Earth Systems Science Chapter 8. Earth Systems Science Chapter 8 11/17/2010 THE CARBON CYCLE

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1 Earth Systems Science Chapter 8 THE CARBON CYCLE The circulations of the osphere, hydrosphere, and lithosphere were studied in previous chapters. Here, we learn how nutrients are recycled in the earth system. We focus on carbon in particular due to its importance for biological activity and for global climate. Nutrients: substances normally in the diet that are essential to organisms. Earth Systems Science Chapter 8 THE CARBON CYCLE 1. carbon cycle: dynamics 2. The short term terrestrial organic carbon cycle. The short term marine organic carbon cycle 4. The long term organic carbon cycle 5. The short term inorganic carbon cycle; interaction with the biological pump 6. The long term inorganic carbon cycle: the carbonatesilicate geochemical cycle Reservoirs gtprate Locations, or types of regions, gross terr prod ocn2 beta switch where the substance you are tracking is stored. o2arate Value of reservoir depends on the net flux living terrestrial bio respiration rrate terr decay a2orate tdrate 2ocn goprate surf ocean weathering oc wocrate gross ocn prod living marine bio lrate dead terrestrial bio litterfall weathering cs revelle switch wcsrate downwell ocn decay urate odrate upwell drate noprate organic sed net ocn prod osrate deep ocean inorg sed organic c sediments carbonate sediments israte STELLA diagram of global C cycle used in our lab, adapted Chameides and Perdue (1997) 1

2 The osphere A variety of processes are related to flux into and out of the osphere. These may vary seasonally, resulting in a seasonal cycle in ospheric carbon concentration. Steady state: same as dynamic equilibrium Residence time, or response time, or efolding time Average amount of time that a substance (e.g. atom of C) remains in a reservoir under steady state conditions Residence time = T = (reservoir size) / outflow rate or (reservoir size) / inflow rate T() = 760 (GtC) / 60 (GtC/yr) = 12.7 yr T = time in which a perturbed system will return to 1/e, or ~8%, of original value rate = 1/T = 1/12.7 (1/yr) = (1/yr) = yr 1 rate photosynthesis Residence time T is calculated at equilibrium using total inflow or total outflow T = (reservoir size) / (total outflow) = (reservoir size) / (total inflow) = (reservoir size) / (flux_out_1 + flux_out_2) = (reservoir size) / (flux_in_1 + flux_in_2) r in 1 r out 1 Rate constant r is calculated using the individual flow r_in_1 = flux_in_1 / reservoir r_in_2 = flux_in_2 / reservoir r_ out_ 1 = flux_ out_ 1 / reservoir r_out_2 = flux_out_2 / reservoir r in 1 r out 1 flux in 1 flux out 1 stock flux in 1 flux out 1 stock flux in 2 flux out 2 flux in 2 flux out 2 r in 2 r out 2 r in 2 r out 2 2

3 THE CARBON CYCLE: preindustrial equ. stocks and flows living terrestrial bio gross terr prod respiration 50 terr decay ocn2 80 2ocn 80 surf ocean weathering oc gross ocn prod 1E 1.8 living marine bio THE CARBON CYCLE: mean residence times (years) living terrestrial bio gross terr prod ocn2 weathering oc 4 respiration 2ocn surf ocean gross ocn prod terr decay living marine bio litterfall weathering cs ocn decay litterfall weathering cs ocn decay dead terrestrial bio 1450 downwell upwell dead terrestrial bio 0 downwell upwell organic sed organic sed 4E4 deep ocean 4 net ocn prod 1E deep ocean net ocn prod 7E7 carbonate sediments inorg sed 0.2 2E7 organic c sediments 1E8 carbonate sediments inorg sed 1E8 organic c sediments Oxidized C that is combined with oxygen examples: CO 2, CaCO Reduced C that is not combined with oxygen, usually combined with other carbon atoms (CC), hydrogen (C H), or nitrogen (CN) example: organic carbon in carbohydrates reduced substances tend to be unstable in the presence of oxygen: organic matter decomposes, metals rust THE SHORTTERM TERRESTRIAL ORGANIC CARBON CYCLE Organic carbon: associated with living organisms; contains CC or CH bonds Photosynthesis: C is removed from the osphere and incorporated into carbohydrate molecule; becomes organic. Primary productivity: amount of organic matter produced by photosynthesis (per year, per area) Primary producers (producers, autotrophs): organisms that store solar energy in chemical bonds (carbohydrates) for other organisms to consume Respiration: C is returned to the osphere; becomes inorganic Net primary productivity (NPP): primary productivity respiration Image Name: North America NDVI Image Date: March 1990November 1990 Image Source: AVHRR Mosaic

4 THE SHORTTERM TERRESTRIAL ORGANIC CARBON CYCLE Photosynthesis: CO 2 + H 2 0 CH (solar energy) Respiration: CO 2 + H 2 0 CH (release energy) Consumers (heterotrophs): organisms that can not use solar energy directly, get their energy by consuming primary producers Image Name: Global Greenness Image Date: June 1992 Image Source: AVHRR NDVI THE SHORTTERM TERRESTRIAL ORGANIC CARBON CYCLE On land, Net Primary Productivity = 0.5 Primary Productivity it Steady state: flux in = flux out THE SHORTTERM TERRESTRIAL ORGANIC CARBON CYCLE THE SHORTTERM TERRESTRIAL ORGANIC CARBON CYCLE Where is the osphere in this model? exogenous to this model root ph roots npp leaf ph branch npp frac stem npp frac leaf npp frac branch ph stem ph leave branches stems branch fall b f rate litter stem fall l f rate s f rate root npp frac leaf fall litter resp litter humification litt dec rate hum factor aerobic: biological process that uses oxygen for metabolism aerobe: an aerobic organism; organism whose metabolism is aerobic metabolism: The chemical processes occurring within a living cell or organism that are necessary for the maintenance of life. In metabolism some substances are broken down to yield energy for vital processes while other substances, necessary for life, are synthesized. (dictionary.com) root humification humus resp humus hum dec rate carbonization root dec rate root resp carb factor charcoal hum factor charcoal oxidation STELLA diagram of terrestrial forest C cycle (adapted from Huggett, 199) 4

5 THE SHORTTERM TERRESTRIAL ORGANIC CARBON CYCLE anaerobic: biological process whose metabolism uses no oxygen anaerobe: an anaerobic organism; organism whose metabolism is anaerobic Methanogenesis: an anaerobic form of metabolism Photosynthesis: CO 2 + H 2 0 CH (solar energy) Respiration: CO 2 + H 2 0 CH (release energy) Methanogenesis: CO 2 + CH 4 2CH 2 0 (release energy) THE SHORTTERM MARINE ORGANIC CARBON CYCLE Plankton: organisms floating in water photic zone: ~mixed layer, upper 100m Diatom (SiO 2, ~50 µm) coccolithophorid (CaCO, ~10 µm) THE SHORTTERM MARINE ORGANIC CARBON CYCLE Plankton: organisms floating in water foraminifer (CaCO, ~600 µm) THE SHORTTERM MARINE ORGANIC CARBON CYCLE The Biological Pump radiolarian (SiO 2, ~50 µm) Thermohaline Circulation photic zone: ~mixed layer, upper 100m 5

6 THE SHORTTERM MARINE ORGANIC CARBON CYCLE The Biological Pump Nutrient Limitation Organisms (i.e. plankton) require a variety of nutrients to grow. These nutrients are obtained from the ambient water. Nutrients are required in certain ratios: Redfield Ratios THE SHORTTERM MARINE ORGANIC CARBON CYCLE SEAWIFS Mean Chlorophyl September 97 August 2000 Center of gyres downwelling few sources of nutrients little biological activity Typically, the organism stops multiplying when one of the required nutrients is depleted. The depleted nutrient is called the limiting nutrient. If more of the nutrient were present, there would be additional growth. Areas with nutrient input from rivers or from upwelling more biological activity High latitudes generally more productive than low latitudes THE LONGTERM ORGANIC CARBON CYCLE On long time scales the processes that are part of the short term cycle are approximately in equilibrium. However, the slower processes associated with geological processes become important. Reservoir value flux T (GtC) (GtC/y) (y) osphere soil/sed sed. rock 1e e08 THE LONGTERM ORGANIC CARBON CYCLE Terrestrial as well as marine organic sediments fill the ocean basins, get buried and lithify, remain in sedimentary rocks until uplift and weathering, or subduction. This is sometimes referred to as a leak from the short term organic C cycle because removal of CO 2 leaves one oxygen molecule (O 2 ) in the osphere: CO 2 + H 2 0 CH

7 THE LONGTERM ORGANIC CARBON CYCLE Fossil fuels are formed from the organic carbon in sedimentary rocks. How does the burning of fossil fuels affect this system diagram? Short circuit the flux from sedimentary rocks to the osphere How does the deforestation affect this system diagram? What about reforestation? Sources and sinks of ospheric carbon that do not depend directly on biological activity exist. source: a reservoir from which h the osphere gains carbon sink: a reservoir to which the osphere loses carbon inorganic: not directly related to biological activity Important reservoirs of inorganic carbon: the osphere, the ocean, sedimentary rocks Sedimentary rock carbon reservoirs consist mostly of: limestone: CaCO dolomite: CaMg(CO ) 2 (older sedimentary rocks) (CO 2 ) g (CO 2 ) g (CO 2 ) aq H 2 CO HCO CO 2 (CO 2 ) aq H 2 CO HCO CO 2 mixed layer r g gaseous phase flux g to aq aqueous phase mixed layer r A Chemical A flux A to BC Chemicals B and C flux aq to g flux BC to A rates of diffusion r aq r BC rates of chemical reactions 7

8 Atmosphere Ocean Carbon Exchange CO2 diffuses between the osphere and the ocean Diffusion: i the free or random movement of a substance from a region in which it is highly concentrated into one in which it is less concentrated. In gases and liquids, it happens spontaneously at the molecular level, and continues until the concentration becomes uniform (Kemp, The Environment Dictionary) CO2 dissolves in water dissolve: when two substances go into solution solution: a homogeneous mixture formed when substances in different states are combined together, and the mixture takes on the state of one of the components (Kemp, The Environment Dictionary) Atmosphere Ocean Carbon Exchange CO2 diffuses between the osphere and the ocean The direction and magnitude of diffusion i depends d on the partial pressure of CO2 in the osphere, the amount of CO2 in solution, the solubility of CO2 in water, and on the rate constant of the diffusion process partial pressure: pressure of one particular gas in the osphere solubility: the maximum amount of a substance that will dissolve in a specified liquid (similar to saturation in the osphere) rate constant: number representing speed with which diffusion occurs (CO2)g (CO2)aq where g=gas, aq=aqueous = dissolved in water Chemistry of Inorganic Carbon in Water dissolved CO2 generates carbonic acid CO 2 + H 2 O H 2 CO this reaction can go either direction, depending on the relative concentrations of reactants and products. Reaction occurs until chemical equilibrium is reached reactants: left hand side of equation products: right hand side of equation chemical equilibrium: when relative concentrations of reactants and products reach the point where no net change in concentrations occurs Chemistry of Inorganic Carbon in Water carbonic acid generates hydrogen ions, bicarbonate ions, carbonate ions H 2 CO H + + HCO (bicarbonate ion) HCO H + + CO 2 (carbonate ion) H + concentration determines the ph of water ph = log[h + ] where [H + ] is the concentration of hydrogen ions. These reactions tend towards chemical equilibrium, depending on the concentrations of bicarbonate and carbonate, the concentration of the H + ion (ph), and the temperature. 8

9 Summary (CO 2 ) g (CO 2 ) aq diffusion ocean. CO 2 + H 2 O H 2 CO CO2 carbonic acid H 2 CO H+ + HCO carbonic acid bicarbonate HCO H+ + CO 2 Interaction with the biological pump bicarbonate carbonate CO 2 + H 2 0 CH photosynthesis/decomposition Ca HCO CaCO + H 2 CO calcium carbonate shells Net Effect: plankton remove CO2 from surface water, drawing more CO2 out of the osphere. The organic material, and calcium carbonate shells, eventually sink into the deep ocean. production decomposition mixed layer (CO 2 ) g Net effect: drawdown of CO2! (CO 2 ) aq H 2 CO HCO CO 2 Diatom (SiO 2, ~50 µm) ) coccolithophorid (CaCO, ~10 µm) blue = inorganic chemistry red = organic carbon dioxide effect green = organic carbonate effect consumption to the deep ocean foraminifer (CaCO, ~600 µm) radiolarian (SiO 2, ~50 µm) (CO 2 ) g Net effect: drawdown of CO2! (CO 2 )g (CO 2 ) aq H 2 CO HCO CO 2 (CO 2 )aq H 2 CO HCO CO 2 mixed layer coccolithophorid (CaCO, ~10 µm) foraminifer (CaCO, ~600 µm) mixed layer H + ion H + ion Equilibrium values depend on ph and temperature ph = log[h + ] Dissolved CO2 contributes to acidification blue = inorganic chemistry red = organic carbon dioxide effect green = organic carbonate effect 9

10 From weathering to deposition on the sea floor Rain drops are slightly acidic to due CO2 dissolving in them, resulting in carbonic acid. Carbonate Weathering: CaCO + H 2 CO Ca HCO calcium carbonic calcium bicarbonate carbonate acid ion ion Silicate Weathering: CaSiO + 2H 2 CO Ca HCO + SiO 2 + H 2 O wollastonite carbonic calcium bicarbonate silica water acid ion ion From weathering to deposition on the sea floor These reactions provide the weathered material that gets washed into the oceans and is available for production of calcium carbonate and silicate shells by plankton in the mixed layer. As the plankton die, and the shells sink into the deep ocean, they do not dissolve much at first. The shallow and middle depths of the ocean are saturated with respect to CaCO: there is little acidity to dissolve the shells. In deeper parts of the ocean they do dissolve more, as these waters often have higher concentrations of dissolved CO2, and therefore carbonic acid, due to the decomposition of organic matter. From weathering to deposition on the sea floor carbonate compensation depth (CCD): depth below which the carbonate shells dissolve faster than the rate of shells settling through the water column. Below the CCD, carbonate shells dissolve, no carbonate is deposited on the ocean floor. From weathering to deposition on the sea floor The net result of weathering to deposition is that some carbon is removed from the osphere and ends up in calcium carbonate on the ocean floor. Thus, weathering removes CO2 from the osphere and stores it in calcium carbonate sediments. This is another CO2 leak from the system. If there were no other source of CO2 into the osphere, CO2 concentrations would drop to zero in about a million years. 10

11 Summary of the cycle What process makes up for the CO2 leakage from the osphere associated with weathering? Volcanism, and emission through midocean ridges THE LONG TERM INORGANIC CARBON CYCLE: The CarbonateSilicate Geochemical Cycle Net effect: return of CO2 to the! Carbonate metamorphism: CaCO + SiO2 CaSiO + CO2 calcite silica wollastonite carbon dioxide THE LONG TERM INORGANIC CARBON CYCLE: The CarbonateSilicate Geochemical Cycle So, ospheric CO2 loss by weathering is compensated for by CO2 emissions associated with plate tectonics (volcanic and midocean ridge emissions). Feedbacks that affect the weathering rate are believed to play a role in regulating ospheric CO2 levels, and therefore climate, over geologic time scales. 11