BeckyAlexander Slides

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-)
Sulfate (SO42-)
Global anthropogenic
US anthropogenic NOx SO2 emissions
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
THE NITROGEN CYCLE: MAJOR
PROCESSES
combustion
ATMOSPHERE lightning
N2 NO
oxidation
HNO3
biofixation
denitri-
fication deposition
orgN decay
BIOSPHERE NH3/NH4+ NO3-

assimilation nitrification
burial
weathering
LITHOSPHERE
Oxidation States of Nitrogen
Increasing oxidation number (oxidation reactions)
-3 0 +1 +2 +3 +4 +5
NH3 N2 N2O NO HONO NO2 HNO3

Ammonia Nitrous Nitric Nitrous Nitrogen Nitric acid
NH4+ oxide oxide acid dioxide NO3-
Ammonium NO2- Nitrate
R1N(R2)R3 Nitrite
Organic N
Decreasing oxidation number (reduction reactions)

THE NITROGEN CYCLE: MAJOR
PROCESSES
combustion
ATMOSPHERE lightning
N2 NO
oxidation
HNO3
biofixation
denitri-
fication deposition
orgN decay
BIOSPHERE NH3/NH4+ NO3-

assimilation nitrification
burial
weathering
LITHOSPHERE
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?

Flows in Tg N yr-1

Flows in Tg N yr-1
NOx emissions (Tg N yr-1) to troposphere
LIGHTNING STRATOSPHERE
5.8 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

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?
Ocean
reservoir
Land reservoir

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

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 anaerobic conditions
Low yield product of nitrification

N2O
and denitrification
Nitrification: NH4+ NO3-
Oceans: nitrification ~ 4 TgN/yr

Soils: nitrification and denitrification ~7 TgN/yr
Anthropogenic impacts on
atmospheric N2O
Important as
• source of NOx radicals in stratosphere  stratospheric ozone depletion
• greenhouse gas
IPCC
~15% increase since pre-industrial times [2001]
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)
Nitrate and sulfate aerosols over the
past 200 years in Greenland
Nitrate (NO3-)
Sulfate (SO42-)
Global anthropogenic
US anthropogenic NOx SO2 emissions
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
Sulfur emissions to the Units of Tg S

atmosphere
11% volcanic
11%
29%
60% anthropogenic
60%
29% biogenic
Charlson et al., 1992

Oxidation states of sulfur
Increasing oxidation number (oxidation reactions)
-2 -1 0 +4 +6
H2S(g) CH3SSCH3(g) CH3SOCH3(g) SO2 (g) H2SO4 (aq)

Hydrogen Dimethyl Dimethyl Sulfur dioxide Sulfuric acid
sulfide disulfide sulfoxide HSO3-(aq) HSO4- (aq)
CS2(g) Bisulfite Bisulfate
Carbon SO32-(aq) SO42- (aq)
disulfide Sulfite Sulfate
CH3SCH3 CH3SO3H (aq)
Dimethyl Methane
sulfide (DMS) sulfonic acid
OCS (MSA)
Carbon sulfide
Decreasing oxidation number (reduction reactions)

The Tropospheric Sulfur Cycle: Major
Processes
OH, NO3 OH
CS2 DMS SO2 SO42-
H2S 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

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
Dry (16%) and

DMS Anthropogenic Volcanoes Dry deposition wet dep (84%)
(18 Tg S yr-1) (67 Tg S yr-1) (14 Tg S yr-1)
One Box Model Atmospheric “box”;
spatial distribution of X
Chemical Chemical within box is not resolved
production loss
Inflow Fin Outflow Fout
P L
X
D
E
Deposition
Emission
dm
Mass balance equation:   sources - sinks  Fin  E  P  Fout  L  D
dt
m
Atmospheric lifetime:   Fout
Fout  L  D Fraction lost by export: f  F  L  D
out
1 Fout L D 1 1 1
Lifetimes add in parallel:      
 m m m  export  chem  dep
1
Loss rate constants add in series: k  kexport  kchem  kdep

Special Case:
Constant source, 1st order sink
dm S
 S  km  m(t )  m(0)e  (1  e  kt )
 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

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Geochemical Cycles

THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS

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

• The elemental composition of the Earth has remained

• Biogeochemical cycling of these elements between the different

Year to year variability may be largely dominated by variability in

BIOSPHERE NH3/NH4+ NO3-

Increasing oxidation number (oxidation reactions)

NH3 N2 N2O NO HONO NO2 HNO3

Decreasing oxidation number (reduction reactions)

BIOSPHERE NH3/NH4+ NO3-

Inventories in Tg N, 1Tg = 1x1012 g

Inventories in Tg N, 1Tg = 1x1012 g

Inventories in Tg N, 1Tg = 1x1012 g

Martin et al. [2002]

Slide courtesy of Shelley Kunasek

Inventories in Tg N, 1Tg = 1x1012 g

Diaz and Rosenberg, Science, 2008

Mahowald, Science, 11 November 2011

Mahowald, Science, 11 November 2011

Denitrification: NO3- N2 anaerobic conditions

Low yield product of nitrification

Nitrification: NH4+ NO3-

Oceans: nitrification ~ 4 TgN/yr

Bouwman et al., 2002.

Year to year variability may be largely dominated by variability in

SO2: Anthropogenic (fossil fuel combustion) source comparable to natural

Sulfur is oxidized in the atmosphere: SO2 ---- > H2SO4

Sulfate is a major component of

Strongly perturbed by human activities!

Sulfur emissions to the Units of Tg S

Charlson et al., 1992

H2S(g) CH3SSCH3(g) CH3SOCH3(g) SO2 (g) H2SO4 (aq)

Decreasing oxidation number (reduction reactions)

DMS from phytoplankton

IPCC, Chapter 5, 2001 kg km-2 hr-1

Owen Bricker, USGS

DMS  3  SO2  22 3  SO42 

Dry (16%) and

Initial condition m(0)

Characteristic time  = 1/k for

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