CHAPTER 6: GEOCHEMICAL CYCLES
THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS
•
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
BIOGEOCHEMICAL CYCLING OF ELEMENTS:
examples of major processes
Physical exchange, redox chemistry, biochemistry are involved
Surface
reservoirs
HISTORY OF EARTH’S ATMOSPHERE
N2
CO2
H2O
O2
oceans CO2
form
dissolves
Outgassing
4.5 Gy
B.P
Life forms
in oceans
4 Gy
B.P.
O2 reaches
current levels;
life invades
continents
Onset of
photosynthesi
s
3.5 Gy
B.P.
0.4 Gy
B.P.
present
EVOLUTION OF O2 AND O3 IN EARTH’S ATMOSPHERE
COMPARING THE ATMOSPHERES
OF EARTH, VENUS, AND MARS
Venus
Earth
Mars
6100
6400
3400
91
1
0.007
0.96
4x10-4
0.95
N2 (mol/mol)
3.4x10-2
0.78
2.7x10-2
O2 (mol/mol)
6.9x10-5
0.21
1.3x10-3
3x10-3
1x10-2
3x10-4
Radius (km)
Surface pressure (atm)
CO2 (mol/mol)
H2O (mol/mol)
RUNAWAY GREENHOUSE EFFECT ON VENUS
EARTH
VENUS
OXIDATION STATES OF NITROGEN
N has 5 electrons in valence shell !9 oxidation states from –3 to +5
Increasing oxidation number (oxidation reactions)
-3
0
+1
+2
+3
+4
NH3
Ammonia
NH4+
Ammonium
R1N(R2)R3
Organic N
N2
N2O
Nitrous
oxide
NO
Nitric
oxide
HONO
NO2
Nitrous acid Nitrogen
dioxide
NO2Nitrite
Decreasing oxidation number (reduction reactions)
+5
HNO3
Nitric acid
NO3Nitrate
THE NITROGEN CYCLE: MAJOR PROCESSES
ATMOSPHERE
N2
combustion
lightning
NO
oxidation
HNO3
biofixation
orgN
BIOSPHERE
burial
LITHOSPHERE
denitrification
deposition
decay
assimilation
NH3/NH4+
nitrification
NO3weathering
BOX MODEL OF THE NITROGEN CYCLE
Inventories in Tg N
Flows in Tg N yr-1
N2O: LOW-YIELD PRODUCT OF BACTERIAL
NITRIFICATION AND DENITRIFICATION
Important as
• source of NOx radicals in stratosphere
• greenhouse gas
IPCC
[2001]
PRESENT-DAY GLOBAL BUDGET
OF ATMOSPHERIC N2O
SOURCES (Tg N yr-1)
Natural
18 (7 – 37)
10 (5 – 16)
Ocean
3 (1 - 5)
Tropical soils
4 (3 – 6)
Temperate soils
2 (1 – 4)
Anthropogenic
8 (2 – 21)
Agricultural soils
4 (1 – 15)
Livestock
2 (1 – 3)
Industrial
1 (1 – 2)
SINK (Tg N yr-1)
Photolysis and oxidation in
stratosphere
12 (9 – 16)
ACCUMULATION (Tg N yr-1)
4 (3 – 5)
IPCC
[2001]
Although a closed budget can be constructed, uncertainties in sources are large!
FAST OXYGEN CYCLE: ATMOSPHERE-BIOSPHERE
• Source of O2: photosynthesis
nCO2 + nH2O " (CH2O)n + nO2
• Sink: respiration/decay
(CH2O)n + nO2 "
nCO2 + nH2O
O2 lifetime: 5000 years
CO2
Photosynthesis
less respiration
O2
orgC
litter
orgC
decay
…however, abundance of organic carbon in
biosphere/soil/ocean reservoirs is too small to control
atmospheric O2 levels
Illustrates long time scale for interhemispheric exchange
SLOW OXYGEN CYCLE: ATMOSPHERE-LITHOSPHERE
O2 lifetime: 3 million years
O2: 1.2x106 Pg O
O2
OCEAN
Photosynthesis
decay
CO2
Fe2O3runoff weathering
H2SO4
FeS2 orgC
CONTINENT
orgC
Uplift
burial
CO2
orgC: 1x107 Pg C
FeS2: 5x106 Pg S
microbes
SEDIMENTS
CO2
O2
orgC
FeS2
Compression
subduction
ATMOSPHERIC CO2 INCREASE OVER PAST 1000 YEARS
RECENT GROWTH IN ATMOSPHERIC CO2
Notice:
• atmospheric increase is ~50% of fossil fuel emissions
• large interannual variability
Arrows
indicate El
Nino events
GLOBAL CO2 BUDGET (Pg C yr-1)
IPCC [2001]
IPCC [2001]
ATMOSPHERE
CO2(g)
KH = 3x10-2 M atm-1
CO2.H2O
OCEAN
K1 = 9x10-7 M CO2.H2O
K2 =
7x10-10
UPTAKE OF CO2 BY THE OCEANS
M
HCO3
-
HCO3- + H+
2-
CO3 +
pH
H+
CO2.H2O
Net uptake:
CO2(g) + CO32-
2HCO3-
pK 1
n
a
2
ce
O pK
HCO3-
CO32-
Equilibrium calculation
for Alk = 2.25x10-3 M
The alkalinity is the excess positive charge
in the ocean to be balanced by carbon:
2.1
2.0
1.9
[CO2.H2O]+[HCO3-]
+[CO32-], 10-3M
1.4
4
3
[HCO3-],
10-3M
[CO32-],
10-4 M
supply of CO32-
Increasing Alk requires dissolution of
sediments:
8.6
8.2
100
It is conserved upon addition of CO2
!uptake of CO2 is limited by the existing
2
8.4
Alk = [Na+] + [K+] + 2[Mg2+] + 2[Ca2+]
- [Cl-] – 2[SO42-] – [Br-]
= [HCO3-] + 2[CO32-]
1.8
1.6
LIMIT ON OCEAN UPTAKE OF CO2:
CONSERVATION OF ALKALINITY
Ocean pH
200 300 400 500
pCO2 , ppm
CaCO3
Ca2+ + CO32-
…which takes place over a time scale
of thousands of years
EQUILIBRIUM PARTITIONING OF CO2
BETWEEN ATMOSPHERE AND GLOBAL OCEAN
Equilibrium for present-day ocean:
N CO 2 ( g )
1
F=
=
= 0.03
V
PK
K
K
K
N CO 2 ( g ) + N CO 2 (aq ) 1 + oc H (1 + 1 + 1 2 )
Na
[ H + ] [ H + ]2
varies roughly as [H+]
# 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?
f =
dN CO 2 ( g )
=
dN CO 2 ( g ) + dN CO 2 (aq ) 1 +
1
= 0.28
Voc PK H K1 K 2
N a [ H + ]([ H + ] + 4 K 2 )
varies roughly as [H+]2
# 28% of added CO2 remains in atmosphere!
FURTHER LIMITATION OF CO2 UPTAKE:
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 UPTAKE
OF CO2 FROM TRENDS IN O2,
1990-2000
NET UPTAKE OF CO2 BY TERRESTRIAL BIOSPHERE
(1.4 Pg C yr-1 in the 1990s; IPCC [2001])
is a small residual of large atm-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 BIOSPHERE
Inventories in PgC
Flows in PgC yr-1
Time scales are short !
net uptake from reforestation is transitory
GLOBAL PREINDUSTRIAL CARBON CYCLE
Inventories in PgC
Flows in PgC yr-1
PROJECTED FUTURE TRENDS IN CO2 UPTAKE
BY OCEANS AND TERRESTRIAL BIOSPHERE
IPCC [2001]
PROJECTIONS OF FUTURE CO2 CONCENTRATIONS
[IPCC, 2001]