1. Geologic carbon cycle Textbook chapter 5, 6 & 14 Global carbon cycle
Long-term stability and feedback

2. Geological carbon cycle
Weathering of
rocks
Volcanic degassing
Sediment burial
Williams and Follows (2011)

3. Volcanic degassing Volcano Hydrothermal vents
Very approximate carbon flux ~ 0.04 GTC/year
Small carbon source relative to human emission, air-sea CO2 exchange, biological productivity
BUT it is dominant over long timescales ~ millions of years+

4. Volcanic degassing Equation for Ocean/Atmosphere Carbon Inventory
Steady State
Timescale
Residence Time

5. Residence time (Residence time) = (Inventory) / (Flux)
Volcanic degassing
0.04 GTC/year
Ocean-atmosphere system
~ 40,000 GTC

6. Weathering Physical Weathering = mechanical breakdown of rocks
Erosion
Formation of sediments
Chemical Weathering = chemical breakdown
Salinity
Some nutrients (silicate, phosphate)
Alkalinity (Ca2+)
Carbonate Rocks
1. Carbon dioxide is removed from the atmosphere by dissolving in water and forming carbonic acid
CO2 + H2O -> H2CO3 (carbonic acid)
2. Carbonic acid is used to weather rocks, yielding bicarbonate ions, other ions, and clays
H2CO3 + H2O + silicate minerals -> HCO3- + cations (Ca++, Fe++, Na+, etc.) + clays
3. Calcium carbonate is precipitated from calcium and bicarbonate ions in seawater by marine organisms like coral
Ca++ + 2HCO3- -> CaCO3 + CO2 + H2O the carbon is now stored on the seafloor in layers of limestone

7. Weathering of Carbonate Rocks
1. Carbon dioxide is removed from the atmosphere by dissolving in water and forming carbonic acid
CO2 + H2O -> H2CO3 (carbonic acid)
2. Carbonic acid is used to weather rocks (e.g. rain), yielding bicarbonate ions, other ions, and clays, which are dumped into ocean (e.g. river runoff)
H2CO3 + H2O + silicate minerals -> HCO3- + cations (Ca++, Fe++, Na+, etc.) + clays
3. Calcium carbonate is precipitated from calcium and bicarbonate ions in seawater by marine organisms like coral, coccolithophores, foraminifera
Ca++ + 2HCO3- -> CaCO3 + CO2 + H2O (form both calcite and aragonite)
the carbon is now stored on the seafloor in layers of limestone

8. Formation of sediments
Erosion and sediment transport
Grain size scales and energy conditions

9. Seafloor sediments
Plankton origin
marine snow
Land origin

10. CCD = Calcite Compensation Depth
Hard shell component of the marine snow
Solubility of calcite depends on the pressure
Calcite tends to dissolve in the deep ocean
Above CCD, calcite is preserved in the sediment
Below CCD, calcite is dissolved and not preserved in the sediment

11. Thermodynamic stability of CaCO3
Solubility product Ksp
Ksp increases with pressure
Supersaturation above the calcite
saturation horizon
Undersaturation below the
Sarmiento and Gruber (2006)

12. Distribution of calcite on the seafloor
Chapter 5, Fig 17

13. Stability of calcite and pH
Combination of DIC and Alk controls the acidity of seawater.
Increasing DIC increases acidity and lowers [CO32-].
Once [CO32-] goes down below the thermodynamic threshold [CO32-]sat, calcite is undersaturated and dissolves in the water

14. Carbonate weathering cycle
CaCO3 (land)  Ca2+ + CO32- (river input to the ocean)  Formation of marine snow  CaCO3 (sediment)
In a steady state (geological), no net addition of alkalinity or DIC to the ocean-atmosphere system

15. Carbonate deposits The sink becomes the source
CaCO3 deposit from coccolith-rich sedimentary rock

16. Silicate weathering cycle
CaSiO3(land)+CO2(air) SiO2 + Ca2+ + CO32- (river input)
 SiO2(sediment) + CaCO3(sediment)
Silicate weathering absorbs CO2 from the atmosphere, and bury it into the sediment  Net removal of CO2

17. Biogenic silica on the seafloor sediments
Chapter 5, Fig 15

18. Silica distribution in the surface ocean
Sarmiento and Gruber (2006)

19. Silicate weathering and CO2
Volcanism  CO2 to the atmosphere
Chemical breakdown of silicate rock  CO2 into the ocean
Burial of CaCO3  Plate tectonics  Subduction zone

20. Faint young sun paradox
Sagan and Mullen (1972): In the early Earth, the solar energy input was only about 70% of today but the climate was as warm as today.
Long-term stability of the Earth’s climate system
Temperature remained within °C and 100°C

21. Weathering-CO2 feedback
Hypothesis: The speed of rocks’ chemical breakdown partly depends on the temperature.
Cold climate tends to slow down chemical weathering
Slow-down of silicate weathering cannot balance volcanic CO2 flux
Climate warms up due to increased atmospheric CO2
Weathering-CO2 feedback tends to stabilize the climate temperature over millions of years
Is this sufficient to explain the early Earth’s warmth? Rosing et al., (2010) Nature: ongoing debate

22. Evidence for the weathering CO2 feedback?
Ice core pCO2 for the last 800,000 years
Very little long term trend

23. Modulation of weathering CO2 feedback
Volcanic CO2 input
The rate of plate subduction
Calcite composition of subducting seafloor
Weathering of silicate rock
Mountain building
Continent distribution
Sea level
Vegetation on land

24. Burial of organic carbon
Sink of atmospheric CO2
Removal of reduced carbon  Source of atmospheric O2
Origin of fossil fuel

25. Photosynthesis and respiration
Simplified representation of photosynthesis
Most of the CH2O will return to CO2 via aerobic respiration
Energy source for living organisms
Small fraction of CH2O is buried on land and in the ocean sediments
Increases atmospheric O2

26. Long-term controls on atmospheric O2
Great O2 event = 2.5 billion years ago
Early atmosphere had no oxygen.
Oxygenation of the planet by biological O2 production
O2 supports more complex, multi-cellular life
Burial of organic matter balances organic C weathering

27. Organic Carbon-O2 feedback
Hypothesis: Burial of organic carbon depends on the oxygen content of the deep ocean
If atmospheric O2 gets low, deep water goes anoxic
Aerobic respiration stops and the respiration of organic matter decreases
More organic matter are preserved in the sediment
Increases atmospheric O2
This feedback can potentially stabilize the atmospheric oxygen
No quantitative model/theory yet

28. CaCO3 – pH feedback Why ocean pH is about 8?  Carbonate chemistry
DIC and alkalinity of seawater set pH of the seawater
[CO32-] (≈ Alk – DIC) increases with pH
CaCO3-pH feedback
If the ocean pH gets low, more CaCO3 dissolves at the seafloor.
Dissolution of CaCO3 increases Alk relative to DIC
pH increases

29. Fate of fossil fuel CO2
CO2 emission into the atmosphere by human activities (decades)
Partial absorption into the land and upper ocean (decades)
O(100-1,000 years)
Equilibration of deep ocean carbon reservoir
Absorbs 85% of carbon emission
O(10,000+ years)
Dissolution of seafloor CaCO3
Increases alkalinity
Absorbs remaining 15%

30. Theme III: long-term carbon-climate relation
Three stabilizing mechanisms for temperature, CO2, alkalinity and pH of the seawater
Operates over plate tectonic timescales, providing stability to the climate and biogeochemical cycles
Weathering-CO2 feedback
Silicate weathering
Organic Carbon-O2 feedback
Preservation of organic matter on the seafloor
CaCO3-pH feedback
Preservation of CaCO3 on the seafloor

31. Changing mood of carbon cycle
O(10-1k years) Ocean carbon cycle acts as a sink of carbon and heat, moderating the climate change
O(100k years) Ocean carbon cycle seems to act as an amplifier of the glacial-interglacial climate change
O(1 million years) Ocean carbon cycle seems to stabilize the climate and cycling of elements through the three feedbacks…
Further reading: D. Archer (2010) “The Global Carbon Cycle”, Princeton University Press.