Geochemical Cycles
• 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
THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS
BIOGEOCHEMICAL CYCLING OF ELEMENTS:examples of major processes
Physical exchange, redox chemistry, biochemistry are involved
Surfacereservoirs
Nitrate and sulfate aerosols over the past 200 years in Greenland
Nitrate (NO3-) Sulfate (SO4
2-)
Mayewski et al., 1990
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.
US anthropogenic NOx emissions
Global anthropogenic SO2 emissions
THE NITROGEN CYCLE: MAJOR PROCESSES
ATMOSPHERE N2 NO
HNO3
NH3/NH4+ NO3
-
orgN
BIOSPHERE
LITHOSPHERE
combustionlightning
oxidation
deposition
assimilation
decay
nitrification
denitri-fication
biofixation
burialweathering
Oxidation States of Nitrogen
-3 0 +1 +2 +3 +4 +5
NH3
AmmoniaNH4
+
AmmoniumR1N(R2)R3
Organic N
N2 N2ONitrousoxide
NONitric oxide
HONONitrous acidNO2
-
Nitrite
NO2
Nitrogen dioxide
HNO3
Nitric acidNO3
-
Nitrate
Decreasing oxidation number (reduction reactions)
Increasing oxidation number (oxidation reactions)
THE NITROGEN CYCLE: MAJOR PROCESSES
ATMOSPHERE N2 NO
HNO3
NH3/NH4+ NO3
-
orgN
BIOSPHERE
LITHOSPHERE
combustionlightning
oxidation
deposition
assimilation
decay
nitrification
denitri-fication
biofixation
burialweathering
BOX MODEL OF THE NITROGEN CYCLE
Inventories in Tg N, 1Tg = 1x1012 gFlows 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 gFlows 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 gFlows 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?
NOx emissions (Tg N yr-1) to troposphere
FOSSIL FUEL23.1
AIRCRAFT0.5
BIOFUEL2.2
BIOMASSBURNING
5.2
SOILS5.1
LIGHTNING5.8
STRATOSPHERE0.2
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?
BOX MODEL OF THE NITROGEN CYCLE
Inventories in Tg N, 1Tg = 1x1012 gFlows in Tg N yr-1
Land reservoir
Ocean reservoir
Nitrate can lead to eutrophication
Diaz and Rosenberg, Science, 2008
Crop N use efficiency typically <40%, so most washes out or is lost to atmosphere (Canfield et al., 2010).
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
N2O
Nitrification: NH4+ NO3
-
Oceans: nitrification ~ 4 TgN/yrSoils: nitrification and denitrification ~7 TgN/yr
Low yield product of nitrification and denitrification
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
Rice fields
Upland crops
Bouwman et al., 2002.
N2O Emissions
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 (SO4
2-)
Mayewski et al., 1990
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.
US anthropogenic NOx emissions
Global anthropogenic SO2 emissions
SULFUR CYCLEMost 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 ---- > H2SO4S(+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
Charlson et al., 1992
Units of Tg S
60%
29%
11%
Sulfur emissions to the atmosphere
60% anthropogenic
29% biogenic
11% volcanic
Oxidation states of sulfur
-2 -1 0 +4 +6
H2S(g)Hydrogen sulfideCS2(g)Carbon disulfideCH3SCH3
Dimethyl sulfide (DMS)OCSCarbon sulfide
CH3SSCH3(g)Dimethyl disulfide
CH3SOCH3(g)Dimethyl sulfoxide
SO2 (g)Sulfur dioxideHSO3
-(aq)BisulfiteSO3
2-(aq)SulfiteCH3SO3H (aq)Methane sulfonic acid (MSA)
H2SO4 (aq)Sulfuric acidHSO4
- (aq)BisulfateSO4
2- (aq)Sulfate
Decreasing oxidation number (reduction reactions)
Increasing oxidation number (oxidation reactions)
Surface
DMSCS2
H2SSO2 SO4
2-OH
O3, H2O2
OH, NO3
MSA
OH
The Tropospheric Sulfur Cycle: Major Processes
Sulfate aerosols:
Natural and anthropogenic
sources
kg km-2 hr-1
DMS from phytoplankton+ SO2 from volcanoes
IPCC, Chapter 5, 2001
Environmental Impacts of Anthropogenic Sulfur
Pollution•Particulate matter (SO4
2-)
•Acid rain (H2SO4)
Climate•Aerosols CCN
Owen Bricker, USGS
Extra slides
Box model of the sulfur cycle
Anthropogenic (67 Tg S yr-1)
DMS (18 Tg S yr-1)
Volcanoes (14 Tg S yr-1)
24
,,2
/ 3223 SOSODMS OOHOHNOOH
Dry depositionDry (16%) and wet dep (84%)
One Box Model
Inflow Fin Outflow FoutXE
Emission Deposition
D
Chemicalproduction
P L
Chemicalloss
Atmospheric “box”;spatial distribution of X within box is not resolved
out
Atmospheric lifetime: mF L D
Fraction lost by export: out
out
FfF L D
Lifetimes add in parallel: export chem dep
1 1 1 1outF L Dm m m
Loss rate constants add in series:export chem dep
1k k k k
Mass balance equation: sources - sinks in outdm F E P F L Ddt
Special Case: Constant source, 1st order sink
( ) (0) (1 )kt ktdm SS km m t m e edt 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