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THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS

GLOBAL BIOGEOCHEMICAL CYCLING: NITROGEN, OXYGEN, CARBON, SULFUR Daniel J. Jacob Harvard University http://www-as.harvard.edu/chemistry/trop. THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS.

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THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS

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  1. GLOBAL BIOGEOCHEMICAL CYCLING:NITROGEN, OXYGEN, CARBON, SULFUR Daniel J. JacobHarvard Universityhttp://www-as.harvard.edu/chemistry/trop

  2. 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 cyclingof these elements between the different reservoirs of the Earth system determines the composition of the Earth’s atmosphere and the evolution of life

  3. BIOGEOCHEMICAL CYCLING OF ELEMENTS:examples of major processes Physical exchange, redox chemistry, biochemistry are involved Surface reservoirs

  4. HISTORY OF THE EARTH’S ATMOSPHERE N2 CO2 H2O O2 O2 reaches current levels; life invades continents oceans form CO2 dissolves Life forms in oceans Onset of photosynthesis Outgassing 4.5 Gy B.P 4 Gy B.P. 0.4 Gy B.P. 3.5 Gy B.P. present

  5. COMPARING THE ATMOSPHERES OF EARTH, VENUS, AND MARS

  6. OXIDATION STATES OF NITROGENN has 5 electrons in valence shell a9 oxidation states from –3 to +5 Increasing oxidation number (oxidation reactions) Decreasing oxidation number (reduction reactions)

  7. THE NITROGEN CYCLE: MAJOR PROCESSES combustion lightning ATMOSPHERE N2 NO oxidation HNO3 denitri- fication biofixation deposition orgN decay NH3/NH4+ NO3- BIOSPHERE assimilation nitrification weathering burial LITHOSPHERE

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

  9. N2O: LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION AND DENITRIFICATION • Important as • source of NOx radicals in stratosphere • greenhouse gas IPCC [2001]

  10. PRESENT-DAY GLOBAL BUDGET OF ATMOSPHERIC N2O IPCC [2001] Although a closed budget can be constructed, uncertainties in sources are large!

  11. FAST OXYGEN CYCLE: ATMOSPHERE-BIOSPHERE • Source of O2: photosynthesis nCO2 + nH2O g (CH2O)n + nO2 • Sink: respiration/decay (CH2O)n + nO2 g nCO2 + nH2O CO2 O2 lifetime: 5000 years Photosynthesis less respiration O2 orgC orgC decay litter

  12. …however, abundance of organic carbon in biosphere/soil/ocean reservoirs is too small to control atmospheric O2 levels

  13. SLOW OXYGEN CYCLE: ATMOSPHERE-LITHOSPHERE O2 lifetime: 3 million years O2: 1.2x106 Pg O CO2 O2 Photosynthesis decay runoff Fe2O3 H2SO4 weathering CO2 O2 orgC FeS2 OCEAN CONTINENT orgC Uplift burial CO2 orgC: 1x107 Pg C FeS2: 5x106 Pg S microbes FeS2 orgC SEDIMENTS Compression subduction

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

  15. ATMOSPHERIC CO2 INCREASE OVER PAST 1000 YEARS

  16. RECENT GROWTH IN ATMOSPHERIC CO2 • Notice: • atmospheric increase is ~50% of fossil fuel emissions • large interannual variability Arrows indicate El Nino events

  17. GLOBAL CO2 BUDGET (Pg C yr-1) IPCC [2001] IPCC [2001]

  18. CO2(g) CO2.H2O CO2.H2O HCO3- + H+ ATMOSPHERE UPTAKE OF CO2 BY THE OCEANS KH = 3x10-2 M atm-1 OCEAN K1 = 9x10-7 M Ocean pH pK1 K2 = 7x10-10 M HCO3- CO32- + H+ pK2 CO2.H2O HCO3- CO32- Net uptake: CO2(g) + CO32- 2HCO3-

  19. EQUILIBRIUM PARTITIONING OF CO2BETWEEN ATMOSPHERE AND GLOBAL OCEAN varies roughly as [H+] Equilibrium for present-day ocean: • only 3% of total inorganic carbon is in the atmosphere But CO2(g) k [ [H+] k [ F k … positive feedback to increasing CO2 Pose problem differently: how does a CO2additiondN partition between the atmosphere and ocean at equilibrium? varies roughly as [H+]2 [ 28% of added CO2 remains in atmosphere!

  20. Equilibrium calculation for Alk = 2.25x10-3 M 100 200 300 400 500 pCO2 , ppm LIMIT ON OCEAN UPTAKE OF CO2:CONSERVATION OF ALKALINITY The alkalinity is the excess positive charge in the ocean to be balanced by carbon: Alk = [Na+] + [K+] + 2[Mg2+] + 2[Ca2+] - [Cl-] – 2[SO42-] – [Br-] = [HCO3-] + 2[CO32-] It is conserved upon addition of CO2 auptake of CO2 is limited by the existing supply of CO32- Increasing Alk requires dissolution of sediments: CaCO3 2.1 2.0 [CO2.H2O]+[HCO3-] +[CO32-], 10-3M 1.9 1.8 [HCO3-], 10-3M 1.6 1.4 4 [CO32-], 10-4 M 3 2 8.6 Ocean pH Ca2+ + CO32- 8.4 8.2 …which takes place over a time scale of thousands of years

  21. FURTHER LIMITATION OF CO2 UPTAKE: SLOW OCEAN TURNOVER (~ 200 years) Uptake by oceanic mixed layer only (VOC= 3.6x1016 m3) would givef = 0.94 (94% of added CO2 remains in atmosphere) Inventories in 1015 m3 water Flows in 1015 m3 yr-1

  22. EVIDENCE FOR LAND UPTAKE OF CO2 FROM TRENDS IN O2,1990-2000

  23. GLOBAL CO2 BUDGET (Pg C yr-1) IPCC [2001] IPCC [2001]

  24. NET UPTAKE OF CO2 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 = CO2 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 CO2 observations show that the net uptake is at northern midlatitudes but cannot resolve American vs. Eurasian contributions

  25. CYCLING OF CARBON WITH TERRESTRIAL BIOSPHERE Inventories in PgC Flows in PgC yr-1 Time scales are short a net uptake from reforestation is transitory

  26. PROJECTED FUTURE TRENDS IN CO2 UPTAKEBY OCEANS AND TERRESTRIAL BIOSPHERE IPCC [2001]

  27. PROJECTIONS OF FUTURE CO2 CONCENTRATIONS[IPCC, 2001]

  28. OXIDATION STATES OF SULFURS has 6 electrons in valence shell a oxidation states from –2 to +6 Increasing oxidation number (oxidation reactions) Decreasing oxidation number (reduction reactions)

  29. THE GLOBAL SULFUR CYCLE SO42- SO2 H2S ATMOSPHERE 2.8x1012 g S t = 1 week SO2 CS2 COS (CH3)2S deposition runoff plankton OCEAN 1.3x1021 g S t = 107 years volcanoes industry SO42- microbes vents uplift SEDIMENTS 7x1021 g S t = 108 years FeS2

  30. GLOBAL SULFUR EMISSION TO THE ATMOSPHERE1990 annual mean Chin et al. [2000]

  31. FURTHER READING • Jacob, D.J., Introduction to Atmospheric Chemistry, Princeton University Press, 1999. …don’t leave home without it! • Warneck, P., Chemistry of the Natural Atmosphere, 2nd 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

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