Earth Systems Science
THE CARBON CYCLE
The circulations of the atmosphere, hydrosphere, and lithosphere were studied in previous chapters. Here, we learn how nutrients are recycled in the earth system. We focus on carbon in particular due to its importance for biological activity and for global climate.
Nutrients: substances normally in the diet that are essential to organisms.
Earth Systems Science
1. carbon cycle: dynamics
2. The short term terrestrial organic carbon cycle
3. The short term marine organic carbon cycle
4. The long term organic carbon cycle
5. The short term inorganic carbon cycle; interaction with the biological pump
6. The long term inorganic carbon cycle: the carbonate-silicate geochemical cycle
THE CARBON CYCLE
THE CARBON CYCLE: DYNAMICS
atm
living terrestrial bio
dead terrestrial bio
surf ocean
deep ocean
living marine bio
organic c sedimentscarbonate sediments
gross terr prod
respiration
litterfall
terr decay
ocn2atm
atm2ocn
downwell upwell
gross ocn prod
ocn decay
net ocn prod
organic sed
inorg sed
weathering cs
gtprate
rrate
tdrate
lrate
drate
a2orate
o2arate
noprate
wcsrateurate odrate
goprate
osrate
israte
weathering oc
wocrate
beta switch
revelle switch
THE CARBON CYCLE: DYNAMICSReservoirsLocations, or types of regions, where the substance you are tracking is stored.
Value of reservoir depends on the net flux
STELLA diagram of global C cycle used in our lab, adapted Chameides and Perdue (1997)
The atmosphere
A variety of processes are related to flux into and out of the atmosphere.
These may vary seasonally, resulting in a seasonal cycle in atmospheric carbon concentration.
Steady state: same as dynamic equilibrium
THE CARBON CYCLE: DYNAMICS
Residence time, or response time, or e-folding time
Average amount of time that a substance (e.g. atom of C) remains in a reservoir under steady state conditions
Residence time = T = (reservoir size) / outflow rateor
(reservoir size) / inflow rate
T(atm) = 760 (Gt-C) / 60 (Gt-C/yr) = 12.7 yr
THE CARBON CYCLE: DYNAMICS
atm
photosynthesis
rate
rate = 1/T = 1/12.7 (1/yr) = .07874 (1/yr) = .07874 yr-1
T = time in which a perturbed system will return to 1/e, or ~38%, of original value
THE CARBON CYCLE: DYNAMICS
Residence time T is calculated at equilibrium using total inflow or total outflow
T = (reservoir size) / (total outflow) = (reservoir size) / (total inflow)
= (reservoir size) / (flux_out_1 + flux_out_2)= (reservoir size) / (flux_in_1 + flux_in_2)
stockflux in 1
flux in 2
flux out 1
flux out 2
r in 1
r in 2
r out 1
r out 2
THE CARBON CYCLE: DYNAMICS
Rate constant r is calculated using the individual flow
r_in_1 = flux_in_1 / reservoir r_in_2 = flux_in_2 / reservoirr_out_1 = flux_out_1 / reservoirr_out_2 = flux_out_2 / reservoir
stockflux in 1
flux in 2
flux out 1
flux out 2
r in 1
r in 2
r out 1
r out 2
THE CARBON CYCLE: DYNAMICS
Oxidized C that is combined with oxygen
examples: CO2, CaCO3
Reduced C that is not combined with oxygen, usually combined with other carbon atoms (C-C), hydrogen (C-H), or nitrogen (C-N)
example: organic carbon in carbohydrates
reduced substances tend to be unstable in the presence of oxygen: organic matter
decomposes, metals rust
Image Name: North America NDVIImage Date: March 1990-November 1990Image Source: AVHRR Mosaichttp://edc.usgs.gov/products/landcover.html
Organic carbon: associated with living organisms; contains C-C or C-H bonds
Photosynthesis: C is removed from the atmosphere and incorporated into carbohydrate molecule; becomes organic.
Primary productivity: amount of organic matter produced by photosynthesis (per year, per area)
Primary producers (producers, autotrophs):
organisms that store solar energy in chemical bonds (carbohydrates) for other organisms to consume
Respiration: C is returned to the atmosphere; becomes inorganic
Net primary productivity (NPP): primary productivity - respiration
THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
Image Name: Global Greenness Image Date: June 1992Image Source: AVHRR NDVIhttp://edc.usgs.gov/products/landcover.html
Photosynthesis: CO2 + H20 CH20 + 02
(solar energy)
Respiration: CO2 + H20 CH20 + 02
(release energy)
Consumers (heterotrophs): organisms that can not use solar energy directly, get their energy by consuming primary producers
THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
On land, Net Primary Productivity = 0.5 Primary Productivity
Steady state:flux in = flux out
THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
npp
leave branches stems
litter
roots
humus
charcoal
leaf ph
branch ph stem ph
root ph
leaf fall
branch fall
stem fall
litter humification
root humification
carbonization
litter resp
humus resp
charcoal oxidation
leaf npp frac
branch npp fracstem npp frac
root npp frac
l f rate
b f rate
s f rate
litt dec rate
root dec rate
hum factor
hum dec rate
root resp
hum factor
carb factor
STELLA diagram of terrestrial forest C cycle (adapted from Huggett, 1993)
Where is the atmosphere in this model?exogenous to this model
THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
aerobic: biological process that uses oxygen for metabolism
aerobe: an aerobic organism; organism whose metabolism is aerobic
metabolism: The chemical processes occurring within a living cell or organism that are necessary for the maintenance of life. In metabolism some substances are broken down to yield energy for vital processes while other substances, necessary for life, are synthesized. (dictionary.com)
THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
anaerobic: biological process whose metabolism uses no oxygen
anaerobe: an anaerobic organism; organism whose metabolism is anaerobic
Methanogenesis: an anaerobic form of metabolism
Photosynthesis: CO2 + H20 CH20 + 02
(solar energy)
Respiration: CO2 + H20 CH20 + 02
(release energy)
Methanogenesis: CO2 + CH4 2CH20 (release energy)
THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
THE SHORT-TERM MARINE ORGANIC CARBON CYCLE
Diatom (SiO2, ~50 m)
coccolithophorid (CaCO3, ~10 m)
Plankton: organisms floating in water
photic zone: ~mixed layer, upper 100m
THE SHORT-TERM MARINE ORGANIC CARBON CYCLE
foraminifer (CaCO3, ~600 m)
radiolarian (SiO2, ~50 m)
Plankton: organisms floating in water
photic zone: ~mixed layer, upper 100m
THE SHORT-TERM MARINE ORGANIC CARBON CYCLE
The Biological Pump
ThermohalineCirculation
THE SHORT-TERM MARINE ORGANIC CARBON CYCLE
The Biological Pump Nutrient Limitation
Organisms (i.e. plankton) require a variety of nutrients to grow. These nutrients are obtained from the ambient water. Nutrients are required in certain ratios: Redfield Ratios
Typically, the organism stops multiplying when one of the required nutrients is depleted. The depleted nutrient is called the limiting nutrient. If more of the nutrient were present, there would be additional growth.
http://seawifs.gsfc.nasa.gov/SEAWIFS/IMAGES/
SEAWIFS Mean Chlorophyl September 97 - August 2000 Center of gyres – downwelling – few sources of nutrients – little biological activity
Areas with nutrient input from rivers – or from upwelling – more biological activity
THE SHORT-TERM MARINE ORGANIC CARBON CYCLE
High latitudes generally more productive than low latitudes
THE LONG-TERM ORGANIC CARBON CYCLE
On long time scales the processes that are part of the short term cycle are approximately in equilibrium. However, the slower processes associated with geological processes become important.
Reservoir value flux T (Gt-C) (Gt-C/y) (y)
atmosphere 760 60 12.7soil/sed. 1600 30 53.3sed. rock 1e07 0.05 2e08
This is sometimes referred to as a “leak” from the short term organic C cycle because removal of CO2 leaves one oxygen molecule (O2 ) in the atmosphere:
CO2 + H20 CH20 + 02
THE LONG-TERM ORGANIC CARBON CYCLE
Terrestrial as well as marine organic sediments fill the ocean basins, get buried and lithify, remain in sedimentary rocks until uplift and weathering, or subduction.
THE LONG-TERM ORGANIC CARBON CYCLE
Fossil fuels are formed from the organic carbon in sedimentary rocks.
How does the burning of fossil fuels affect this system diagram?
Short circuit the flux from sedimentary rocks to the atmosphereHow does the deforestation affect this system diagram?What about reforestation?
THE INORGANIC CARBON CYCLE
Sources and sinks of atmospheric carbon that do not depend directly on biological activity exist.
source: a reservoir from which the atmosphere gains carbon
sink: a reservoir to which the atmosphere loses carbon
inorganic: not directly related to biological activity
Important reservoirs of inorganic carbon:the atmosphere, the ocean, sedimentary rocks
Sedimentary rock carbon reservoirs consist mostly of:limestone: CaCO3
dolomite: CaMg(CO3)2 (older sedimentary rocks)
THE INORGANIC CARBON CYCLE:
(CO2)g
(CO2)aq H2CO3 HCO3- CO3
2-
mixedlayer
atm
gaseous phase aqueous phase
r g
r aq
flux g to aq
flux aq to g
rates of diffusion
THE INORGANIC CARBON CYCLE:
(CO2)g
(CO2)aq H2CO3 HCO3- CO3
2-
mixedlayer
atm
Chemical A Chemicals B and C
r A
r BC
flux A to BC
flux BC to A
rates of chemical reactions
THE INORGANIC CARBON CYCLE
Atmosphere – Ocean Carbon Exchange
CO2 diffuses between the atmosphere and the ocean
Diffusion: the free or random movement of a substance from a region in which it is highly concentrated into one in which it is less concentrated. In gases and liquids, it happens spontaneously at the molecular level, and continues until the concentration becomes uniform … (Kemp, The Environment Dictionary)
CO2 dissolves in water
dissolve: when two substances go into solutionsolution: a homogeneous mixture formed when substances in different states … are combined together, and the mixture takes on the state of one of the components (Kemp, The Environment Dictionary)
THE INORGANIC CARBON CYCLE
Atmosphere – Ocean Carbon Exchange
CO2 diffuses between the atmosphere and the ocean
The direction and magnitude of diffusion depends on the partial pressure of CO2 in the atmosphere, the amount of CO2 in solution, the solubility of CO2 in water, and on the rate constant of the diffusion process
partial pressure: pressure of one particular gas in the atmospheresolubility: the maximum amount of a substance that will dissolve in a
specified liquid (similar to saturation in the atmosphere)rate constant: number representing speed with which diffusion occurs
(CO2)g (CO2)aqwhere g=gas, aq=aqueous = dissolved in water
THE INORGANIC CARBON CYCLE
Chemistry of Inorganic Carbon in Water
dissolved CO2 generates carbonic acid
CO2 + H2O H2CO3
this reaction can go either direction, depending on the relative concentrations of reactants and products. Reaction occurs until chemical equilibrium is reached
reactants: left hand side of equationproducts: right hand side of equationchemical equilibrium: when relative concentrations of reactants and
products reach the point where no net change in concentrations occurs
THE INORGANIC CARBON CYCLE
Chemistry of Inorganic Carbon in Water
carbonic acid generates hydrogen ions, bicarbonate ions, carbonate ions
H2CO3 H+ + HCO3-
(bicarbonate ion)
HCO3- H+ + CO32-
(carbonate ion)H+ concentration determines the pH of water
pH = -log[H+]where [H+] is the concentration of hydrogen ions.
These reactions tend towards chemical equilibrium, depending on the concentrations of bicarbonate and carbonate, the concentration of the H+ ion (pH), and the temperature.
Summary
(CO2)g (CO2)aq diffusion ocean - atm.
CO2 + H2O H2CO3 CO2 - carbonic acid
H2CO3 H+ + HCO3- carbonic acid - bicarbonate
HCO3- H+ + CO3
2- bicarbonate - carbonate
Interaction with the biological pump
CO2 + H20 CH20 + 02 photosynthesis/decomposition
Ca2+ + 2HCO3- CaCO3 + H2CO3 calcium carbonate shells
Net Effect: plankton remove CO2 from surface water, drawing more CO2 out of the atmosphere. The organic material, and calcium carbonate shells, eventually sink into the deep ocean.
THE INORGANIC CARBON CYCLE
THE INORGANIC CARBON CYCLE:interaction with the biological pump
(CO2)g
(CO2)aq H2CO3 HCO3- CO3
2-
mixedlayer
atm
decomposition coccolithophorid (CaCO3, ~10 m)
Diatom (SiO2, ~50 m)
production
foraminifer (CaCO3, ~600 m)
radiolarian (SiO2, ~50 m)
consumption
to the deep oceanblue = inorganic chemistryred = organic carbon dioxide effectgreen = organic carbonate effect
Net effect: drawdown of atm CO2!Net effect: drawdown of atm CO2!
THE INORGANIC CARBON CYCLE:interaction with the biological pump
(CO2)g
(CO2)aq H2CO3 HCO3- CO3
2-
mixedlayer
atm
coccolithophorid (CaCO3, ~10 m)
foraminifer (CaCO3, ~600 m)
blue = inorganic chemistryred = organic carbon dioxide effectgreen = organic carbonate effect
Net effect: drawdown of atm CO2!Net effect: drawdown of atm CO2!
THE INORGANIC CARBON CYCLE:interaction with the biological pump
(CO2)g
(CO2)aq H2CO3 HCO3- CO3
2-
mixedlayer
atm
Equilibrium values depend on pH and temperature
pH = -log[H+]
Dissolved CO2 contributes to acidification
H+ ionH+ ion
THE INORGANIC CARBON CYCLE:interaction with the biological pump
From weathering to deposition on the sea floor
Rain drops are slightly acidic to due atm CO2 dissolving in them, resulting in carbonic acid.
Carbonate Weathering:CaCO3 + H2CO3 Ca2+ + 2HCO3-
calcium carbonic calcium bicarbonatecarbonate acid ion ion
Silicate Weathering:CaSiO3 + 2H2CO3 Ca2+ + 2HCO3
- + SiO2 + H2Owollastonite carbonic calcium bicarbonate silica water acid ion ion
THE INORGANIC CARBON CYCLE:interaction with the biological pump
From weathering to deposition on the sea floor
These reactions provide the weathered material that gets washed into the oceans and is available for production of calcium carbonate and silicate shells by plankton in the mixed layer.
As the plankton die, and the shells sink into the deep ocean, they do not dissolve much at first. The shallow and middle depths of the ocean are saturated with respect to CaCO3: there is little acidity to dissolve the shells.
In deeper parts of the ocean they do dissolve more, as these waters often have higher concentrations of dissolved CO2, and therefore carbonic acid, due to the decomposition of organic matter.
THE INORGANIC CARBON CYCLE:interaction with the biological pump
From weathering to deposition on the sea floor
carbonate compensation depth (CCD): depth below which the carbonate shells dissolve faster than the rate of shells settling through the water column.
Below the CCD, carbonate shells dissolve, no carbonate is deposited on the ocean floor.
THE INORGANIC CARBON CYCLE:interaction with the biological pump
From weathering to deposition on the sea floor
The net result of weathering to deposition is that some carbon is removed from the atmosphere and ends up in calcium carbonate on the ocean floor.
Thus, weathering removes CO2 from the atmosphere and stores it in calcium carbonate sediments. This is another CO2 “leak” from the system. If there were no other source of CO2 into the atmosphere, CO2 concentrations would drop to zero in about a million years.
THE INORGANIC CARBON CYCLE:interaction with the biological pump
Summary of the cycle
What process makes up for the CO2 leakage from the atmosphere associated with weathering?
Volcanism, and emission through mid-ocean ridges
THE LONG TERM INORGANIC CARBON CYCLE:The Carbonate-Silicate Geochemical Cycle
Carbonate metamorphism:CaCO3 + SiO2 CaSiO3 + CO2 calcite silica wollastonite carbon dioxide
Net effect: return of CO2 to the atm!Net effect: return of CO2 to the atm!
THE LONG TERM INORGANIC CARBON CYCLE:The Carbonate-Silicate Geochemical Cycle
So, atmospheric CO2 loss by weathering is compensated for by CO2 emissions associated with plate tectonics (volcanic and mid-ocean ridge emissions).
Feedbacks that affect the weathering rate are believed to play a role in regulating atmospheric CO2 levels, and therefore climate, over geologic time scales.
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