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

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

Volcanic degassing Volcano Hydrothermal vents Very approximate carbon flux ~ 0.04 GTC/year Small carbon source relative to human emission, air-sea CO 2 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 degassing 0.04 GTC/year

Weathering Physical Weathering = mechanical breakdown of rocks Erosion Formation of sediments Chemical Weathering = chemical breakdown Salinity Some nutrients (silicate, phosphate) Alkalinity (Ca 2+ )

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

Seafloor sediments Plankton origin marine snow Land 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 CaCO 3 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 [CO 3 2- ]. Once [CO 3 2- ] goes down below the thermodynamic threshold [CO 3 2- ] sat, calcite is undersaturated and dissolves in the water

Carbonate weathering cycle Carbonate weathering CaCO 3 (land)  Ca 2+ + CO 3 2- (river input to the ocean)  Formation of marine snow  CaCO 3 (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 CaSiO 3 (land)+CO 2 (air)  SiO 2 + Ca 2+ + CO 3 2- (river input)  SiO 2 (sediment) + CaCO 3 (sediment) Silicate weathering absorbs CO 2 from the atmosphere, and bury it into the sediment  Net removal of CO 2

Biogenic silica on the seafloor sediments Chapter 5, Fig 15

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

Silicate weathering and CO 2 Volcanism  CO 2 to the atmosphere Chemical breakdown of silicate rock  CO 2 into the ocean Burial of CaCO 3  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-CO 2 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 CO 2 flux Climate warms up due to increased atmospheric CO 2 Weathering-CO 2 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 CO 2 feedback? Ice core pCO 2 for the last 800,000 years Very little long term trend

Modulation of weathering CO 2 feedback Volcanic CO 2 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 CO 2 Removal of reduced carbon  Source of atmospheric O 2 Origin of fossil fuel

Photosynthesis and respiration Simplified representation of photosynthesis Most of the CH 2 O will return to CO 2 via aerobic respiration Energy source for living organisms Small fraction of CH 2 O is buried on land and in the ocean sediments Increases atmospheric O 2

Long-term controls on atmospheric O 2 Great O 2 event = 2.5 billion years ago Early atmosphere had no oxygen. Oxygenation of the planet by biological O 2 production O 2 supports more complex, multi-cellular life Burial of organic matter balances organic C weathering

Organic Carbon-O 2 feedback Hypothesis: Burial of organic carbon depends on the oxygen content of the deep ocean If atmospheric O 2 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 O 2 This feedback can potentially stabilize the atmospheric oxygen No quantitative model/theory yet

CaCO 3 – pH feedback Why ocean pH is about 8?  Carbonate chemistry DIC and alkalinity of seawater set pH of the seawater [CO 3 2- ] (≈ Alk – DIC) increases with pH CaCO 3 -pH feedback If the ocean pH gets low, more CaCO 3 dissolves at the seafloor. Dissolution of CaCO 3 increases Alk relative to DIC pH increases

Fate of fossil fuel CO 2 CO 2 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 CaCO 3 Increases alkalinity Absorbs remaining 15%

Theme III: long-term carbon-climate relation Three stabilizing mechanisms for temperature, CO 2, alkalinity and pH of the seawater Operates over plate tectonic timescales, providing stability to the climate and biogeochemical cycles Weathering-CO 2 feedback Silicate weathering Organic Carbon-O 2 feedback Preservation of organic matter on the seafloor CaCO 3 -pH feedback Preservation of CaCO 3 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.