1. No category

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
THE CARBON CYCLE
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: DYNAMICS
THE CARBON CYCLE: DYNAMICS
Reservoirs
Locations, or types of regions,
where the substance you are
tracking is stored.
gtprate
beta switch
gross terr prod
atm
ocn2atm
weathering oc
o2arate
wocrate
respiration
Value of reservoir depends on the
net flux
liv ing terrestrial bio
atm2ocn
rrate
terr decay
goprate
surf ocean
gross ocn prod
a2orate
liv ing marine bio
tdrate
weathering cs
ocn decay
litterf all
lrate
rev elle switch
wcsrate
urate
dead terrestrial bio
downwell
odrate
upwell
drate
noprate
organic sed
osrate
net ocn prod
deep ocean
inorg sed
organic c sediments
carbonate sediments
israte
STELLA diagram of global C cycle used in our
lab, adapted Chameides and Perdue (1997)
THE CARBON CYCLE: DYNAMICS
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 rate
or
(reservoir size) / inflow rate
T(atm) = 760 (Gt-C) / 60 (Gt-C/yr) = 12.7 yr
T = time in which a perturbed system will return to 1/e, or ~38%, of original value
rate = 1/T = 1/12.7 (1/yr) = .07874 (1/yr) = .07874 yr-1
rate
atm
photosy nthesis
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)
r in 1
f lux in 1
f lux in 2
r out 1
stock
f lux out 1
f lux out 2
r in 2
r out 2
THE CARBON CYCLE: DYNAMICS
Rate constant r is calculated using the individual flow
r_in_1
r_in_2
r_out_1
r_out_2
=
=
=
=
flux_in_1 / reservoir
flux_in_2 / reservoir
flux_out_1 / reservoir
flux_out_2 / reservoir
r in 1
f lux in 1
f lux in 2
r out 1
stock
f lux out 1
f lux out 2
r in 2
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 (CH), 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
THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
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
Image Name: North America NDVI
Image Date: March 1990-November 1990
Image Source: AVHRR Mosaic
http://edc.usgs.gov/products/landcover.html
THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
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
Image Name: Global Greenness Image Date: June 1992
Image Source: AVHRR NDVI
http://edc.usgs.gov/products/landcover.html
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
Where is the
atmosphere in
this model?
exogenous to
this model
npp
leaf ph
branch npp f rac
leaf npp f rac
stem npp f rac
branch ph
leav e
stem ph
branches
stems
branch f all
b f rate
litter
stem f all
l f rate
s f rate
root ph
root npp f rac
leaf f all
litter resp
litter humif ication
litt dec rate
hum f actor
roots
root humif ication
humus resp
humus
hum dec rate
carbonization
root dec rate
root resp
carb f actor
hum f actor
charcoal
charcoal oxidation
STELLA diagram of terrestrial forest C cycle (adapted from Huggett, 1993)
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 MARINE
ORGANIC CARBON CYCLE
Diatom (SiO2, ~50 mm)
Plankton: organisms floating in water
coccolithophorid (CaCO3,
~10 mm)
photic zone: ~mixed layer, upper 100m
foraminifer (CaCO3, ~600 mm)
THE SHORT-TERM MARINE
ORGANIC CARBON CYCLE
Plankton: organisms floating in water
radiolarian (SiO2, ~50 mm)
photic zone: ~mixed layer, upper 100m
THE SHORT-TERM MARINE ORGANIC CARBON CYCLE
The Biological Pump
Thermohaline
Circulation
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.
THE SHORT-TERM MARINE ORGANIC CARBON CYCLE
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
High latitudes generally more productive than low latitudes
http://seawifs.gsfc.nasa.gov/SEAWIFS/IMAGES/
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
soil/sed. 1600
30
sed. rock 1e07 0.05
12.7
53.3
2e08
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.
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
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 atmosphere
How 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:
atm
(CO2)g
(CO2)aq
H2CO3
HCO3-
rg
mixed
layer
gaseous phase
f lux g to aq
aqueous phase
f lux aq to g
r aq
rates of diffusion
CO32-
THE INORGANIC CARBON CYCLE:
atm
(CO2)g
(CO2)aq
H2CO3
HCO3-
rA
mixed
layer
Chemical A
f lux A to BC
Chemicals B and C
f lux BC to A
r BC
rates of chemical reactions
CO32-
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 solution
solution: 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 atmosphere
solubility: 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)aq
where 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 equation
products: right hand side of equation
chemical 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.
THE INORGANIC CARBON CYCLE
Summary
(CO2)g  (CO2)aq
diffusion ocean - atm.
CO2 + H2O  H2CO3
CO2 - carbonic acid
H2CO3
 H+ + HCO3-
HCO3-  H+ + CO32-
carbonic acid - bicarbonate
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.
atm
THE INORGANIC CARBON CYCLE:
interaction with the biological pump
(CO2)g
Net effect: drawdown of atm CO2!
production (CO )
2 aq
decomposition
mixed
layer
H2CO3
HCO3-
CO32-
coccolithophorid
(CaCO3,
~10 mm)
Diatom
(SiO2, ~50
mm)
consumption
foraminifer
(CaCO3, ~600 mm)
radiolarian
(SiO2, ~50 mm)
to the deep ocean
blue = inorganic chemistry
red = organic carbon dioxide effect
green = organic carbonate effect
atm
THE INORGANIC CARBON CYCLE:
interaction with the biological pump
(CO2)g
Net effect: drawdown of atm CO2!
(CO2)aq
mixed
layer
blue = inorganic chemistry
red = organic carbon dioxide effect
green = organic carbonate effect
H2CO3
HCO3-
CO32-
coccolithophorid foraminifer
(CaCO3, ~600 mm)
(CaCO3,
~10 mm)
atm
THE INORGANIC CARBON CYCLE:
interaction with the biological pump
(CO2)g
(CO2)aq
mixed
layer
H2CO3
H+ ion
HCO3-
CO32-
H+ ion
Equilibrium values depend on pH and temperature
pH = -log[H+]
Dissolved CO2 contributes to acidification
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+ + 2HCO3calcium carbonic calcium bicarbonate
carbonate acid
ion
ion
Silicate Weathering:
CaSiO3 + 2H2CO3  Ca2+ + 2HCO3- + SiO2 + H2O
wollastonite 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
Net effect: return of CO2 to the atm!
Carbonate metamorphism:
CaCO3 + SiO2  CaSiO3 + CO2
calcite
silica wollastonite carbon
dioxide
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 midocean 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.