Biogeochemical Cycles: What controls CO2 and O2 levels in the atmosphere?
1. What controls CO2 in the atmosphere?
2. What controls O2 in the atmosphere?
These questions are linked (see Keeling plots)
Why is O2 changing seasonally?
Why is it out of phase between N and S hemispheres?
Why is it slowly decreasing?
We're seeing the seasonal shift between photosynthesis and aerobic respiration.
To understand survival of either gas at Earth's surface, must examine the cycling of C
around the surface and between the crust and surface. Must consider four concepts
1. Reservoirs: temporary repositories for material that flows through them.
At steady-state for mass, a reservoir is neither growing nor decreasing.
Either it has no inflows or outflows, or those flows are balanced.
Reservoir size is indicated either by mass units or in moles.
2. Residence Time: the average length of time a substance spends in a given reservoir.
Residence Time = Reservoir Size at Steady State/Inflow or Outflow Rate
Residence time gives an indication of how fast a reservoir will respond to perturbations in its
inflow or outflow.
The atmosphere contains 760 Gt C as CO2. Every year it exchanges ~60 Gt C with the
ocean, and 60 Gt with the land biosphere via photosynthesis-respiration. The residence
time of C in the atmosphere is ~6.3 years. If the activity of the terrestrial biosphere
dropped, it would take ~6 years for atmospheric CO2 to reach a new steady state.
Gt = Gigaton = 1 billion (109) metric tons. A metric ton = 1000 kg, so a Gt = 1015 gm
3. Feedbacks – linkage of two or more system components to form a round-trip flow of
information. Some maintain balanced flow (i.e., steady state) (negative feedback). Others
lead to run away acceleration or deceleration of flow from system (positive feedback).
Negative feedback example: thermostats, photosynthesis-pCO2
Positive feedback example: CO2 solubility-temperature
4. Reduced vs. Oxidized Carbon
Oxidized substances are electron-poor; reduced substances are electron-rich.
Organic Carbon, which is rich in C-C, C-H, and C-N bonds, is reduced.
Inorganic Carbon, which is rich in C-O bonds, is oxidized.
Knowing these concepts, we can understand what it takes to build O2 in the atmosphere.
Once O2 is created, its survival is controlled by reaction with reduced organic C.
Organic C must build up in a reservoir with a long residence time if O 2 is to accumulate.
Large C reservoirs at the Earth's surface are:
1. Dissolved marine CO2 & HCO3-: 37,740 GtC (oxidized)
600 GtC in the surface ocean, remainder in the deep ocean
2. Marine carbonate sediments: 2500 GtC (oxidized
3. Organic C in marine and terrestrial sediments: 1600 GtC (reduced)
4. Atmospheric CO2: 760 GtC
5. Primary producers (marine and land plants): 600 GtC (reduced)
6. Atmospheric methane: 10 GtC (reduced)
Large C reservoirs in the crust are:
1. Carbonate sedimentary rocks: 40x106 GtC (oxidized)
2. Organic C in sedimentary rocks: 10x106 GtC (reduced)
Main processes affecting organic C reservoirs are:
Short-term processes: i.e., those that involve surface reservoirs w/short residence times.
1. Photosynthesis: CO2 & H2O links oxidized atmosphere to primary producers
2. Death of Primary Producers: Deposition in shallow sediments or eaten by consumers.
a. Consumption leads to respiration, which converts reduced C back to CO2.
b. Deposition of organic matter in sediments. Nearly all of this is immediately decomposed
by microbes and from organic C back to CO2.
Long-term processes: i.e., those that involve large reservoirs w/long residence times.
3. Burial of Organic C: A small amount of organic C (0.05 GtC/yr) survives short-term cycling
to be buried in sedimentary rocks.
4. Weathering of Sedimentary Rocks: Long-term steady state is maintained by the oxidative
weathering of a small amount of sedimentary organic C every year.
BOTTOM LINE FOR OXYGEN: O2 builds up in the atmosphere because of this leak of organic
C (and other reduced substances) from the rapidly cycling Earth surface reservoirs to the
slowly reacting crustal reservoir. If we lost this flux, or if sedimentary rock organic C was
oxidized faster than organic C was buried, the Earth's atmosphere would quickly be stripped of
O2. Steady state is maintained by this balance.
What feedbacks link organic C burial and weathering to maintain balance?
More efficient burial of organic matter at times of low O 2?
More efficient weathering/combustion of organic matter at time of high O 2?
Less efficient photosynthesis at times of high O 2?
What might knock these feedbacks out of whack?
Initial increase in O2 required that these feedbacks were not complete – greater rate of
burial than weathering in the late Archean/early Proterozoic.
Dramatic increase in sedimentation rate, sequesters organic C, increases O 2.
New compound classes that are hard to degrade, increases org C burial, increases O2.
Example: Coal forest of Carboniferous. Nothing could eat them. Giant insects.
Burial of organic carbon also removes carbon from the Earth's surface, which has some
effect on the amount of CO2 in the atmosphere. Atmospheric CO2 are also influenced by
the inorganic cycling of carbon.
Short-term processes:
1. CO2 exchange between surface ocean and atmosphere
2. Dissolution of CO2 in the ocean and carbonate equilibria
CO2 + H2O  2CO3
H2CO3  H + + HCO3-  2H+ + CO323. Biogenic precipitation of calcium carbonate
Ca2+ + 2HCO3-  CaCO3 + CO2 + H2O
4. Deposition of carbonate in marine sediments
5. Dissolution of carbonate from marine sediments
Bottom Line: Little inorganic C survives trip to ocean floor and corrosive bottom waters.
Only 0.2 GtC/yr is converted into CO3 sedimentary rock.
Long-term processes:
Carbonate-Silicate Geochemical Cycle
1. Carbonate weathering
CaCO3 + CO2 + H2O Ca2+ + 2HCO32. Silicate weathering
CaSiO3 + H2CO3 + CO2 Ca2+ + 2HCO3- + SiO2
3. Metamorphism & Volcanism
CaCO3 + SiO2  CaSiO3 + CO2
Bottom line: Silicate weathering, followed by marine carbonate precipitation, consumes
CO2 at Earth's surface. Carbonate weathering, followed by marine carbonate
precipitation, has no net effect on CO2 levels.
On the long-term, weathering of silicates balances volcanic input of CO2. Steady state is
maintained by this balance.
What negative feedbacks link the rate of silicate weathering with the amount of
volcanism, via the CO2 concentration of the atmosphere?
Why is silicate weathering sensitive to the amount of atmospheric CO2?
Increased CO2, stronger greenhouse, increased T, faster reactions.
Increased CO2, stronger greenhouse, increased rainfall, faster reactions.
Increased CO2, more acidic rainwater, faster reactions.