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 composition 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
Nitrate and sulfate aerosols over the
past 200 years in Greenland
Nitrate (NO3-)
US anthropogenic NOx
emissions
Sulfate (SO42-)
Global anthropogenic
SO2 emissions
Year to year variability may be largely dominated by variability in
atmospheric circulation, the trend in background concentrations
reflects source strengths (anthropogenic) in the recent record.
Mayewski et al., 1990
1
THE NITROGEN CYCLE: MAJOR
PROCESSES
ATMOSPHERE
combustion
lightning
N2
NO
oxidation
HNO3
biofixation
denitrification
orgN
BIOSPHERE
deposition
decay
assimilation
NH3/NH4+
nitrification
NO3-
burial
weathering
LITHOSPHERE
Oxidation States of Nitrogen
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
Nitrous
acid
NO2Nitrite
NO2
HNO3
Nitrogen Nitric acid
dioxide
NO3Nitrate
+5
Decreasing oxidation number (reduction reactions)
THE NITROGEN CYCLE: MAJOR
PROCESSES
ATMOSPHERE
N2
combustion
lightning
NO
oxidation
HNO3
biofixation
denitrification
orgN
BIOSPHERE
burial
deposition
decay
assimilation
NH3/NH4+
nitrification
NO3weathering
LITHOSPHERE
2
BOX MODEL OF THE NITROGEN CYCLE
Inventories in Tg N, 1Tg = 1x1012 g
Flows in Tg N yr-1
What if denitrification shut off while N2 fixation still operated? How long
would it take for the atmosphere to be depleted of N2?
BOX MODEL OF THE NITROGEN CYCLE
Inventories in Tg N, 1Tg = 1x1012 g
Flows in Tg N yr-1
What if denitrification shut off while N2 fixation still operated? How long
would it take for the atmosphere to be depleted of N2?
BOX MODEL OF THE NITROGEN CYCLE
Inventories in Tg N, 1Tg = 1x1012 g
Flows in Tg N yr-1
What if denitrification shut off while N2 fixation still operated? How long
would it take for the atmosphere to be depleted of N2?
3
NOx emissions (Tg N yr-1) to troposphere
LIGHTNING
5.8
STRATOSPHERE
0.2
SOILS
5.1
FOSSIL FUEL
23.1
BIOMASS
BURNING
BIOFUEL
5.2
AIRCRAFT
2.2
0.5
Mapping of tropospheric NO2 from the
GOME satellite instrument (July 1996)
Martin et al. [2002]
Tropospheric Nitrate Formation
Slide courtesy of Shelley Kunasek
4
Question
1. Together, industrial fertilizer and fossil
fuel combustion contribute double the
natural rate of terrestrial nitrogen
fixation. Industrial fertilizer has
increased the land biofixation rate by
130 Tg N yr-1, and fossil fuel
combustion by 25 Tg N yr-1. Does this
significantly impact the land and ocean
biota reservoirs?
BOX MODEL OF THE NITROGEN CYCLE
Land reservoir
Ocean
reservoir
Inventories in Tg N, 1Tg = 1x1012 g
Flows in Tg N yr-1
Nitrate can lead to eutrophication
Crop N use efficiency typically <40%, so most washes out or is lost to
atmosphere (Canfield et al., 2010).
Diaz and Rosenberg, Science, 2008
5
Aerosol Indirect Effect
(Biogeochemical Cycles)
Which direction
and why?
Mahowald, Science, 11 November 2011
Aerosol Indirect Effect
(Biogeochemical Cycles)
Mahowald, Science, 11 November 2011
Nitrification and denitrification:
microbial source of N2O in soils and
oceans
Denitrification: NO3-
N2
N2O
Nitrification:
NH4+
anaerobic conditions
Low yield product of nitrification
and denitrification
NO3-
Oceans: nitrification ~ 4 TgN/yr
Soils: nitrification and denitrification ~7 TgN/yr
6
Anthropogenic impacts on
atmospheric N2O
Important as
• source of NOx radicals in stratosphere  stratospheric ozone depletion
• greenhouse gas
IPCC
[2001]
~15% increase since pre-industrial times
N2O Emissions
Upland crops
Rice fields
Bouwman et al., 2002.
Environmental Impacts of
Anthropogenic Fixed Nitrogen
Pollution
•Photochemical smog (NOx)
•Acid rain (HNO3)
•Eutrophication (HNO3, NH3)
•Nitrogen fertilization and species diversity (HNO3, NH3)
•Stratospheric ozone depletion (N2O)
Climate
•Greenhouse gas (N2O)
•Atmospheric chemistry and the lifetime of greenhouse gases
(such as CH4)
7
Nitrate and sulfate aerosols over the
past 200 years in Greenland
Nitrate (NO3-)
Sulfate (SO42-)
Global anthropogenic
SO2 emissions
US anthropogenic NOx
emissions
Year to year variability may be largely dominated by variability in
atmospheric circulation, the trend in background concentrations
reflects source strengths (anthropogenic) in the recent record.
Mayewski et al., 1990
SULFUR CYCLE
Most sulfur is tied up in sediments and soils. There are large fluxes to the
atmosphere, but with short atmospheric lifetimes, the atmospheric S burden is
small.
SO2: Anthropogenic (fossil fuel combustion) source comparable to natural
sources (soils, sediments, volcanoes)
Sulfur is oxidized in the atmosphere:
SO2 ---- > H2SO4
S(+IV)
S(+VI)
Sulfate is an important contributor to
acidity of precipitation. Sulfuric acid has
a low Pvap and thus partitions primarily
to aerosol/aqueous phase
Sulfate is a major component of
atmospheric aerosol and contributes to the
formation of new aerosol particles, with
both direct and indirect climate impacts.
Strongly perturbed by human activities!
Major Sulfur Reservoirs on Earth
Units of Tg S
Sulfur emissions to the
atmosphere
11% volcanic
11%
29%
60% anthropogenic
60%
29% biogenic
Charlson et al., 1992
8
Oxidation states of sulfur
Increasing oxidation number (oxidation reactions)
-2
-1
H2S(g)
CH3SSCH3(g)
Hydrogen
Dimethyl
sulfide
disulfide
CS2(g)
Carbon
disulfide
CH3SCH3
Dimethyl
sulfide (DMS)
OCS
Carbon sulfide
0
+4
+6
CH3SOCH3(g)
Dimethyl
sulfoxide
SO2 (g)
Sulfur dioxide
HSO3-(aq)
Bisulfite
SO32-(aq)
Sulfite
CH3SO3H (aq)
Methane
sulfonic acid
(MSA)
H2SO4 (aq)
Sulfuric acid
HSO4- (aq)
Bisulfate
SO42- (aq)
Sulfate
Decreasing oxidation number (reduction reactions)
The Tropospheric Sulfur Cycle: Major
Processes
CS2 DMS
H2S
OH, NO3
SO2
OH
SO42-
O3, H2O2
OH
MSA
Surface
Sulfate
aerosols:
Natural and
anthropogenic
sources
DMS from phytoplankton
+
SO2 from volcanoes
IPCC, Chapter 5, 2001
kg km-2 hr-1
9
Environmental Impacts of
Anthropogenic Sulfur
Pollution
•Particulate matter (SO42-)
•Acid rain (H2SO4)
Owen Bricker, USGS
Climate
•Aerosols  CCN
Extra slides
Box model of the sulfur cycle
DMS  3  SO2  22 3  SO42
OH / NO
OH , H O ,O
DMS
Anthropogenic
Volcanoes
(18 Tg S yr-1)
(67 Tg S yr-1)
(14 Tg S yr-1)
Dry (16%) and
Dry deposition wet dep (84%)
10
One Box Model
Chemical
production
Inflow Fin
P
X
Atmospheric “box”;
spatial distribution of X
Chemical within box is not resolved
loss
Outflow Fout
L
D
E
Deposition
Emission
dm
  sources - sinks  Fin  E  P  Fout  L  D
Mass balance equation:
dt
m
Fout
Atmospheric lifetime:  
Fout  L  D Fraction lost by export: f  F  L  D
out
Lifetimes add in parallel:
1


Fout L D
1
1
1
  


m m m  export  chem  dep
Loss rate constants add in series:
k
1

 kexport  kchem  kdep
Special Case:
Constant source, 1st order sink
dm
S
 S  km  m(t )  m(0)e  kt  (1  e  kt )
dt
k
Steady state
solution
(dm/dt = 0)
Initial condition m(0)
Characteristic time  = 1/k for
• reaching steady state
• decay of initial condition
If S, k are constant over t >> , then dm/dt = 0 and m=S/k: quasi steady state
11