OCN 401-10 Nov. 16, 2010 KCR Global Carbon Cycle - I Systematics: Reservoirs and Fluxes
The Global carbon cycle Reservoirs: biomass on land in the oceans, atmosphere, soil and rocks, waters Processes: atmospheric exchange, weathering and the rock cycle, the physical chemistry of the oceans Linked to the global cycles of oxygen, nitrogen and phosphorus
Photosynthesis links the C & O Cycles Photosynthetic uptake of C to synthesize tissues releases O 2 : CO 2 + H 2 O CH 2 O + O 2 Weathering of rocks consumes O 2 : CH 2 O + O 2 CO 2 + H 2 O - Burial of organic matter (reduced C) equates to an increase in the atmospheric O 2 reservoir - Photosynthesis and weathering impact form and locus and transformations of bioelements N, P and S - Anthropogenic activity has impacted the global C cycle, and global climate change through the greenhouse effect
Earth s Carbon Reservoirs Total Sedimentary Rocks Surficial Active Pools *Largest near-surface pool of C; ocean contains 56 X more C than the atmosphere; therefore has an enormous capacity to buffer changes in atmospheric CO 2 via Henry s Law, which describes partitioning of gases between the gaseous and dissolved phase (eqn. 2.7): S = kp 1 x 10 23 g 8 x 10 22 g 4 x 10 19 g TDC in Ocean 3.8 x 10 19 g (Fig. 11.1)* Extractable Fossil Fuels 4 x 10 18 g Soil Organic Carbon 1.5 x 10 12 g (Table 5.3) S is solubility of gas in a liquid k = solubility constant P = overlying pressure in atmosphere
The Contemporary Global Carbon Cycle [(surficial active pools (10 15 g C) and fluxes between pools (10 15 g C yr -1 )] *Largest fluxes link atm CO 2 to land vegetation and the surface ocean. * * * * *
Fluxes and Residence Times (all units x 10 15 g) Global NPP = GPP - R p = 60 g/yr T R of Atm-CO 2 wrt terrestrial vegetation: 750 g / 60 g yr -1 = 12.5 yr Thus, each molecule of CO 2 in the atm has the potential to be taken up in terrestrial NPP every 12.5 years. T R of Atm-CO 2 wrt the ocean: 750 g / 90 g yr -1 = 8 yr Mean T R of Atm-CO 2 wrt the ocean: 750 g / (60 + 90) g yr -1 = 5 yr. Mean T R only slightly longer than the mixing time of the atmosphere, so only minor seasonal variations are evident about the mean global average concentration of 360 ppm (in 1996).
Seasonal Variations in Atmospheric CO 2 Seasonal uptake of CO 2 results in oscillations in atm CO 2 content
Seasonal Variations in Atmospheric CO 2 (cont d.) Carbon dioxide is only a small fraction of the Earth s surficial Carbon reservoir, but its role in photosynthesis, climate regulation and rock weathering make it a critical component of the system Globally, 2/3 of terrestrial vegetation occurs in regions with seasonal biomass growth Atm CO 2 fluctuations are greatest in the N. Hemisphere, where most of the continental landmass resides S. Hemisphere fluctuations believed due to exchange with surface ocean
Carbon Reservoirs
Listed in order of size (oceans): g C Carbonate sediments 6.53 x 10 22 Organic matter in seds 1.25x 10 22 DIC in Ocean 3.74 x 10 19 DOC in oceans 1.00 x 10 18 Ocean biota 3.00 x 10 15 Carbonate sediments are the largest reservoir larger than organic matter reservoir by ~ 5:1 Ocean water is next largest reservoir Inorganic (DIC) is ~40 x organic (DOC) ocean reservoir Listed in order of size (land + atmosphere)): CaCO 3 in soils 7.2 x 10 17 Land biota 7.0 x 10 17 Soil organic matter 2.5 x 10 17 Atmosphere CO 2 6 x 10 17 Soils are next largest reservoir Living biotic reservoir is ~same as inorganic reservoir Dead organic matter is 1/3 of the inorganic reservoir Phytomass ~100 x bacteria and animal reservoirs Atmosphere is the smallest reservoir, similar to size of all living biomass
Transfers between organic reservoirs (on land and in the oceans) can occur on short time scales Buried reservoirs of organic carbon are large relative to atmosphere
Reactions linking carbon and oxygen C + O 2 = CO 2 Under conditions at Earth s surface this inorganic reaction proceeds to the right, i.e. CO 2 favored over C and O 2 thermodynamically --lots of Gibbs free energy Burning of fossil fuels or forest fires --> oxidation of carbon But kinetics are slow, activation energy is needed to promote the reaction
Reactions linking C and O: photosynthesis and respiration CO 2 + 2H 2 O = CH 2 O + H 2 O + O 2 Photosynthesis rxn proceeding to the right, releases oxygen to the atmosphere Respiration, rxn proceeding to the left, consumes oxygen from the atmosphere Both of these reactions proceed rapidly on annual cycles
Organic C burial links CO 2 to atmospheric O 2 cycle
Carbonate and Silicate Rock Cycle Weathering on land CaCO 3 + CO 2 + H 2 O = Ca 2+ + 2HCO 3 - CaSiO 3 +2CO 2 +3H 2 O = Ca 2+ + 2HCO 3 - + H 4 SIO 4 --> Uptake of atmospheric CO 2 during weathering on land, delivery of dissolved form to oceans Deposition in the oceans Ca 2+ + 2HCO 3 - = CaCO 3 + CO 2 + H 2 O H 4 SiO 4 = SiO 2 + 2H 2 O --> Release of CO 2 during carbonate precipitation Metamorphic reactions CaCO 3 + SiO 2 = CaSiO 3 + CO 2 --> Release of CO 2 and return to atm via volcanic/hydrothermal activity
Carbonate and Silicate Rock Cycle CaCO 3 weathering on land and re-precipitation in ocean has no net effect on atmospheric CO 2 Weathering of silicates on land and re-precipitation in ocean results in net uptake of atmospheric CO 2 Balance of weathering types affects atmospheric CO 2 Subduction of sediments and volcanic activity returns CO 2 to atmosphere In the absence of recycling, weathering would remove all CO 2 from atmosphere in ~ 1 million years Residence time of CO 2 in atmosphere relative to weathering and volcanic input is ~ 6,000 years, i.e. the rock cycle exerts long term control on atm CO 2 Rock cycle does not control decadal to century scale changes seen in modern C cycle
History of the C cycle Long term changes in atmospheric CO 2 driven by rock cycle and biological cycle Initial high levels of atmospheric CO 2 Weak sun Silicate weathering and carbonate precipitation in ocean reduced atmospheric CO 2 levels Evolution of life ~3.9 Ga sequestered organic C Leads to production of oxygen
Initial production of O 2 used up by oxidation of Fe in seawater Terrestrial weathering also consumed early O 2 production Multi-cellular organisms appear as O 2 levels in atmosphere increase
Phanerozoic C cycle Driven by evolution of land plants - C storage 400-500 Ma 30 x 10 12 moles C buried in sediments yr -1 releases oxygen to atmosphere Atmos. reservoir O 2 38 x 10 18 moles, T r = 1 x 10 6 yrs wrt sedimentary C Uplift of rocks and weathering of kerogen balances process Atmospheric O 2 is balance of organic C burial and its weathering
Evolution of angiosperms ~ 150 Ma Deeper roots increase Si weathering rates, leads to drop in atm CO 2
from wikipedia.org
O 2 levels in atmosphere track maximum burial of organic C ~ 350 Ma Microbial processes then pull down O 2 levels as organic C is oxidized
Fluxes of C