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