Earth Systems Science Chapter 8 THE CARBON CYCLE The circulations of the atmosphere, 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 carbon cycle: dynamics The short term terrestrial organic carbon cycle The short term marine organic carbon cycle The long term organic carbon cycle The short term inorganic carbon cycle; interaction with the biological pump The long term inorganic carbon cycle: the carbonate-silicate geochemical cycle

THE CARBON CYCLE: DYNAMICS

THE CARBON CYCLE: DYNAMICS Reservoirs Locations, or types of regions, where the substance you are tracking is stored. Value of reservoir depends on the net flux STELLA diagram of global C cycle used in our lab, adapted Chameides and Perdue (1997)

THE CARBON CYCLE: DYNAMICS The atmosphere A variety of processes are related to flux into and out of the atmosphere. These may vary seasonally, resulting in a seasonal cycle in atmospheric carbon concentration. Steady state: same as dynamic equilibrium

THE CARBON CYCLE: DYNAMICS Residence time, or response time, or e-folding 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(atm) = 760 (Gt-C) / 60 (Gt-C/yr) = 12.7 yr T = time in which a perturbed system will return to 1/e, or ~38%, of original value rate = 1/T = 1/12.7 (1/yr) = .07874 (1/yr) = .07874 yr-1

THE CARBON CYCLE: DYNAMICS 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)

THE CARBON CYCLE: DYNAMICS 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

THE CARBON CYCLE: DYNAMICS Oxidized C that is combined with oxygen examples: CO2, CaCO3 Reduced C that is not combined with oxygen, usually combined with other carbon atoms (C-C), hydrogen (C-H), or nitrogen (C-N) example: organic carbon in carbohydrates reduced substances tend to be unstable in the presence of oxygen: organic matter decomposes, metals rust

THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE Organic carbon: associated with living organisms; contains C-C or C-H bonds Photosynthesis: C is removed from the atmosphere 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 atmosphere; becomes inorganic Net primary productivity (NPP): primary productivity - respiration Image Name: North America NDVI Image Date: March 1990-November 1990 Image Source: AVHRR Mosaic http://edc.usgs.gov/products/landcover.html

THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE Photosynthesis: CO2 + H20  CH20 + 02 (solar energy) Respiration: CO2 + H20  CH20 + 02 (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 http://edc.usgs.gov/products/landcover.html

THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE On land, Net Primary Productivity = 0.5 Primary Productivity Steady state: flux in = flux out

THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE Where is the atmosphere in this model? exogenous to this model STELLA diagram of terrestrial forest C cycle (adapted from Huggett, 1993)

THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE 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)

THE SHORT-TERM 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: CO2 + H20  CH20 + 02 (solar energy) Respiration: CO2 + H20  CH20 + 02 (release energy) Methanogenesis: CO2 + CH4  2CH20 (release energy)

Plankton: organisms floating in water Diatom (SiO2, ~50 mm) coccolithophorid (CaCO3, ~10 mm) THE SHORT-TERM MARINE ORGANIC CARBON CYCLE Plankton: organisms floating in water photic zone: ~mixed layer, upper 100m

Plankton: organisms floating in water foraminifer (CaCO3, ~600 mm) radiolarian (SiO2, ~50 mm) THE SHORT-TERM MARINE ORGANIC CARBON CYCLE Plankton: organisms floating in water photic zone: ~mixed layer, upper 100m

THE SHORT-TERM MARINE ORGANIC CARBON CYCLE The Biological Pump Thermohaline Circulation

THE SHORT-TERM 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 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.

THE SHORT-TERM MARINE ORGANIC CARBON CYCLE SEAWIFS Mean Chlorophyl September 97 - August 2000 Center of gyres – downwelling – few sources of nutrients – little biological activity Areas with nutrient input from rivers – or from upwelling – more biological activity High latitudes generally more productive than low latitudes http://seawifs.gsfc.nasa.gov/SEAWIFS/IMAGES/

THE LONG-TERM 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 (Gt-C) (Gt-C/y) (y) atmosphere 760 60 12.7 soil/sed. 1600 30 53.3 sed. rock 1e07 0.05 2e08

THE LONG-TERM 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 CO2 leaves one oxygen molecule (O2 ) in the atmosphere: CO2 + H20  CH20 + 02

THE LONG-TERM ORGANIC CARBON CYCLE Short circuit the flux from sedimentary rocks to the atmosphere Fossil fuels are formed from the organic carbon in sedimentary rocks. How does the burning of fossil fuels affect this system diagram? How does the deforestation affect this system diagram? What about reforestation?

THE INORGANIC CARBON CYCLE Sources and sinks of atmospheric carbon that do not depend directly on biological activity exist. source: a reservoir from which the atmosphere gains carbon sink: a reservoir to which the atmosphere loses carbon inorganic: not directly related to biological activity Important reservoirs of inorganic carbon: the atmosphere, the ocean, sedimentary rocks Sedimentary rock carbon reservoirs consist mostly of: limestone: CaCO3 dolomite: CaMg(CO3)2 (older sedimentary rocks)

THE INORGANIC CARBON CYCLE: rates of diffusion atm (CO2)g (CO2)aq H2CO3 HCO3- CO32- mixed layer

THE INORGANIC CARBON CYCLE: atm (CO2)g (CO2)aq H2CO3 HCO3- CO32- rates of chemical reactions mixed layer

THE INORGANIC CARBON CYCLE Atmosphere – Ocean Carbon Exchange CO2 diffuses between the atmosphere and the ocean Diffusion: 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)

THE INORGANIC CARBON CYCLE Atmosphere – Ocean Carbon Exchange CO2 diffuses between the atmosphere and the ocean The direction and magnitude of diffusion depends on the partial pressure of CO2 in the atmosphere, 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 atmosphere solubility: the maximum amount of a substance that will dissolve in a specified liquid (similar to saturation in the atmosphere) rate constant: number representing speed with which diffusion occurs (CO2)g  (CO2)aq where g=gas, aq=aqueous = dissolved in water

THE INORGANIC CARBON CYCLE Chemistry of Inorganic Carbon in Water dissolved CO2 generates carbonic acid CO2 + H2O  H2CO3 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

THE INORGANIC CARBON CYCLE Chemistry of Inorganic Carbon in Water carbonic acid generates hydrogen ions, bicarbonate ions, carbonate ions H2CO3  H+ + HCO3- (bicarbonate ion) HCO3-  H+ + CO32- (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.

THE INORGANIC CARBON CYCLE Summary (CO2)g  (CO2)aq diffusion ocean - atm. CO2 + H2O  H2CO3 CO2 - carbonic acid H2CO3  H+ + HCO3- carbonic acid - bicarbonate HCO3-  H+ + CO32- bicarbonate - carbonate Interaction with the biological pump CO2 + H20  CH20 + 02 photosynthesis/decomposition Ca2+ + 2HCO3-  CaCO3 + H2CO3 calcium carbonate shells Net Effect: plankton remove CO2 from surface water, drawing more CO2 out of the atmosphere. The organic material, and calcium carbonate shells, eventually sink into the deep ocean.

THE INORGANIC CARBON CYCLE: interaction with the biological pump atm (CO2)g (CO2)aq H2CO3 HCO3- CO32- Net effect: drawdown of atm CO2! coccolithophorid (CaCO3, ~10 mm) Diatom (SiO2, ~50 mm) production decomposition mixed layer foraminifer (CaCO3, ~600 mm) radiolarian (SiO2, ~50 mm) consumption to the deep ocean blue = inorganic chemistry red = organic carbon dioxide effect green = organic carbonate effect

THE INORGANIC CARBON CYCLE: interaction with the biological pump atm (CO2)g (CO2)aq H2CO3 HCO3- CO32- Net effect: drawdown of atm CO2! mixed layer foraminifer (CaCO3, ~600 mm) coccolithophorid (CaCO3, ~10 mm) blue = inorganic chemistry red = organic carbon dioxide effect green = organic carbonate effect

THE INORGANIC CARBON CYCLE: interaction with the biological pump atm (CO2)g (CO2)aq H2CO3 HCO3- CO32- H+ ion H+ ion mixed layer Equilibrium values depend on pH and temperature pH = -log[H+] Dissolved CO2 contributes to acidification

THE INORGANIC CARBON CYCLE: interaction with the biological pump From weathering to deposition on the sea floor Rain drops are slightly acidic to due atm CO2 dissolving in them, resulting in carbonic acid. Carbonate Weathering: CaCO3 + H2CO3  Ca2+ + 2HCO3- calcium carbonic calcium bicarbonate carbonate acid ion ion Silicate Weathering: CaSiO3 + 2H2CO3  Ca2+ + 2HCO3- + SiO2 + H2O wollastonite carbonic calcium bicarbonate silica water acid ion ion

THE INORGANIC CARBON CYCLE: interaction with the biological pump 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 CaCO3: 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.

THE INORGANIC CARBON CYCLE: interaction with the biological pump 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.

THE INORGANIC CARBON CYCLE: interaction with the biological pump From weathering to deposition on the sea floor The net result of weathering to deposition is that some carbon is removed from the atmosphere and ends up in calcium carbonate on the ocean floor. Thus, weathering removes CO2 from the atmosphere 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 atmosphere, CO2 concentrations would drop to zero in about a million years.

THE INORGANIC CARBON CYCLE: interaction with the biological pump Summary of the cycle What process makes up for the CO2 leakage from the atmosphere associated with weathering? Volcanism, and emission through mid-ocean ridges

THE LONG TERM INORGANIC CARBON CYCLE: The Carbonate-Silicate Geochemical Cycle Net effect: return of CO2 to the atm! Carbonate metamorphism: CaCO3 + SiO2  CaSiO3 + CO2 calcite silica wollastonite carbon dioxide

THE LONG TERM INORGANIC CARBON CYCLE: The Carbonate-Silicate Geochemical Cycle So, atmospheric CO2 loss by weathering is compensated for by CO2 emissions associated with plate tectonics (volcanic and mid-ocean ridge emissions). Feedbacks that affect the weathering rate are believed to play a role in regulating atmospheric CO2 levels, and therefore climate, over geologic time scales.