earth systems science chapter 8

Earth Systems ScienceChapter 8

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 ScienceChapter 8

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


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


atm

photosynthesis

raterate = 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


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


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: preindustrial equ. stocks and flows

atm



surf ocean

deep ocean

living marine bio


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

weathering oc

600

1E3 1.8

2E77E7

4E4

800

1450

100

5050

0.01

80

80 40

30

0.01

4

3733

0.2

0.2

THE CARBON CYCLE: mean residence times (years)

atm



surf ocean

deep ocean

living marine bio


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

weathering oc

3-4

6-7 .05

1E81E8

1E3

8

30


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


On land, Net Primary Productivity = 0.5 Primary Productivity

Steady state:flux in = flux out


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 fracbranch npp frac

stem 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


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)


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 m)

coccolithophorid (CaCO3, ~10 m)

Plankton: organisms floating in water

photic zone: ~mixed layer, upper 100m


foraminifer (CaCO3, ~600 m)

radiolarian (SiO2, ~50 m)

Plankton: organisms floating in water

photic zone: ~mixed layer, upper 100m


The Biological Pump

ThermohalineCirculation


The Biological Pump Nutrient LimitationOrganisms (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


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


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.


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


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


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)


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


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


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:interaction with the biological pump

(CO2)g


2-

mixedlayer

atm

decomposition coccolithophorid (CaCO3, ~10 m)

Diatom (SiO2, ~50 m)

production


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!


(CO2)g


2-

mixedlayer

atm

coccolithophorid (CaCO3, ~10 m)


blue = inorganic chemistryred = organic carbon dioxide effectgreen = organic carbonate effect

Net effect: drawdown of atm CO2!Net effect: drawdown of atm CO2!


(CO2)g


2-

mixedlayer

atm

Equilibrium values depend on pH and temperature

pH = -log[H+]

Dissolved CO2 contributes to acidification

H+ ionH+ ion


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



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.



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 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.


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.

earth systems science chapter 8

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