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

Wahlen, 1993

Geochemical Approaches Four R’s of Geochemistry (Dayton Carritt)Four R’s of Geochemistry (Dayton Carritt) RoutesRoutes RatesRates ReactionsReactions ReservoirsReservoirs Inverse Chemical Engineering (W. S. Broecker)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. 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.

Rate Measurements Flux Measurements (chamber, eddy flux) Sulfate Reduction 35 SO 4 -2 H 2 35 S (1.4 Ci mmole -1 ) (carrier-free) Rate Measurements Flux Measurements (chamber, eddy flux) Sulfate Reduction 35 SO 4 -2 H 2 35 S (1.4 Ci mmole -1 ) (carrier-free) Methane Oxidation Aerobic and Anaerobic Aerobic and Anaerobic Carbon ( 14 C) Carbon ( 14 C) 14 C-CH 4 14 CO 2 (55 mCi mmol -1 ) 14 C-CH 4 14 CO 2 (55 mCi mmol -1 ) Hydrogen ( 3 H) Hydrogen ( 3 H) 3 H-CH 4 3 H 2 0 (3 Ci mmol -1 ) 3 H-CH 4 3 H 2 0 (3 Ci mmol -1 )

Methane Sources Microbial Microbial Competitive substrates (anoxic conditions) Competitive substrates (anoxic conditions) CO 2 reduction CO 2 reduction CO 2 + 4H 2 CH 4 + 2H 2 O CO 2 + 4H 2 CH 4 + 2H 2 O Acetate fermentation Acetate fermentation CH 3 COOH CH 4 + CO 2 CH 3 COOH CH 4 + CO 2 Non-competitive substrates (oxic conditions?) Non-competitive substrates (oxic conditions?) Methylated Compounds Methylated Compounds (methylamines, DMS, DMDS, methane thiol, (methylamines, DMS, DMDS, methane thiol, methyl phosphonate) methyl phosphonate)

Methane Sources Abiotic “Serpentinization Reaction” “Serpentinization Reaction” 6[(Mg 1.5 Fe 0.5 )SiO 4 ] + 7H 2 O olivine olivine 3[Mg 3 Si 2 O 5 (OH) 4 ] + Fe 3 O 4 + H 2 3[Mg 3 Si 2 O 5 (OH) 4 ] + Fe 3 O 4 + H 2 serpentine magenetite serpentine magenetiteand CO 2 + 4H 2 (300 C, 500bar) CH 4 + 2H 2 O “Thermal Cracking”, Pyrolysis “Thermal Cracking”, Pyrolysis 14 CH 4 added by PWR’s 14 CH 4 added by PWR’s

Methane Sinks Microbial Aerobic Oxidation 2CH 4 + O 2 2CO 2 + 2H 2 0 2CH 4 + O 2 2CO 2 + 2H 2 0 (decreases pH, dissolves carbonates) (decreases pH, dissolves carbonates) Anaerobic Oxidation (AOM or AMO) (AOM or AMO) CH 4 + SO 4 -2 HCO HS - + H 2 0 CH 4 + SO 4 -2 HCO HS - + H 2 0 (increases alkalinity; isotopically light (increases alkalinity; isotopically light carbonates precipitate.) carbonates precipitate.) “Reverse Methanogenesis” “Reverse Methanogenesis” CH 4 2H 2 0 CO 2 + 4H 2 CH 4 + 2H 2 0 CO 2 + 4H 2

Methane Sinks Photochemical Oxidation (principal atmospheric sink) (principal atmospheric sink) O 3 + h O( 1 D) + O 2 = 315 nm O( 1 D) + H 2 O 2OH CH 4 + OH H CH 3

Cicerone & Oremland, 1988

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.

Inversions Fung et al., 1997, JGR Hein et al., 1997, GBC Mikalof-Fletcher et al., 2004, GBC (CH 4 & 13 C-CH 4 ) Butler et al., 2005, JGR Van der Werf et al., 2004, Science (wildfire contributions) Bousquet et al., 2000, Nature

Recently Reported CH 4 Sources Aerobic Methane Production by Plants Siberian thaw lakes/Yedoma soils *Ocean Vent Additions: CH 4 -consuming benthic communities communities *Methane Clathrate Hydrate, Mud Volcano Additions *Large “Fossil CH 4 ” Additions to Anoxic Basins & Ocean * oxidized in ocean; not emitted to atmosphere * oxidized in ocean; not emitted to atmosphere

Aerobic Production

Aerobic Production? Production?

Aerobic Production?

Siberian thaw lakes/Yedoma soils

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

Treude et al., 2003

Michaelis et al. (2002) m height

5  m Boetius et al. (2000)

Clathrate Hydrates Clathrate Hydrates

Mud Volcanoes

Fossil CH 4 Additions Cariaco Basin

Fossil CH 4 Additions Black Sea

Future Work Add  2 H-CH 4 and  13 C-CH 4 to NOAA time series Natural hydrate dissociation rate? More ocean measurements of natural 14 CH 4 Ocean mixed layer maximum? Identify/isolate anaerobic methane oxidizer(s) Determine determine mechanism for anaerobic oxidizer(s). oxidizer(s).

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

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

Clathrate Hydrates Clathrate Hydrates

Approaches to Estimating the Clathrate-Derived Methane Flux to the Ocean Global Methane Budget Sinks Aerobic oxidation of methane Aerobic oxidation of methane 2CH 4 + O 2 2CO 2 + 2H 2 0 2CH 4 + O 2 2CO 2 + 2H 2 0 (decreases pH, dissolves carbonates) (decreases pH, dissolves carbonates) Anaerobic oxidation of methane (AOM or AMO) Anaerobic oxidation of methane (AOM or AMO) CH 4 + SO 4 -2 HCO HS - + H 2 0 CH 4 + SO 4 -2 HCO HS - + H 2 0 (increases alkalinity; carbonates w/light (increases alkalinity; carbonates w/light isotopic signature ppt.) isotopic signature ppt.)