Global Carbon Cycle - I OCN 401 - Biogeochemical Systems Reading: Schlesinger, Chapter 11
1. Overview of global C cycle 2. Global C reservoirs Outline 3. The contemporary global C cycle 4. Fluxes and residence times 5. Global seasonal variations in atmospheric CO 2 6. Linkages between the C and O Cycles 7. History of the global C cycle
Reservoirs: biomass on land biomass in the oceans atmosphere soil and rocks waters The Global Carbon Cycle Processes atmospheric exchange organic matter cycling weathering and the rock cycle the physical chemistry of the oceans Linked to the global cycles of oxygen, nitrogen and phosphorus
Global Carbon Reservoirs Total 1 x 10 23 g Sedimentary Rocks 8 x 10 22 g Surficial Active Pools 4 x 10 19 g Dissolved C 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) *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): C = kp C = concentration of gas in seawater k = solubility constant P = partial pressure in atm
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 (continental surface + 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
Buried Organic Carbon vs. Atmospheric Carbon 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
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 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 / 92 g yr -1 = 8 yr (all units x 10 15 g) Mean T R of Atm-CO 2 wrt the ocean + land: 750 g / (60 + 92) 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 ~380 ppm
Global Seasonal Variations in Atmospheric CO 2 Seasonal uptake of CO 2 results in oscillations in atm CO 2 content Effect is greater in Northern Hemisphere
Carbon dioxide is only a small fraction of the Earth s surficial C 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
www.esrl.noaa.gov/gmd/ccgg/trends/
Mauna Loa Observatory www.esrl.noaa.gov/gmd/photo_gallery/field_sites/mlo/
Photosynthesis Links the C and O Cycles Photosynthetic uptake of C to synthesize organic matter releases O 2 : CO 2 + H 2 O CH 2 O + O 2 Oxidation of organic matter 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 - Note: photosynthesis and weathering also impact speciation, distribution and transformations of bioelements N, P and S
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
The Global Cycle of Weathering Ions carried by Rivers to oceans Weathering of Silicate rocks The Rock Cycle: Primary minerals at Earth surface exposed to acidic forms of C, N, S from atmos Products of weathering reactions are carried to the ocean via rivers Weathering products accumulate as dissolved salts or sediments Subduction carries sediments back into the deep earth -CO 2 released - Primary minerals re-formed at high T and P
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 Thus, 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 -- thus the rock cycle exerts long term control on atm CO 2 Rock cycle does not control decade- 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 and a weak sun Silicate weathering and carbonate precipitation in ocean reduced atmospheric CO 2 levels Evolution of life ~3.9 Ga stored organic C and lead to production of O 2
Initial production of O 2 consumed by oxidation of Fe in seawater Terrestrial weathering also consumed early O 2 production
Phanerozoic C Cycle Driven by evolution of land plants (and start of C storage) 400-500 Ma 30 x 10 12 moles-c/yr buried in sediment -- releases O 2 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
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
The C Cycle Since the Industrial Revolution Since 1800, by burning fossil fuel and cutting forests, we have released more than 400 billion tons of carbon - half of it during the last 30 years only (upper part of the graph). This extra CO 2 accumulates in the atmosphere, vegetation and ocean (lower part of the graph). (Global Carbon Project, 2008)