Global Carbon Cycle
Why study the C cycle?
• Key element of life – so fundamental
– Fossil fuel burning and global warming
• Perturbation by humans (atm CO2)
• Complex cycle – long & short term cycles; organic
and inorganic components
• Geological processes operating over millions of
years
• Biological processes operating on annual time scales
• Interactions between long and short term cycles
“short”-term
“long”-term
anthropogenic
Combined Inventories
Atmosphere
770 Gt C
Terr. Systems
~2400 Gt C
Oceans
~39,000 Gt C
Sed. Rocks
50,000,000 Gt C
1.
2.
3.
4.
Fig. 8-3
Major Inventories
Majority of C tied up in rock
cycles – large reservoirs with
long residence times
Reservoirs active on short
time scales are ocean, atm,
& land
Large exchange fluxes to and
from atm – atm has short
residence time (3 yr); small
net fluxes due to biology
(most PP is respired)
Problem with adding fossil
fuel CO2 to atm –
transferring C from long
term geologic reservoir to a
short term reservoir – may
affect short term feedback
control mechanisms
50.3
Atmospheric CO2
Respiration
Air-Sea Exchange
Terrestrial primary
production and respiration
Physical
weathering
59.6 - 59.7
50
CO2
POC
Marine primary prod.
River transport
60
0.4
Upwelling
remineralization
Kerogen
Land Plants
Particle Rain
Oceans
Humification
Uplift of
sedimentary
rocks
POC Deposition
CO2
Benthic Fluxes
Soil humus
0.1
Recent Sediments
0.1
Carbon Burial
Sedimentary Rocks
A model – little transfer of biological C to ocean (from land) or sediments (from water)
Atmosphere
• Most C in atm as CO2
– Some methane and CO
• Atm CO2 shows rapid increase in recent time
– Beginning with Industrial Revolution
• See seasonal variations in recent increase
– Uptake in Spring due to plant growth (N hemisphere)
– Release in fall from net respiration
Northern hemisphere
-More land
-More terrestrial prod
Figs. 1-2 and 1-3
Southern amplitude is lower
Seasonality offset by 6 mos.
400
South Pole
Barrow, AK
CO2 (p p m )
380
360
340
320
300
1975
1980
1985
1990
1995
2000
Year
Northern hemisphere has more extensive seasonal forests
Close tracking between N & S hemispheres
2005
Prior to humans, the system showed natural variability
(50 – 80 ppm glacial-interglacial)
Smaller Holocene changes
Interglacial
Glacial
Holocene changes
• Recent high resolution ice core
• Natural variability in Holocene is second
order change when compared with glacialinterglacial excursions and anthropogenic
increase
• Allows us to think about a nearly constant
pre-industrial interglacial CO2 level of ~280
ppm
Recent increases in Atm CO2
• Some due to land use changes (preindustrial)
– Deforestation – two-fold problem
• Decrease PS uptake of CO2
• Burn the wood - charcoaling
• Mainly due to fossil fuel burning (postindustrial)
• Deforestation in tropics may be partially balanced
by N hemisphere forest expansion/regrowth
Atmos. increase
(3.3 PgC/yr)
IPCC – Intergovernmental Panel on Climate Change
Land use change (1.7 PgC/yr)
Emissions
(5.5 PgC/yr)
Atmosphere
Residual terrestrial sink
(1.9 PgC/yr)
Ocean uptake (2 PgC/yr)
Atmosphere
CO2
∆pCO2 > 0
(primarily upwelling
regions)
∆pCO2 < 0
(primarily high latitudes)
CO2
CO2
Surface Ocean
CO2 + CO32- + H2O  2HCO3-
CO2 + H2O  Organic Matter + O2
Ca2+ + 2HCO32-  CaCO3 + CO2 + H2O
CO2 + CO32- + H2O  2HCO3-
Upwelling
and vertical mixing
Sinking particulate
organic matter
(“biological pump”)
CO2 + H2O  Organic Matter + O2
Deep Ocean
HCO3-
Bottom water formation
(high latitudes)
(“solubility pump”)
CO2
CO2 + H2O  Organic Matter + O2
Ca2+ + 2HCO32-  CaCO3 + CO2 + H2O
Sediments
Oceans are largest “active” reservoir in the carbon cycle – primarily DIC
Oceans
• Link the “active” or short-term cycles with long-term
geological cycles – sink for fossil fuel CO2
• Ocean processes
– Biological cycle
– Weathering reactions and long term controls
– Atm CO2  riverine bicarb  neutralized in ocean 
returned to atm or buried in seds
• Processes that remove CO2 from atm
– Gas exchange – equilibration of sfc ocean with atm
– Biological pump
– Bottom water formation
Gas exchange
• If CO2 were a simple gas, ocean could only take
up ~3% of fossil fuel input
• Acid-base chemistry enhances ocean uptake
• Remember carbonate buffering system?
– CO32- + H2O + CO2  2 HCO3– Buffering rxn drives CO2 to bicarb
• Surface waters reach equilibrium with atm in
about 1 year
– Can keep pace with human activity
– But surface ocean too small to have capacity to remove
it all
Biological Pump
• PP and calcite ppt consume DIC
• Removed from surface ocean via particle flux
• Through interactions with carbonate system, this
lowers partial pressure (pCO2) in surface ocean
which enhances gas exchange (DpCO2< 0)
• Transports CO2 to deep ocean in the form of OM
or calcite shells
• Limitations of biological pump
– Availability of other nutrients (N, P, Fe)
– More CO2 doesn’t necessarily lead to more PP
Bottom water formation
• Removes CO2 by physical movement of
water away from surface
• Solubility pump
• CO2 is more soluble in cold water
Intermediate and deep water
• Can add CO2 through oxidation of OM
• Calcite dissolution – excess CO2 from OM
oxidation reacts with sinking calcite
Upwelling
• Intermediate waters are enriched in DIC
– Mixing with deep waters, OM oxidation &
calcite dissolution, yields some CO2 increase
• Upwelling results in excess pCO2 in surface
waters (DpCO2 > 0)
– Oceans outgas CO2
– High productivity upwelling can still be net
CO2 sinks
Global oceanic C sources and sinks
for atm C
- reflect upwelling and deep water formation
and high productivity
IPCC calculations
• Integrate data on ocean flux data
• Calculation attempts to assess short-term
sinks for excess atm CO2 due to
anthropogenic activities
Time scales of ocean C cyle
• Ocean processes slow relative to rate of fossil fuel
burning
• Bottom water circulation on timescales of 100’s of
years so equilibration with atm is slow
• Deep sea seds equilibrate with atm on timescales of
1000’s of years – where the bulk of the ocean’s
neutralizing capacity resides
• Oceans respond too slowly to take up all excess CO2
– so atm CO2 is increasing
• But, oceans have helped! Oceans have taken up 1/3
to ½ of added CO2
Atmosphere
∆pCO2 > 0
(North Pacific and
upwelling regions)
CO2
CO2
∆pCO2 < 0
(primarily high latitudes)
Equilibration
time ~1 yr
CO2
Surface Ocean
CO2 + H2O  Organic Matter + O2
CO2 + CO32- + H2O  2HCO3-
Ca2+ + 2HCO32-  CaCO3 + CO2 + H2O
CO2 + CO32- + H2O  2HCO3-
Upwelling
and vertical mixing
Deep Ocean
Sinking particulate
organic matter
(“biological pump”)
Equilibration time
~500-1000 yr
CO2 + H2O  Organic Matter + O2
HCO3-
Bottom water
formation
(high latitudes)
CO2
CO2 + H2O  Organic Matter + O2
Ca2+ + 2HCO32-  CaCO3 + CO2 + H2O
Sediments
Equilibration time
~103-104 yr
Terrestrial systems
• Variety of reservoirs that turnover on different
timescales
– Soil humus – altered remains of plants
– Land plant biomass
– Methane – source of atm methane
• Terrestrial PP ~ = to Marine PP
• Terrestrial systems store excess CO2 differently –
humus versus bicarb
• Imp for understanding system responses to increasing
CO2. Increasing CO2:
– might increase PP (neg feedback)
– might increase rates of decomposition (pos feedback)
50.3
Atmospheric CO2
Respiration
Air-Sea Exchange
Terrestrial primary
production and respiration
59.6 - 59.7
Physical
weathering
50
CO2
POC
Marine primary prod.
River transport
60
Upwelling
0.4
remineralization
Kerogen
Land Plants
Particle Rain
Oceans
Humification
Uplift of
sedimentary
rocks
POC Deposition
CO2
Benthic Fluxes
Soil humus
0.1
Recent Sediments
0.1
Carbon Burial
Sedimentary Rocks
Comparable terrestrial & marine PP
negative feedback
(temperature and
CO2 fertilization)
Terrestrial system responses
to rising CO2 and global
warming
positive feedback
(temperature enhancement of
soil respiration)
Controls on atm CO2
• Break down overall cycle to components
• Look at effects on particular components
Atmospheric CO
50.3
Air-Sea Exchange
Respiration
Terrestrial primary
production and respiration
50
CO 2
POC
Marine primary prod.
59.6 - 59.7
Physical
weathering
2
60
River transport
Upwelling
0.4
remineralization
Kerogen
Land Plants
Particle Rain
Oceans
Humification
Uplift of
sedimentary
rocks
POC Deposition
CO 2
Benthic Fluxes
Soil humus
0.1
Recent Sediments
0.1
Sedimentary Rocks
Carbon Burial
Short-term biological cycle
•
•
•
•
•
Years to decades
Does not include calcite ppt/dissolution
Does not include anthropogenic inputs
PS versus respiration nearly balanced – little loss
Some transport of org C from land to oceans
– Most gets oxidized in the ocean
• Small amount of marine OM buried in seds
– Leaves behind some O2 in atm
• Short-term cycles process a lot of CO2
- 30-50% of atm CO2 consumed per year
O2
Net
productivity
Q uickTim e™ and a
TI FF ( LZW) decom pr essor
ar e needed t o see t his pict ur e.
CO2
Organic matter
O2
Net
productivity
Q uickTim e™ and a
TI FF ( LZW) decom pr essor
ar e needed t o see t his pict ur e.
CO2
Organic matter
O2
Net
productivity
CO2
Uplift and kerogen
oxidation
O2
Q uickTim e™ and a
TI FF ( LZW) decom pr essor
ar e needed t o see t his pict ur e.
CO2
Organic matter
CO 2  H 2 O  CH 2 O  O 2
O2
Net
productivity

CO2
Uplift and kerogen
oxidation
O2
Q uickTim e™ and a
TI FF ( LZW) decom pr essor
ar e needed t o see t his pict ur e.
CO2
Organic matter
Long term org C cycles
• Millions of years
• Components include: OM in sediments, fossil fuels, atm
O2 versus CO2
• Burial of OM from Short-term cycle
– Inc P and T; most ends up as kerogen,
– Some winds up in fossil fuels (oil, coal)
– OM in shale is largest reservoir on earth (long t)
• Removal balanced by kerogen oxidation/weathering
• Affects atm O2
– Net burial leaves O2 in the atm
Also linked to pyrite burial/oxidation which requires OM
as an intermediate to catalyze the sulfate reduction
CO 2  H 2 O  CH 2 O  O 2
2 Fe 2 O 3  16 Ca

2
 16 HCO

3
2
 8 SO 4 
4 FeS 2  16 CaCO
3
 8 H 2 O  15 O 2
Produced by bacterial sulfate reduction
- linked to carbon oxidation
O2 in atm controlled by a balance between pyrite and OM
burial in seds and later oxidation on land
Without this balance atm O2 would increase to 150% of present
Levels and depletion of atm CO2 in < 10,000 years (see text)
Long-term inorg C cycle
• 100’s of millions of years
• Balance between weathering and plate tectonics
– Weathering silicate rocks consumes CO2, transferred to
the ocean as bicarb, removal of bicarb by organisms &
calcite, burial in seds, subduction, vulcanism (also
affects other cations – Ca, Mg, Na)
• Cycle is a balance between weathering (takes up
CO2) and tectonics (releases CO2)
• Plate tectonics – more vigorous then more CO2
release
• Climate sensitivity (weathering)
CaCO3 ppt.
Bicarbonate
transport
CO2 removal
CaCO3
(“regenerates” CO2)
Figs. 8-17
(“regenerates” CO2)
Short-term
Long term (organic
Long term
(inorganic/tectoni
Link with short term C cycle
In surface oceans
“adds” back CO2
Onset of modern plate tectonics
“turns this on”
Increase in surface temperature due to increase in solar luminosity
Drop in CO2 by increased weathering at higher temp
Decrease greenhouse – increase ppt of carbonates?
Bob Berner’s calculations of changes in CO2 over the Phanerozoic
Bob Berner’s calculations of changes in CO2 over the Phanerozoic
“Hot” houses
Bob Berner’s calculations of changes in CO2 over the Phanerozoic
“Hot” houses
“Ice” houses
Fig. 8-18
Effect of humans
• Pre-industrial
– Steady state on decadal to century timescales
– Ocean a net source of CO2
• Neutralizes river bicarb and oxidation of OM from rivers
– Burial of org C
• That which escaped oxidation and marine OM
• Humans
– Oceans a sink for CO2
– Increase sediment and nutrient load to rivers/ocean
– Eutrophication, hypoxia, denitrification
Fig. 10-16 A portion of the biogeochemical cycles of inorganic carbon (Cing) and organic carbon (Corg), nutrient N
and P, and suspended solids (SS) in the land–ocean system. (a) Geological, long-term system; (b) one possible
situation today. In (b), the fluxes of organic and inorganic carbon and suspended solids to the seafloor are increased
over their pristine geological values in (a). These increases are due to human activities. Notice the net heterotrophic
nature of the ocean giving rise to a net flux of carbon dioxide to the atmosphere prior to human interference in the
carbon cycle. Now more carbon dioxide enters the ocean because of the burning of fossil fuels and deforestation
practices (see Chapter 12). Fluxes are in millions of tons of C, N, P, and suspended solids per year. (After Wollast
and Mackenzie, 1989.)
CO2
All fluxes are millions
of tons of C per year
460 (net)
oxidation
Corg (terr.)
360
200
400
Riverine inputs
Cing
100 (net; approx.
50,000 (r) - 50,100(pp))
The Ocean
400
rxn. (1)
Corg (marine +terr)
Burial in sediments
Cing
140
200
rxn. (1) Ca
2
 2 HCO

3

CaCO
3
 CO 2  H 2 O
Fossil fuel burning
• Transferring large amounts of CO2 from rock cycle
to atm with no equivalent rapid uptake mechanisms!
– Ocean uptake limited by the biological pump (nuts)
– Uptake by terr. systems not rapid enough
– Accumulates in atm
• How does increase affect climate?
– Depends on time scales of increase in atm conc versus time
scales of changes in earth’s heat balance (via its circulatory
system)
– Positive and negative feedback responses
N and P Cycles
80 y-1
Global nitrogen reservoirs, fluxes and turnover times. Major reservoirs are underlined, pool
sizes and fluxes are given in Tg (1012 g) N and Tg N yr-1. Turnover times (reservoir divided
by largest flux to or from reservoir ) are in parentheses.
Atmosphere
120 (NF)
98 (DN)
121 (NF)
172 (DN)
Land
27 (RT)
Oceans
The Pre-Industrial N Cycle (fluxes = Tg N/yr)
(1860’s numbers from Galloway et al., 2004)
Greenhouse effect
Ice volume
Global sea level
Atmospheric CO2
(+)
Oceanic primary
productivity
Area of continental
shelves
Shelf denitrification
Oceanic fixed-N
inventory
Stimulates N-fixation
Fig. 14-13 The iron fertilization hypothesis for the intensification of the biological pump
during glaciations.
Stratospheric
Effects
Atmosphere
PM &
Visibility
Effects
Ozone
Effects
NOx
Energy
Production
Food
Production
NOx
NHx
NH3
Agroecosystem Effects
Crop
People
(Food; Fiber)
Human Activities
The Nitrogen
Cascade
--Indicates denitrification potential
Animal
Soil
Norg
Terrestrial
Ecosystems
GH
Effects
N2O
Forests &
Grassland
Soil
NO3
N2O
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Ocean
Effects
Aquatic Ecosystems
(t≈ 100 yr)
3.4 x (14 to 32) = 50-110
Retained in soils or denitrified
Anthro. N fixation = 140 Tg N/yr
- 41
- 8.5
-9
-3.4
61.9 Tg N/yr
~80 Tg N/yr missing ?
(?)
Nr and the Atmosphere
 NOx emissions contribute to OH,
which defines the oxidizing
capacity of the atmosphere
 NOx emissions are responsible for
tens of thousands of excess-deaths
per year in the United States
 O3 and N2O contribute to
atmospheric warming
 N2O emissions contribute to
stratospheric O3 depletion
Nr and Freshwater Ecosystems
• Surface water acidification
– Tens of thousands of lakes and
streams
– Biodiversity losses
• As reductions in SO2 emissions
continue, Nr deposition
becomes more important.
Nr and Coastal Ecosystems
• Increased algal productivity
• Shifts in community structure
• Harmful algal blooms
• Degradation of seagrass and algal
beds
• Formation of nuisance algal mats
• Coral reef destruction
• Increased oxygen demand and
hypoxia
• Increased nitrous oxide (greenhouse
gas)
Sybil Seitzinger, 2003
There are significant effects
of Nr accumulation within each
reservoir
These effects are linked temporally
and biogeochemically in the
Nitrogen Cascade
Atmosphere
Terrestrial
Ecosystems
Food
Production
NHx
Agroecosystem Effects
Crop
People
(Food; Fiber)
Animal
Soil
Norg
Human Activities
The Nitrogen
Cascade
Galloway et al., 2003a
Aquatic Ecosystems
Atmosphere
PM &
Visibility
Effects
Food
Production
NHx
NH3
Agroecosystem Effects
Crop
People
(Food; Fiber)
Human Activities
The Nitrogen
Cascade
Galloway et al., 2003a
Animal
Soil
Terrestrial
Ecosystems
Forests &
Grassland
Soil
Norg
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Ocean
Effects
Aquatic Ecosystems
Atmosphere
PM &
Visibility
Effects
Food
Production
NHx
NH3
Agroecosystem Effects
Crop
People
(Food; Fiber)
Human Activities
The Nitrogen
Cascade
Galloway et al., 2003a
Animal
Soil
Norg
Terrestrial
Ecosystems
Forests &
Grassland
Soil
NO3
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Ocean
Effects
Aquatic Ecosystems
Atmosphere
PM &
Visibility
Effects
Ozone
Effects
NOx
Energy
Production
Food
Production
NOx
NHx
NH3
Agroecosystem Effects
Crop
People
(Food; Fiber)
Human Activities
The Nitrogen
Cascade
Galloway et al., 2003a
Animal
Soil
Norg
Terrestrial
Ecosystems
Forests &
Grassland
Soil
NO3
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Ocean
Effects
Aquatic Ecosystems
Atmosphere
PM &
Visibility
Effects
Ozone
Effects
NOx
Energy
Production
Food
Production
NOx
NHx
NH3
Agroecosystem Effects
Crop
People
(Food; Fiber)
Human Activities
The Nitrogen
Cascade
--Indicates denitrification potential
Animal
Soil
Norg
Terrestrial
Ecosystems
Forests &
Grassland
Soil
NO3
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Ocean
Effects
Aquatic Ecosystems
Stratospheric
Effects
Atmosphere
PM &
Visibility
Effects
Ozone
Effects
NOx
Energy
Production
Food
Production
NOx
NHx
NH3
Agroecosystem Effects
Crop
People
(Food; Fiber)
Human Activities
The Nitrogen
Cascade
--Indicates denitrification potential
Animal
Soil
Norg
Terrestrial
Ecosystems
GH
Effects
N2O
Forests &
Grassland
Soil
NO3
N2O
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Ocean
Effects
Aquatic Ecosystems
Ind. N fix.
Population
Total react. N
Crop N fix.
Fossil fuel N F
Fig. 10-16 A portion of the biogeochemical cycles of inorganic carbon (Cing) and organic carbon (Corg), nutrient N
and P, and suspended solids (SS) in the land–ocean system. (a) Geological, long-term system; (b) one possible
situation today. In (b), the fluxes of organic and inorganic carbon and suspended solids to the seafloor are increased
over their pristine geological values in (a). These increases are due to human activities. Notice the net heterotrophic
nature of the ocean giving rise to a net flux of carbon dioxide to the atmosphere prior to human interference in the
carbon cycle. Now more carbon dioxide enters the ocean because of the burning of fossil fuels and deforestation
practices (see Chapter 12). Fluxes are in millions of tons of C, N, P, and suspended solids per year. (After Wollast
and Mackenzie, 1989.)
Sulfur cycle
Nr and Terrestrial Ecosystems
•
N is the limiting nutrient in most
temperate and polar ecosystems
•
Nr deposition increases and then
decreases forest and grassland
productivity
•
Nr additions probably decrease
biodiversity across the entire range
of deposition
Sulfate
Pyrite uplift and
weathering
Pyrite burial
Hydrothermal
uptake
2 Fe 2 O 3  16 Ca
2
 16 HCO

3
2
 8 SO 4 
4 FeS 2  16 CaCO
3
 8 H 2 O  15 O 2
CO 2  H 2 O  CH 2 O  O 2

2 Fe 2 O 3  16 Ca
2
 16 HCO

3
2
 8 SO 4 
4 FeS 2  16 CaCO
3
 8 H 2 O  15 O 2
CO 2  H 2 O  CH 2 O  O 2

2 Fe 2 O 3  16 Ca
2
 16 HCO

3
2
 8 SO 4 
4 FeS 2  16 CaCO
3
 8 H 2 O  15 O 2
Sulfate
SO2
DMS
H2S
H2S
SO2
Sulfate
SO2
SO2
DMS
H2S
H2S
SO2
William Turner, “The fighting Téméraire tugged
to her last berth to be broken up” (Tambora)
Ash and debris
from volcanic
eruptions
Edvard Munch “The Scream”
(possibly inspired by Krakatoa)
The CLAW Hypothesis
(neg. feedback; reflectivity)
Clouds
Temp.
(+)
? (+/-)
Cloud condensation
nuclei
(+)
DMS
(+)
Plankton
Fig. 14-18
Fig. 14-19