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The C-cycle

Carbon Reservoir Dynamics atmospheric reservoir. Seasonal fluctuation in atmospheric CO 2 from Mauna Loa. The C-cycle. Definitions. Nutrients —substances essential to life Biosphere —the part of the Earth that supports life, including the oceans, atmosphere, land surface and soils

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The C-cycle

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  1. Carbon Reservoir Dynamicsatmospheric reservoir Seasonal fluctuation in atmospheric CO2 from Mauna Loa The C-cycle Definitions • Nutrients—substances essential to life • Biosphere—the part of the Earth that supports life, including the oceans, atmosphere, land surface and soils • Organic carbon—associated with compounds of biologic systems (C-C and C-H bonds) • Inorganic carbon—associated with compounds of inorganic systems • a re-cycling system • Biosphere subcycle—terrestrial and marine realms, inorganic and organic pathways, fast cycling • Lithosphere subcycle—long term storage and tectonic re-cycling, slow cycling Elements of the C-cycle volcanism re-cycles C One atom of C cycles for about 30 Ky before leaking into sedimentary storage weathering re-cycles C uplift metamorphism subduction • Reservoirs are temporary repositories for mass that flows through them, their sizes depending on imbalances between inflow and outflow • Steady-state—no change in state of the system with time Decay in soils (oxidation) nMy Carbon Reservoir Dynamics • Carbon reservoirs • How does the size of each carbon reservoir respond to perturbations

  2. Carbon Reservoir Dynamicsresidence time of C in the atmosphere Carbon Reservoir Dynamicsperturbation of the atmospheric reservoir 760 Gton carbon / 60 Gton carbon/yr = 12.7 yr Organic C-cycle (short term) • Photosynthesis CO2 + H2O = CH2O + O2 • Primary producers • Consumers • Biomass = primary producers + consumers • Consumers are 1% of biomass CO2 fertilization negative feedback loop  atm CO2 — photosynthetic rate (reduced organic carbon)— atm CO2 Residence time • The average time that an element remains in a reservoir at steady-state • The time required to fill a reservoir to the steady-state concentration Primary producers in the seaphytoplankton (upper 100m in photic zone) Diatom with sliceous test (50mm diameter) Coccolithoforid with calcite test (10mm diameter)

  3. Nutrient limitation The biological pump Planktonic consumers in the seazooplankton Foraminifera with calcite test (600mm diameter) Radiolarian with siliceous test (50mm diameter) The biological pump • Primary production of organic C in surface waters — oxidation of organic C in deeper waters • Settling organic particles by-pass fluid advection • Balanced by upwelling Nutrient and oxygen distribution in the abyss Nutrient limitation Concentration of some elements in seawater limit productivity (P, Si, Fe)

  4. Ocean productivity measured by satellite (chlorophyll) Organic C-cycle (long term) • Geological processes control atmospheric CO2 on longer time scales • 0.1% of marine productivity leaks into long term geologic storage • This leak controls the O2 content of the atmosphere CO2 + H2O =  CH2O +  O2 • organic C in sedimentary rocks is the largest reservoir on earth (108 Gtons) • Residence time is 200 Ma Summer Thermohaline circulation Nutrient limitation Vertical distribution of Fe Winter Upwelling and nutrient recycling

  5. Seawater-atmosphere exchange of CO2 Inorganic C-cycle • Describes sources and sinks of carbon other than respiration decomposition and weathering • CO2 reacts with water CO2 (g)+ H2O(l) = H2CO3(aq) H2CO3(aq) = H+ + HCO3– HCO3–= H+ + CO3–2 • Limestone (CaCO3) and dolostone (CaMg(CO3)2 are long term sedimentary archives of inorganic carbon Ca+2 + 2HCO3(aq)– = CaCO3 (s)+ H2CO3(aq) Carbonate deposition in the oceans Chemical weathering of Ca-bearing minerals • Carbonate weathering CaCO3 (s)+ H2CO3(aq) = Ca+2 + 2HCO3(aq)– • Silicate weathering CaSiO3 (s)+ 2H2CO3(aq) = Ca+2 + 2HCO3(aq)– +SiO2 (aq) +H2O • Carbonate weathering on land consumes 1 mole of CO2 per mole of Ca released—the net effect on CO2(atm) is zero • Silicate weathering on land consumes twice the amount of CO2 as Ca—the net effect on CO2(atm) is a reduction of 1 mole equivalent

  6. Net result of silicate weathering CaSiO3 (s)+ 2H2CO3(aq) = Ca+2 + 2HCO3(aq)– +SiO2 (aq) +H2O Ca+2 + 2HCO3(aq)– = CaCO3 (s)+ H2CO3(aq) CO2 (g)+ H2O(l) = H2CO3(aq) Carbonate-silicate cyclelong term climate stability CaSiO3 (s)+ CO2 (g) = CaCO3 (s) + SiO2 (aq) Carbonate metamorphism (reverse of silicate weathering) CaCO3 (s)+ SiO2 (s) = CaSiO3 (s)(wollastonite) + CO2 (g) Net result of silicate weathering CaSiO3 (s)+ 2H2CO3(aq) = Ca+2 + 2HCO3(aq)– +SiO2 (aq) +H2O Ca+2 + 2HCO3(aq)– = CaCO3 (s)+ H2CO3(aq) CO2 (g)+ H2O(l) = H2CO3(aq) CaSiO3 (s)+ CO2 (g) = CaCO3 (s) + SiO2 (aq)

  7. Carbonate-silicate cyclelong term feedbacks ensure stability of the Earth’s climate system C-leak Limestone deposition Carbonate-silicate cyclelong term feedbacks ensure stability of the Earth’s climate system C-leak volcanism limestone metamorphism

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