William S. Reeburgh Earth System Science University of California Irvine Reeburgh@uci

1. Atmospheric Methane: How well can we apportion present sources and predict future changes? William S. Reeburgh Earth System Science University of California Irvine Reeburgh@uci.edu

4. Wahlen, 1993

8. GeochemicalApproaches • Four R’s of Geochemistry(Dayton Carritt) • Routes • Rates • Reactions • Reservoirs • Inverse Chemical Engineering(W. S. Broecker) Considers Earth as a chemical plant with no blueprints. Task of geochemistry is to produce the missing blueprints with measurements of concentrations, fluxes, reaction rates, etc.

10. Rate MeasurementsFlux Measurements(chamber, eddy flux)Sulfate Reduction 35SO4-2 H235S (1.4 Ci mmole-1) (carrier-free) Methane Oxidation Aerobic and Anaerobic Carbon (14C) 14C-CH414CO2 (55 mCi mmol-1) Hydrogen (3H) 3H-CH4 3H20 (3 Ci mmol-1)

11. Methane Sources Microbial Competitive substrates (anoxic conditions) CO2 reduction CO2 + 4H2 CH4 + 2H2O Acetate fermentation CH3COOH CH4 + CO2 Non-competitive substrates (oxic conditions?) Methylated Compounds (methylamines, DMS, DMDS, methane thiol, methyl phosphonate)

12. Methane Sources Abiotic “Serpentinization Reaction” 6[(Mg1.5Fe0.5)SiO4] + 7H2O olivine 3[Mg3Si2O5(OH)4] + Fe3O4 + H2 serpentinemagenetite and CO2 + 4H2(300 C, 500bar) CH4 + 2H2O “Thermal Cracking”, Pyrolysis 14CH4 added by PWR’s

13. Methane Sinks Microbial Aerobic Oxidation 2CH4 + O2 2CO2 + 2H20 (decreases pH, dissolves carbonates) Anaerobic Oxidation (AOM or AMO) CH4 + SO4-2 HCO3- + HS- + H20 (increases alkalinity; isotopically light carbonates precipitate.) “Reverse Methanogenesis” CH4 + 2H20 CO2 + 4H2

14. Methane Sinks Photochemical Oxidation (principal atmospheric sink) O3 + h O(1D) + O2 = 315 nm O(1D) + H2O 2OH CH4 + OH H20 + CH3

16. Cicerone & Oremland, 1988

18. Methane budget is well-constrained. We know the total well, but individual source terms are uncertain to a factor of 2 or more. A “bird’s eye” budget; considers net additions to the atmosphere. A net atmospheric budget. We can consider consumption or oxidation, but the previous constraints do not apply. Oxidation before emission to atmosphere has a large effect.

19. Inversions Fung et al., 1997, JGR Hein et al., 1997, GBC Mikalof-Fletcher et al., 2004, GBC (CH4 & 13C-CH4) Butler et al., 2005, JGR Van der Werf et al., 2004, Science (wildfire contributions) Bousquet et al., 2000, Nature

23. Recently Reported CH4 Sources Aerobic Methane Production by Plants Siberian thaw lakes/Yedoma soils *Ocean Vent Additions: CH4-consuming benthic communities *Methane Clathrate Hydrate, Mud Volcano Additions *Large “Fossil CH4” Additions to Anoxic Basins & Ocean *oxidized in ocean; not emitted to atmosphere

24. Aerobic Production

25. Aerobic Production

26. Aerobic Production?

27. Aerobic Production?

28. Siberian thaw lakes/Yedoma soils Siberian thaw lakes/Yedoma soils

29. Lost City Hydrothermal Field Kelley et al. (2005) Boetius (2005)

30. Treude et al., 2003

31. Michaelis et al. (2002) 3 - 4 m height

32. 5 mm Boetius et al. (2000)

34. Clathrate Hydrates

35. Mud Volcanoes http://www.crimea-info.org

37. Fossil CH4 Additions Cariaco Basin

38. Fossil CH4 Additions Black Sea

40. Future Work Add 2H-CH4 and 13C-CH4 to NOAA time series Natural hydrate dissociation rate? More ocean measurements of natural 14CH4 Ocean mixed layer maximum? Identify/isolate anaerobic methane oxidizer(s) Determine determine mechanism for anaerobic oxidizer(s).

41. Resources (2003) In Vol. 4 (The Atmosphere) Treatise on Geochemistry, Eds. Turekian and Holland, Elsevier-Pergamon, Oxford. 2003 (2006 update for on-line version)

42. Acknowledgements Support: NSF Ocean Sciences W. M. Keck Foundation - MS & AMS Students: David Heggie - Australian. Geol. Survey Org. Marc Alperin - UNC Chapel Hill Jennifer King - Univ. of Minnesota David Valentine - UC Santa Barbara John Kessler - Princeton postdoc Mary Pack - UCI current