Atmospheric Methane: How well Atmospheric Methane: How well can wecan we apportion present sources apportion present sources and predictand predict future changes? future changes?

William S. ReeburghWilliam S. Reeburgh

Earth System ScienceEarth System Science

University of California IrvineUniversity of California Irvine

Reeburgh@uci.eduReeburgh@uci.edu

Wahlen, 1993

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GeochemicalGeochemical ApproachesApproaches

• Four R’s of GeochemistryFour R’s of Geochemistry (Dayton Carritt)(Dayton Carritt)• RoutesRoutes• Rates Rates • ReactionsReactions • ReservoirsReservoirs

• Inverse Chemical EngineeringInverse Chemical Engineering (W. S. Broecker) (W. S. Broecker) Considers Earth as a chemical plant with no blueprints. Task of Considers Earth as a chemical plant with no blueprints. Task of geochemistry is to produce the missing blueprints with measurements of geochemistry is to produce the missing blueprints with measurements of concentrations, fluxes, reaction rates, etc. concentrations, fluxes, reaction rates, etc.

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Rate MeasurementsRate Measurements

Flux MeasurementsFlux Measurements (chamber, eddy (chamber, eddy flux)flux)

Sulfate Reduction Sulfate Reduction 3535SOSO44

-2-2 H H223535S (1.4 Ci S (1.4 Ci

mmolemmole-1-1) ) (carrier-free)(carrier-free)

Methane OxidationMethane Oxidation Aerobic and AnaerobicAerobic and Anaerobic

Carbon (Carbon (1414C)C) 1414C-CHC-CH44 1414COCO2 2 (55 (55 mCi mmolmCi mmol-1-1))

Hydrogen (Hydrogen (33H)H) 33H-CHH-CH4 4 33HH220 (3 0 (3 Ci mmolCi mmol-1-1))

Methane SourcesMethane Sources

MicrobialMicrobial

Competitive substrates (anoxic Competitive substrates (anoxic conditions)conditions) COCO22 reduction reduction COCO22 + 4H + 4H22 CH CH44 + 2H + 2H22O O

Acetate fermentationAcetate fermentation CHCH33COOH CHCOOH CH44 + CO + CO22

Non-competitive substrates (oxic Non-competitive substrates (oxic conditions?)conditions?) Methylated Compounds Methylated Compounds (methylamines, DMS, DMDS, (methylamines, DMS, DMDS, methane thiol, methane thiol, methyl phosphonate)methyl phosphonate)

Methane SourcesMethane Sources

AbioticAbiotic “ “Serpentinization Reaction”Serpentinization Reaction”

6[(Mg6[(Mg1.51.5FeFe0.50.5)SiO)SiO44] + 7H] + 7H22O O olivineolivine

3[Mg3[Mg33SiSi22OO55(OH)(OH)44] + ] + FeFe33OO44 + H + H22

serpentineserpentine magenetitemagenetite

and and

COCO22 + 4H + 4H22 (300 C, 500bar)(300 C, 500bar) CH CH44 + 2H + 2H22OO

“ “Thermal Cracking”, PyrolysisThermal Cracking”, Pyrolysis

1414CHCH44 added by PWR’s added by PWR’s

Methane Sinks

Microbial Aerobic Oxidation 2CH2CH44 + O + O2 2 2CO2CO2 2 + 2H+ 2H2200

(decreases pH, dissolves (decreases pH, dissolves carbonates)carbonates)

Anaerobic Oxidation (AOM or AMO)(AOM or AMO) CHCH44 + SO + SO44

-2 -2 HCO HCO33-- + HS + HS--

+ H+ H2200 (increases alkalinity; (increases alkalinity; isotopically light isotopically light carbonates precipitate.)carbonates precipitate.)

““Reverse Methanogenesis”Reverse Methanogenesis” CHCH44 + 2H2H220 CO0 CO2 2 + 4H+ 4H22

Methane SinksMethane Sinks

Photochemical Oxidation Photochemical Oxidation (principal atmospheric sink)(principal atmospheric sink)

OO33 + h + h O( O(11D) + OD) + O22 = 315 nm= 315 nm

O(O(11D) + HD) + H22O 2OHO 2OH

CHCH44 + OH H + OH H220 + CH0 + CH33

Cicerone & Oremland, 1988

Methane budget is well-constrained. Methane budget is well-constrained. We know the total well, but We know the total well, but individual source terms are individual source terms are uncertain to a factor of 2 or more. uncertain to a factor of 2 or more. A “bird’s eye” budget; considers net A “bird’s eye” budget; considers net additions to the atmosphere. A net additions to the atmosphere. A net atmospheric budget.atmospheric budget.

We can consider consumption or We can consider consumption or oxidation, but the previous oxidation, but the previous constraints do not apply. Oxidation constraints do not apply. Oxidation before emission to atmosphere has a before emission to atmosphere has a large effect.large effect.

InversionsInversions

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

Recently Reported CHRecently Reported CH44 Sources Sources

Aerobic Methane Production by PlantsAerobic Methane Production by Plants

Siberian thaw lakes/Yedoma soilsSiberian thaw lakes/Yedoma soils

*Ocean Vent Additions: CH*Ocean Vent Additions: CH44-consuming benthic-consuming benthic communitiescommunities

*Methane Clathrate Hydrate, Mud Volcano Additions*Methane Clathrate Hydrate, Mud Volcano Additions

*Large “Fossil CH*Large “Fossil CH44” Additions to Anoxic Basins & Ocean” Additions to Anoxic Basins & Ocean

**oxidized in ocean; not emitted to atmosphereoxidized in ocean; not emitted to atmosphere

Aerobic ProductionAerobic Production

Aerobic ProductionAerobic Production

AerobicAerobic Production?Production?

Aerobic Production?Aerobic Production?

Siberian thaw lakes/Yedoma soilsSiberian thaw lakes/Yedoma soils

Siberian thaw lakes/Yedoma soilsSiberian thaw lakes/Yedoma soils

Lost City Hydrothermal FieldLost City Hydrothermal Field

Kelley Kelley et al.et al. (2005) (2005)Boetius (2005)Boetius (2005)

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Treude et al., 2003Treude et al., 2003

Michaelis Michaelis et al.et al. (2002) (2002)

3 - 4 m height3 - 4 m height

5 5 mm

Boetius Boetius et al.et al. (2000) (2000)

Clathrate HydratesClathrate Hydrates

Mud VolcanoesMud Volcanoes

http://www.crimea-info.orghttp://www.crimea-info.org

Fossil CHFossil CH44 Additions Additions

Cariaco BasinCariaco Basin

Fossil CHFossil CH44 Additions Additions

Black SeaBlack Sea

Future WorkFuture Work

Add Add 22H-CHH-CH44 and and 1313C-CHC-CH44 to NOAA time to NOAA time seriesseries

Natural hydrate dissociation rate?Natural hydrate dissociation rate?

More ocean measurements of natural More ocean measurements of natural 1414CHCH44

Ocean mixed layer maximum?Ocean mixed layer maximum?

Identify/isolate anaerobic methane Identify/isolate anaerobic methane oxidizer(s)oxidizer(s)

Determine determine mechanism for Determine determine mechanism for anaerobicanaerobic oxidizer(s). oxidizer(s).

ResourcesResources

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

AcknowledgementsAcknowledgements

Support:Support: NSF Ocean SciencesNSF Ocean Sciences W. M. Keck Foundation - MS & AMSW. M. Keck Foundation - MS & AMS

Students:Students: David Heggie - Australian. Geol. Survey Org.David Heggie - Australian. Geol. Survey Org. Marc Alperin - UNC Chapel HillMarc Alperin - UNC Chapel Hill Jennifer King - Univ. of MinnesotaJennifer King - Univ. of Minnesota David Valentine - UC Santa BarbaraDavid Valentine - UC Santa Barbara John Kessler - Princeton postdocJohn Kessler - Princeton postdoc Mary Pack - UCI currentMary Pack - UCI current

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Clathrate HydratesClathrate Hydrates

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Approaches to Estimating the Clathrate-Derived

Methane Flux to the Ocean

Global Methane Budget SinksGlobal Methane Budget Sinks

Aerobic oxidation of methaneAerobic oxidation of methane 2CH2CH44 + O + O2 2 2CO2CO2 2 + 2H+ 2H2200

(decreases pH, dissolves carbonates)(decreases pH, dissolves carbonates)

Anaerobic oxidation of methane (AOM or AMO)Anaerobic oxidation of methane (AOM or AMO) CHCH44 + SO + SO44

-2 -2 HCO HCO33-- + HS + HS-- + H + H2200

(increases alkalinity; carbonates w/light(increases alkalinity; carbonates w/light isotopic signature ppt.)isotopic signature ppt.)