GLOBAL BIOGEOCHEMICAL CYCLING: NITROGEN, OXYGEN, CARBON, SULFUR Daniel J. Jacob Harvard University

THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS Most abundant elements: iron (core), silicon (mantle), hydrogen (oceans), nitrogen, oxygen, 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 THE EARTH’S ATMOSPHERE Outgassing N 2 CO 2 H 2 O oceans form CO 2 dissolves Life forms in oceans Onset of photosynthesis O2O2 O 2 reaches current levels; life invades continents 4.5 Gy B.P 4 Gy B.P. 3.5 Gy B.P. 0.4 Gy B.P. present

COMPARING THE ATMOSPHERES OF EARTH, VENUS, AND MARS VenusEarthMars Radius (km) Surface pressure (atm) CO 2 (mol/mol)0.964x N 2 (mol/mol)3.4x x10 -2 O 2 (mol/mol)6.9x x10 -3 H 2 O (mol/mol)3x x x10 -4

OXIDATION STATES OF NITROGEN N has 5 electrons in valence shell  9 oxidation states from –3 to NH 3 Ammonia NH 4 + Ammonium R 1 N(R 2 )R 3 Organic N N2N2 N 2 O Nitrous oxide NO Nitric oxide HONO Nitrous acid NO 2 - Nitrite NO 2 Nitrogen dioxide HNO 3 Nitric acid NO 3 - Nitrate Decreasing oxidation number (reduction reactions) Increasing oxidation number (oxidation reactions)

THE NITROGEN CYCLE: MAJOR PROCESSES ATMOSPHERE N2N2 NO HNO 3 NH 3 /NH 4 + NO 3 - orgN BIOSPHERE LITHOSPHERE combustion lightning oxidation deposition assimilation decay nitrification denitri- fication biofixation burial weathering

BOX MODEL OF THE NITROGEN CYCLE Inventories in Tg N Flows in Tg N yr -1

N 2 O: LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION AND DENITRIFICATION Important as source of NO x radicals in stratosphere greenhouse gas IPCC [2001]

PRESENT-DAY GLOBAL BUDGET OF ATMOSPHERIC N 2 O SOURCES (Tg N yr -1 )18 (7 – 37) Natural10 (5 – 16) Ocean3 (1 - 5) Tropical soils4 (3 – 6) Temperate soils2 (1 – 4) Anthropogenic8 (2 – 21) Agricultural soils4 (1 – 15) Livestock2 (1 – 3) Industrial1 (1 – 2) SINK (Tg N yr -1 ) Photolysis and oxidation in stratosphere 12 (9 – 16) ACCUMULATION (Tg N yr -1 )4 (3 – 5) Although a closed budget can be constructed, uncertainties in sources are large! IPCC [2001]

FAST OXYGEN CYCLE: ATMOSPHERE-BIOSPHERE Source of O 2 : photosynthesis nCO 2 + nH 2 O  (CH 2 O) n + nO 2 Sink: respiration/decay (CH 2 O) n + nO 2  nCO 2 + nH 2 O O2O2 CO 2 orgC litter Photosynthesis less respiration decay O 2 lifetime: 5000 years

…however, abundance of organic carbon in biosphere/soil/ocean reservoirs is too small to control atmospheric O 2 levels

SLOW OXYGEN CYCLE: ATMOSPHERE-LITHOSPHERE O2O2 CO 2 Compression subduction Uplift CONTINENT OCEAN FeS 2 orgC weathering Fe 2 O 3 H 2 SO 4 runoff O2O2 CO 2 Photosynthesis decay orgC burial SEDIMENTS microbes FeS 2 orgC CO 2 orgC: 1x10 7 Pg C FeS 2 : 5x10 6 Pg S O 2 : 1.2x10 6 Pg O O 2 lifetime: 3 million years

GLOBAL PREINDUSTRIAL CARBON CYCLE Inventories in PgC Flows in PgC yr -1

ATMOSPHERIC CO 2 INCREASE OVER PAST 1000 YEARS

RECENT GROWTH IN ATMOSPHERIC CO 2 Arrows indicate El Nino events Notice: atmospheric increase is ~50% of fossil fuel emissions large interannual variability

GLOBAL CO 2 BUDGET (Pg C yr -1 ) IPCC [2001]

UPTAKE OF CO 2 BY THE OCEANS CO 2 (g) CO 2. H 2 O HCO H + HCO 3 - CO H + K H = 3x10 -2 M atm -1 K 1 = 9x10 -7 M K 2 = 7x M pK 1 Ocean pH pK 2 Net uptake: CO 2 (g) + CO HCO 3 - CO 2. H 2 O HCO 3 - CO 3 2- OCEAN ATMOSPHERE

EQUILIBRIUM PARTITIONING OF CO 2 BETWEEN ATMOSPHERE AND GLOBAL OCEAN Equilibrium for present-day ocean:  only 3% of total inorganic carbon is in the atmosphere But CO 2 (g)  [H + ]  F  … positive feedback to increasing CO 2 Pose problem differently: how does a CO 2 addition dN partition between the atmosphere and ocean at equilibrium?  28% of added CO 2 remains in atmosphere! varies roughly as [H + ] varies roughly as [H + ] 2

LIMIT ON OCEAN UPTAKE OF CO 2 : CONSERVATION OF ALKALINITY Equilibrium calculation for Alk = 2.25x10 -3 M pCO 2, ppm Ocean pH [CO 3 2- ], M [HCO 3 - ], M [CO 2. H 2 O]+[HCO 3 - ] +[CO 3 2- ], M The alkalinity is the excess positive charge in the ocean to be balanced by carbon: Alk = [Na + ] + [K + ] + 2[Mg 2+ ] + 2[Ca 2+ ] - [Cl - ] – 2[SO 4 2- ] – [Br - ] = [HCO 3 - ] + 2[CO 3 2- ] It is conserved upon addition of CO 2  uptake of CO 2 is limited by the existing supply of CO 3 2- Increasing Alk requires dissolution of sediments: CaCO 3 Ca 2+ + CO 3 2- …which takes place over a time scale of thousands of years

FURTHER LIMITATION OF CO 2 UPTAKE: SLOW OCEAN TURNOVER (~ 200 years) Inventories in m 3 water Flows in m 3 yr -1 Uptake by oceanic mixed layer only (V OC = 3.6x10 16 m 3 ) would give f = 0.94 (94% of added CO 2 remains in atmosphere)

EVIDENCE FOR LAND UPTAKE OF CO 2 FROM TRENDS IN O 2,

GLOBAL CO 2 BUDGET (Pg C yr -1 ) IPCC [2001]

NET UPTAKE OF CO 2 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 = CO 2 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 CO 2 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

PROJECTED FUTURE TRENDS IN CO 2 UPTAKE BY OCEANS AND TERRESTRIAL BIOSPHERE IPCC [2001]

PROJECTIONS OF FUTURE CO 2 CONCENTRATIONS [IPCC, 2001]

OXIDATION STATES OF SULFUR S has 6 electrons in valence shell  oxidation states from –2 to FeS 2 Pyrite H 2 S Hydrogen sulfide (CH 3 ) 2 S Dimethylsulfide (DMS) CS 2 Carbon disulfide COS Carbonyl sulfide SO 2 Sulfur dioxide H 2 SO 4 Sulfuric acid SO 4 2- Sulfate Decreasing oxidation number (reduction reactions) Increasing oxidation number (oxidation reactions)

THE GLOBAL SULFUR CYCLE SO 2 H 2 S volcanoesindustry SO 2 CS 2 SO 4 2- OCEAN 1.3x10 21 g S  10 7 years deposition runoff SO 4 2- plankton COS (CH 3 ) 2 S microbes vents FeS 2 uplift ATMOSPHERE 2.8x10 12 g S  1 week SEDIMENTS 7x10 21 g S  10 8 years

GLOBAL SULFUR EMISSION TO THE ATMOSPHERE 1990 annual mean Chin et al. [2000]

FURTHER READING Jacob, D.J., Introduction to Atmospheric Chemistry, Princeton University Press, …don’t leave home without it! Warneck, P., Chemistry of the Natural Atmosphere, 2 nd ed., Academic Press, 1999 Great chapters on biogeochemical cycles Intergovernmental Panel on Climate Change (IPCC), Climate Change 2001: The Scientific Basis, 2001 The latest on the carbon cycle