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CARBON CYCLE (Supporting Life (Stores (Carbonate rocks e.g. limestone,…

- - - - Atmosphere, 600
      - Oceans, 38,700
      - Sedimentary (carbonate) rocks, 60,000 - 100,000,000
      - Sea floor sediments, 6,000
      - Fossil fuels, 4,130
      - Land plants, 560
      - Soils/peat 2,300
    - - But these stores represent most of the C in circulation at any one time
  - - - Typical residence times for C held in rocks is 150 million years
      - Some C-rich sed. rocks subducted into the upper mantle at SDZ, are vented into atmosphere in volcanic eruptions
      - Other C-rich sed. rocks at/near the surface by erosion and tectonic movements are attacked by chemical weathering
        
        Chemical weathering processes
        
        e.g. carboantion
        
        Result of precipitation charged with CO2 from the atmosphere = weak acid
        
        Acid rain attacks carbonate minerals in rocks, releasing CO2 to the atmosphere, and to streams, rivers and oceans in dissolved form
      - Carbonaceous rocks
        
        When partly decomposed land organic matter is buried beneath younger sediments
        
        e.g. oil, coal, lignite and natural gas
        
        These fossil fuels act as C-sinks that endure for millions of years
  - - - Atmosphere
      - Ocean
      - Biosphere
      - Soils
    - - Land plants
      - Phytoplankton
      - They absorb CO2 thru atmosphere and combine it with water to make carbohydrates during photosynthesis
    - - Atmospheric CO2 dissolves in ocean surface water
      - Oceans ventilate CO2 back into atmosphere
      - Thru this exchange, indv. C atmos are stored by natural sequestration for 350 years
- - - - Precipitation
        
        Atmospheric CO2 dissolves in rainwater = weak carbonic acid
        
        Rising concs, of CO2 in atmosphere from anthropogenic emissions = increased acidity = increased acidity of ocean waters - marine life
      - Photsynthesis
        
        Flux of C from atmosphere to plants plants and phytoplankton via photosynthesis is 120 GT/year
        
        Photosynthesis uses CO2 from atmosphere to make glucose
      - Weathering
        
        In situ breakdown of rocks at/near the surface by chemical, physical and biological processes
        
        Most weathering involves rainwater that contains dissolved CO2, derived from the soil as well as atmosphere
        
        Rainwater is a weak carbonic acid that dissolves limestone and chalk via carbonation
        
        Carbonation releases C from rock to water sources + the atmosphere
        
        Process is most effective beneath a soil cover bc the higher conc. pf CO2 in the soil makes rainwater highly acidic
        
        Chemical weathering transfers 0.3 billion tonnes of C to atmosphere+ oceans each year
        
        Physical weathering by freeze-thaw breaks down rocks into smaller particles = larger surface area exposed to chemical attack
        
        Biological processes e.g. chelation - rainwater + decaying organic matter = humid acids in soil that attack mineral rocks
        
        Imp. process in humid + tropical conditions where decomposition is rapid and forest trees provide abundant leaf litter e.g. TROPICAL RAINFORESTS
      - Respiration
        
        Absorbs oxygen and release CO2
        
        Respiration and photosyn. are the 2 most important processes in the fast C cycle
        
        The vol. of C exchanged by resp. + photosyn. is 1000 times greater than moving thru the slow cycle
      - Decomposition
        
        Decomposer orgs. (bac, fungi) breakdown dead organic matter, extracting energy and releasing CO2 to atmosphere and mineral nutrients to the soil
        
        Rates depend on climatic conds.
        
        Fastest rates occur in warm. human environs. e.g. TROPICAL RAINFORESTS
      - Combustion
        
        When organic matter reacts or burns in the rpesence of O2
        
        Releases CO2, SO2 + nitrogen oxides
        
        Uses - coniferous forests, Rocky Mts.
        
        Long, cold winters slow decomposition rates of forest litter which builds up on forest floor
        
        Fires free C and nutrients previously inaccessible to forest trees+ opens up the forest canopy = new habitats + increased BD
        
        Human activities
        
        Firing of forests/grassland to clear for cultivation
        
        Combustion of fossil fuels
        
        Transfers nearly 10 GT of CO2/year from geological store to the atmosphere, oceans + biosphere
    - - Oceans take up C by 2 mechanisms:
        
        Physical (inorganic) Pump
        
        Mixing of surface and deep ocean waters by vertical currents
        
        More even distribution of C - both geographically and vertically
        
        CO2 enters the oceans from atmosphere via diffusion
        
        Surface ocean currents transport the water and its dissolved CO2 polewards, where it cools, becomes more dense and sinks
        
        This downwelling occurs only in a few places of the oceans e.g. between Greenland + Iceland
        
        Downwelling carries dissolved C to the ocean depths where indv. C molecules may remain for centuries
        
        Eventually deep ocean currents transport the C to areas of upwelling
        
        There, cold, C-rich water rises to the surface and CO2 diffuses back into the atmosphere
        
        Biological (organic) Pump
        
        C is exchanged between oceans and atmosphere thru actions of marine orgs.
        
        Nearly 50% of all C fixation by photosyn. occurs in oceans
        
        Around 50 GT of C is drawn from the atmosphere by the biological pump/year
        
        Marine orgs. drive the biological pump
        
        Phytoplankton at ocean surface combine sunlight, water + dissovled CO2 to produce organic material
        
        Other orgs. (molluscs + crustaceans) extract carbonate and calcium ions from sea water to manufacture plates, shells and skeletons of CaCO3
        
        Most of this C-rich material end sup in ocean sediments and is lithified to form chalk and limestone
        
        C locked in the phytoplankton is either accumulates in sediments on the floor or is decomposed and released into the ocean as CO2
      - Vegetation
        
        Land plants contain huge stores of C
        
        Esp. rainforests + boreal forests
        
        Most of this C, extracted from atmospheric Co2 through photsyn., is locked away for decades