GEOLOGIC CARBON CYCLE• Textbook chapter 5, 6 & 14• Global carbon cycle• Long-term stability and feedback

Geological carbon cycle

Williams and Follows (2011)

Volcanic degassingWeathering ofrocks

Sediment burial

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+

Residence time• (Residence time) = (Inventory) / (Flux)

Ocean-atmosphere system~ 40,000 GTC

Volcanic degassing0.04 GTC/year

Weathering• Physical Weathering = mechanical breakdown of rocks

• Erosion• Formation of sediments

• Chemical Weathering = chemical breakdown• Salinity• Some nutrients (silicate, phosphate)• Alkalinity (Ca2+)

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

Seafloor sediments

Plankton originmarine snowLand origin

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

Thermodynamic stability of CaCO3

• Solubility product Ksp

• Ksp increases with pressure• Supersaturation above the calcite

saturation horizon

• Undersaturation below the

saturation horizon

Sarmiento and Gruber (2006)

Distribution of calcite on the seafloor

Chapter 5, Fig 17

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

Carbonate weathering cycle• Carbonate weathering

• CaCO3 (land) Ca2+ + CO32- (river input to the ocean)

Formation of marine snow CaCO3 (sediment)

• In a steady state, no net addition of alkalinity or DIC to the ocean-atmosphere system

Carbonate deposits• The sink becomes the source

CaCO3 deposit from coccolith-rich sedimentary rock

Silicate weathering cycle• Silicate weathering

• 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

Biogenic silica on the seafloor sediments

Chapter 5, Fig 15

Silica distribution in the surface ocean

Sarmiento and Gruber (2006)

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

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

0°C and 100°C

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

Evidence for the weathering CO2 feedback?

• Ice core pCO2 for the last 800,000 years

• Very little long term trend

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

Burial of organic carbon

• Sink of atmospheric CO2

• Removal of reduced carbon Source of atmospheric O2

• Origin of fossil fuel

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

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

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

CaCO3 – pH feedback

• Why ocean pH is about 8?

Carbonate chemistry• DIC and alkalinity of seawater set pH of the seawater• [CO3

2-] (≈ 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

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%

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

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.