CHAPTER 6: GEOCHEMICAL CYCLESCHAPTER 6: GEOCHEMICAL CYCLES

• Most abundant elements: oxygen (in solid earth!), iron (core),

silicon (mantle), hydrogen (oceans), nitrogen, carbon, sulfur…

• The elemental compostion of the Earth has remained essentially

unchanged over its 4.5 Gyr history

– Extraterrestrial inputs (e.g., from meteorites, cometary

material) have been relatively unimportant

– Escape to space has been restricted by gravity

• Biogeochemical cycling of these elements between the different

reservoirs of the Earth system determines the composition of the

Earth’s atmosphere and the evolution of life

THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTSTHE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS

BIOGEOCHEMICAL CYCLING OF ELEMENTS:BIOGEOCHEMICAL CYCLING OF ELEMENTS:

examples of major processesexamples of major processesPhysical exchange, redox chemistry, biochemistry are involved

Surface

reservoirs

HISTORY OF EARTHHISTORY OF EARTH’’S ATMOSPHERES ATMOSPHERE

Outgassing

N2

CO2

H2O

oceans

formCO2

dissolves

Life forms

in oceans

Onset of

photosynthesi

s

O2 O2 reaches

current levels;

life invades

continents

4.5 Gy

B.P

4 Gy

B.P.3.5 Gy

B.P.

0.4 Gy

B.P. present

EVOLUTION OF OEVOLUTION OF O22 AND O AND O33 IN EARTH IN EARTH’’S ATMOSPHERES ATMOSPHERE

COMPARING THE ATMOSPHERESCOMPARING THE ATMOSPHERES

OF EARTH, VENUS, AND MARSOF EARTH, VENUS, AND MARS

3x10-41x10-23x10-3H2O (mol/mol)

1.3x10-30.216.9x10-5O2 (mol/mol)

0.007191Surface pressure (atm)

2.7x10-20.783.4x10-2N2 (mol/mol)

0.954x10-40.96CO2 (mol/mol)

340064006100Radius (km)

MarsEarthVenus

RUNAWAY GREENHOUSE EFFECT ON VENUSRUNAWAY GREENHOUSE EFFECT ON VENUS

EARTH VENUS

OXIDATION STATES OF NITROGENOXIDATION STATES OF NITROGEN

N has 5 electrons in valence shell N has 5 electrons in valence shell !!9 oxidation states from 9 oxidation states from ––3 to +53 to +5

HNO3

Nitric acid

NO3-

Nitrate

+5

NO2

Nitrogen

dioxide

+4

HONO

Nitrous acid

NO2-

Nitrite

NO

Nitric

oxide

N2O

Nitrous

oxide

N2NH3

Ammonia

NH4+

Ammonium

R1N(R2)R3

Organic N

+3+2+10-3

Decreasing oxidation number (reduction reactions)

Increasing oxidation number (oxidation reactions)

THE NITROGEN CYCLE: MAJOR PROCESSESTHE NITROGEN CYCLE: MAJOR PROCESSES

ATMOSPHEREN2 NO

HNO3

NH3/NH4+ NO3

-

orgN

BIOSPHERE

LITHOSPHERE

combustion

lightning

oxidation

deposition

assimilation

decay

nitrification

denitri-

ficationbiofixation

burial weathering

BOX MODEL OF THE NITROGEN CYCLEBOX MODEL OF THE NITROGEN CYCLE

Inventories in Tg N

Flows in Tg N yr-1

NN22O: LOW-YIELD PRODUCT OF BACTERIALO: LOW-YIELD PRODUCT OF BACTERIAL

NITRIFICATION AND DENITRIFICATIONNITRIFICATION AND DENITRIFICATION

Important as

• source of NOx radicals in stratosphere

• greenhouse gas

IPCC

[2001]

PRESENT-DAY GLOBAL BUDGETPRESENT-DAY GLOBAL BUDGET

OF ATMOSPHERIC NOF ATMOSPHERIC N22OO

12 (9 – 16)SINK (Tg N yr-1)

Photolysis and oxidation in

stratosphere

4 (3 – 5)ACCUMULATION (Tg N yr-1)

1 (1 – 2)Industrial

2 (1 – 3)Livestock

4 (1 – 15)Agricultural soils

8 (2 – 21)Anthropogenic

2 (1 – 4)Temperate soils

4 (3 – 6)Tropical soils

3 (1 - 5)Ocean

10 (5 – 16)Natural

18 (7 – 37)SOURCES (Tg N yr-1)

Although a closed budget can be constructed, uncertainties in sources are large!

IPCC

[2001]

FAST OXYGEN CYCLE: ATMOSPHERE-BIOSPHEREFAST OXYGEN CYCLE: ATMOSPHERE-BIOSPHERE

• Source of O2: photosynthesis

nCO2 + nH2O " (CH2O)n + nO2

• Sink: respiration/decay

(CH2O)n + nO2 " nCO2 + nH2O

O2

CO2

orgC

orgClitter

Photosynthesis

less respiration

decay

O2 lifetime: 5000 years

……however, abundance of organic carbon inhowever, abundance of organic carbon in

biosphere/soil/ocean reservoirs is too small to controlbiosphere/soil/ocean reservoirs is too small to control

atmospheric Oatmospheric O2 2 levelslevels

Illustrates long time scale for interhemispheric exchange

SLOW OXYGEN CYCLE: ATMOSPHERE-LITHOSPHERESLOW OXYGEN CYCLE: ATMOSPHERE-LITHOSPHERE

O2CO2

Compression

subduction

Uplift

CONTINENTOCEAN

FeS2orgC

weatheringFe2O3

H2SO4

runoff

O2CO2

Photosynthesis

decay

orgC

burial

SEDIMENTS

microbes

FeS2orgC

CO2orgC: 1x107 Pg C

FeS2: 5x106 Pg S

O2: 1.2x106 Pg OO2 lifetime: 3 million years

ATMOSPHERIC COATMOSPHERIC CO22 INCREASE OVER PAST 1000 YEARS INCREASE OVER PAST 1000 YEARS

RECENT GROWTH IN ATMOSPHERIC CORECENT GROWTH IN ATMOSPHERIC CO22

Arrows

indicate El

Nino events

Notice:

• atmospheric increase is ~50% of fossil fuel emissions

• large interannual variability

GLOBAL COGLOBAL CO22 BUDGET (Pg C yr BUDGET (Pg C yr-1-1))

IPCC [2001]IPCC [2001]

UPTAKE OF COUPTAKE OF CO22 BY THE OCEANS BY THE OCEANS

CO2(g)

CO2.H2O

CO2.H2O HCO3

- + H+

HCO3- CO3

2- + H+

KH = 3x10-2 M atm-1

K1 = 9x10-7 M

K2 = 7x10-10 M pK 1

Ocean p

H

pK 2

Net uptake:

CO2(g) + CO32- 2HCO3

-

CO2.H2O HCO3

- CO32-

OCEAN

ATMOSPHERE

LIMIT ON OCEAN UPTAKE OF COLIMIT ON OCEAN UPTAKE OF CO22::

CONSERVATION OF ALKALINITYCONSERVATION OF ALKALINITYEquilibrium calculation

for Alk = 2.25x10-3 M

pCO2 , ppm100 200 300 400 500

8.6

8.4

8.2

2

3

41.4

1.6

1.8

1.9

2.0

2.1

Ocean pH

[CO32-],

10-4 M

[HCO3-],

10-3M

[CO2.H2O]+[HCO3

-]

+[CO32-], 10-3M

The alkalinity is the excess positive charge

in the ocean to be balanced by carbon:

Alk = [Na+] + [K+] + 2[Mg2+] + 2[Ca2+]

- [Cl-] – 2[SO42-] – [Br-]

= [HCO3-] + 2[CO3

2-]

It is conserved upon addition of CO2

!uptake of CO2 is limited by the existing

supply of CO32-

Increasing Alk requires dissolution of

sediments:

CaCO3Ca2+ + CO3

2-

…which takes place over a time scale

of thousands of years

EQUILIBRIUM PARTITIONING OF COEQUILIBRIUM PARTITIONING OF CO22

BETWEEN ATMOSPHERE AND GLOBAL OCEANBETWEEN ATMOSPHERE AND GLOBAL OCEAN

Equilibrium for present-day ocean:

# only 3% of total inorganic carbon is in the atmosphere

But CO2(g) $ # [H+] $ # F ! … positive feedback to increasing CO2

Pose problem differently: how does a CO2 addition dN partition between

the atmosphere and ocean at equilibrium?

# 28% of added CO2 remains in atmosphere!

2

1 1 22 22

( ) 10.03

( ) ( )1 (1 )

[ ] [ ]

CO

oc HCO CO

a

N gF

V PK K K KN g N aq

N H H+ +

= = =+

+ + +

varies roughly as [H+]

2

1 22 2

2

( ) 10.28

( ) ( )1

[ ]([ ] 4 )

CO

oc HCO CO

a

dN gf

V PK K KdN g dN aq

N H H K+ +

= = =+

++

varies roughly as [H+]2

FURTHER LIMITATION OF COFURTHER LIMITATION OF CO2 2 UPTAKE:UPTAKE:

SLOW OCEAN TURNOVER (~ 200 years)SLOW OCEAN TURNOVER (~ 200 years)

Inventories in 1015 m3 water

Flows in 1015 m3 yr-1

Uptake by oceanic mixed layer only (VOC= 3.6x1016 m3)

would give f = 0.94 (94% of added CO2 remains in atmosphere)

EVIDENCE FOR LAND UPTAKEEVIDENCE FOR LAND UPTAKE

OF COOF CO22 FROM TRENDS IN O FROM TRENDS IN O22,,

1990-20001990-2000

NET UPTAKE OF CONET UPTAKE OF CO22 BY TERRESTRIAL BIOSPHERE BY TERRESTRIAL BIOSPHERE

(1.4 Pg C yr(1.4 Pg C yr-1-1 in the 1990s; IPCC [2001]) in the 1990s; IPCC [2001])

is a small residual of large is a small residual of large atmatm-bio exchange-bio exchange

• Gross primary production (GPP):

GPP = CO2 uptake by photosynthesis = 120 PgC yr-1

• Net primary production (NPP):

NPP = GPP – “autotrophic” respiration by green plants = 60 PgC yr-1

• Net ecosystem production (NEP):

NEP = NPP – “heterotrophic” respiration by decomposers = 10 PgC yr-1

• Net biome production (NBP)

NBP = NEP – fires/erosion/harvesting = 1.4 PgC yr-1

Atmospheric CO2 observations show that the net uptake is at northern

midlatitudes but cannot resolve American vs. Eurasian contributions

CYCLING OF CARBON WITH TERRESTRIAL BIOSPHERECYCLING OF CARBON WITH TERRESTRIAL BIOSPHERE

Inventories in PgC

Flows in PgC yr-1

Time scales are short ! net uptake from reforestation is transitory

GLOBAL PREINDUSTRIAL CARBON CYCLEGLOBAL PREINDUSTRIAL CARBON CYCLE

Inventories in PgC

Flows in PgC yr-1

PROJECTED FUTURE TRENDS IN COPROJECTED FUTURE TRENDS IN CO22 UPTAKE UPTAKE

BY OCEANS AND TERRESTRIAL BIOSPHEREBY OCEANS AND TERRESTRIAL BIOSPHERE

IPCC [2001]

PROJECTIONS OF FUTURE COPROJECTIONS OF FUTURE CO2 2 CONCENTRATIONSCONCENTRATIONS

[IPCC, 2001][IPCC, 2001]