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Global Carbon Cycle - I
Reservoirs and Fluxes
OCN 401 - Biogeochemical Systems
13 November 2012
Reading: Schlesinger, Chapter 11
1. Overview of global C cycle
2. Global C reservoirs
3. The contemporary global C cycle
4. Fluxes and residence times
5. Global seasonal variations in atmospheric CO2
6. Linkages between the C and O Cycles
7. History of the global C cycle
Outline
• Major reservoirs:
• biomass on land
• biomass in the oceans and marine sediments
• atmosphere
• soil and rocks
• Processes
• atmospheric exchange with oceans and plants
• organic matter cycling
• weathering and the rock cycle
• the physical chemistry of the oceans
• Linked to the global cycles of oxygen, nitrogen and phosphorus
The Global Carbon Cycle
Global Carbon Reservoirs
*Largest near-surface pool of C; ocean contains 56x more C than the atmosphere
Total 1 x 1023g
Sedimentary Rocks 8 x 1022 g
Surficial Active Pools 4 x 1019 g
Dissolved C in Ocean 3.8 x 1019g*
Extractable Fossil Fuels 4 x 1018 g
Soil Organic Carbon 1.5 x 1012 g (Table 5.3)
Listed in order of size (oceans): g C
Carbonate sediments 6.5 x 1022
Organic matter in seds 1.2 x 1022
DIC in Ocean 3.7 x 1019
DOC in oceans 1.0 x 1018
Ocean biota 3.0 x 1015
Carbonate sediments are the largest reservoir…
larger than organic matter reservoir by ~ 5:1
Ocean water is next largest reservoir:
Inorganic (DIC) is ~40x organic (DOC) ocean reservoir
Listed in order of size (land + atmosphere)):
CaCO3 in soils 7.2 x 1017
Land biota 7.0 x 1017
Atmosphere CO2 6 x 1017
Soil organic matter 2.5 x 1017
Soils are the next-largest reservoir
Living biotic reservoir is ca. same as inorganic soil reservoir
Dead organic matter is 1/3 of the inorganic soil reservoir
Phytomass 100x bacteria and animal reservoirs
Atmosphere is the smallest reservoir, similar to size of all living biomass
Buried reservoirs of
organic carbon are
large relative to
atmosphere
Transfers between
organic reservoirs (on
land and in the oceans)
can occur on short time
scales
Organic Carbon vs. Atmospheric Carbon
The Contemporary Global Carbon Cycle
Surficial active pools (1015g C) and fluxes between pools (1015g C yr-1)
*
*
*
* *
*Largest
fluxes link
atm CO2 to
land
vegetation
and the
surface
ocean
Fluxes and Residence Times
• Global NPP = GPP - Rp = 60 g/yr
• TR of Atm-CO2 wrt terrestrial
vegetation: 750 g / 60 g yr-1 = 12.5
yr
• Thus, each molecule of CO2 in the
atm has the potential to be taken up
in terrestrial NPP every 12.5 years.
• TR of Atm-CO2 wrt the ocean: 750
g / 92 g yr-1 = 8 yr
• Mean TR of Atm-CO2 wrt the ocean
+ land: 750 g / (60 + 92) g yr-1 = 5 yr
• Mean TR 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
(all units x 1015 g)
Global Seasonal Variations in Atmospheric CO2
• Seasonal uptake of CO2 results in oscillations in atm CO2 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 CO2 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/
“…large interannual changes in CO2 growth rates can mostly be explained
by natural climate variability” (e.g., ENSO). (Patra et al. (2005))
www.esrl.noaa.gov/gmd/Photo_Gallery/Field_Sites/MLO/
Mauna Loa Observatory
• Photosynthetic uptake of C to synthesize organic matter releases O2:
• Oxidation of organic matter consumes O2 :
- Burial of organic matter (reduced C) equates to an increase in the
atmospheric O2 reservoir:
Photosynthesis Links the C and O Cycles
CO2 + H2O CH2O + O2
CH2O + O2 CO2 + H2O
Organic C Burial Links CO2 to Atmospheric O2 Cycle A Simple Model
Carbonate and Silicate Rock Cycle A More Detailed Approach
Weathering on land
CaCO3 + CO2 + H2O = Ca2+ + 2HCO3-
CaSiO3 +2CO2 +3H2O = Ca2+ + 2HCO3- + H4SIO4
--> Uptake of atmospheric CO2 during weathering on land; delivery
of dissolved form to oceans
Deposition in the oceans
Ca2+ + 2HCO3- = CaCO3 + CO2 + H2O
H4SiO4 = SiO2 + 2H2O
--> Release of CO2 during carbonate precipitation
Metamorphic reactions
CaCO3 + SiO2 = CaSiO3 + CO2
--> Release of CO2 via volcanic/hydrothermal activity
Oxidation of uplifted organic matter
CH2O + O2 = CO2 + H2O
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
- CO2 released
- Primary minerals re-formed at high T and P
Weathering of
Silicate rocks
Ions carried by
Rivers to oceans
The Global Cycle of Weathering
• CaCO3 weathering on land and re-precipitation in ocean has no net effect on
atmospheric CO2
• Weathering of silicates on land and re-precipitation in ocean results in net
uptake of atmospheric CO2
• Balance of weathering types affects atmospheric CO2
• Subduction of sediments and volcanic activity returns CO2 to atmosphere
• In the absence of recycling, weathering would remove all CO2 from atmosphere
in ~ 1 million years
• Residence time of CO2 in atmosphere relative to weathering and volcanic input
is ~ 6,000 years, i.e. the rock cycle exerts long term control on atm CO2
• Rock cycle does not control decade- to century-scale changes seen in modern C
cycle
Long-term changes in atmospheric
CO2 driven by rock cycle and
biological evolution
Initial high levels of atmospheric CO2
and a “weak” sun
Silicate weathering and carbonate
precipitation in ocean reduced
atmospheric CO2 levels
Evolution of life ~3.9 Ga
sequestered organic C
Oxygenic photosynthesis ~2.8 Ga
accumulation of O2
History of the C Cycle
• Initial production of O2 consumed by oxidation of Fe in seawater
• Terrestrial weathering also consumed early O2 production
Phanerozoic C Cycle (last 500 million years)
• Driven by evolution of land plants (and C storage) 400-500 Ma
• 30 x 1012 moles C buried in sediments yr-1 -- increases O2 in atmosphere
• Atmospheric O2 = 38 x 1018 moles; Tr = 1 x 106 yrs (wrt sedimentary C)
• Uplift of rocks and weathering of kerogen balances process
• Atmospheric O2 results from balance of organic C burial and its weathering
Evolution of angiosperms ~ 150 Ma
Deeper roots increase Si weathering rates, leads to drop in atm CO2
• O2 levels in atmosphere track maximum burial of organic C ~ 350 Ma
• Microbial processes then pull down O2 levels as organic C is oxidized
White rot fungi appear
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 CO2 accumulates
in the atmosphere, vegetation and ocean (lower part of the graph). (Global Carbon Project, 2008)
The C Cycle Since the Industrial Revolution