis creates organic matter; this sinks to the
deep ocean, where it decays back to CO2
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http://www.liv.ac.uk/~ric/
The Biological Pump
transfer of CO2 to the deep ocean:
Photosynthesis creates organic matter; this sinks to the
deep ocean, where it decays back to CO2
North Atlantic
Pacific Ocean
Deep water
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The Biological Pump
transfer of CO2 to the deep ocean:
Photosynthesis creates organic matter; this sinks to the
deep ocean, where it decays back to CO2
North Atlantic
photosynthesis
Pacific Ocean
Transfer of carbon
Deep water
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The Biological Pump
transfer of CO2 to the deep ocean:
Deep water becomes enriched in CO2
The carbon is recycled to the surface ca. 1,000 years
photosynthesis
North Atlantic
Pacific Ocean
Transfer of carbon
Deep water
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Photosynthesis
CO2 + H2O → CH2O + O2
surface
water
sinking particles
Respiration
CH2O + O2 → CO2 + H2O
deep
water
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surface
water
Photosynthesis
CO2 + H2O → CH2O + O2
sinking particles
Respiration
CH2O + O2 → CO2 + H2O
deep
water
This pumps up the CO2 partial pressure of deep water…
Atm. CO2
pCO2 = 370 ppmv
Surface Ocean
pCO2 = 370 ppmv
DIC
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Biological pump
Deep Ocean
DIC
pCO2 ≅ 1000 ppmv
• Deep water has a
higher CO2 partial
pressure than does
surface water
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Atm. CO2
Air-sea exchange
60 Gt(C)/yr
Ocean
Dissolved inorganic carbon
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Carbon cycle: reservoirs and couplings
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The Long-term Inorganic Carbon Cycle:
0.03
GtonC/yr
CaSiO3 + CO2
CaCO3 + SiO2
0.03
GtonC/yr
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What controls silicate weathering rates?
•Time
•Temperature
•Rainfall
•Exposure of fresh rock surfaces
•Vegetation (roots provide acid)
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Weathering Feedback Loop:
Complete the feedback…
Atm. CO2
Silicate
weathering
Rates
Surface
Temperature
As temperatures go up, weathering rates increase
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Weathering Feedback Loop:
Positive or negative? Stabililizing?…
Atm. CO2
Silicate
weathering
Rates
Surface
Temperature
Weathering reactions remove CO2, and as CO2
declines, planet temperatures go down
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Weathering Feedback Loop:
Atm. CO2
Silicate
weathering
Rates
Surface
Temperature
This is a negative feedback loop, or a stable system. This
loop is a key control on climate over long time scales (i.e.,
millions of years).
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What is the RT of CO2 in sedimentary rock?
Reservoir is 40,000,000 Gt C
Sedimentation and burial rate is 0.2 Gt C/yr
RT = 40,000,000 Gt C/0.2 Gt C/yr
= 200 million yrs
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The Inorganic
Carbon Cycle
Atm. CO2
Fast
Air-sea
exchange
Med.
Slow
Marine
sediments
Sedimentary
rocks
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The Inorganic
Carbon Cycle
Atm. CO2
1-10s-100s years
Fast
Air-sea
exchange
Med.
Slow
Marine
sediments
Sedimentary
rocks
100s-10,000s years
Geologic - millions
years
Problem:
Acceleration of this
part of cycle by
large factor
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Note the imbalance in the net fluxes!
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Summary
Reservoirs with residence times
Organic/Inorganic cycles
Timescales from annual cycles to geologic
Changes in sources/sinks leads to changes
in atmospheric reservoir
Changes in reservoirs changes balance ie Ocn <--> Atmos
Feedbacks in other parts of the cycle – ie
Carbon uptake in ocn -> more alkaline
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