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]
