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