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
Wahlen, 1993
T
I
a
Q
F
F
r
e
u
ic
k
U
(
n
e
T
n
e
im
e
c
o
m
d
e
d
™
p
t
r
o
a
e
n
s
s
d
s
e
e
e
T
Q
F
I
a
r
u
ic
F
e
k
U
(
n
e
T
n
e
im
e
c
o
m
d
e
d
™
p
t
r
o
a
e
n
s
s
d
s
e
e
e
d
Geochemical Approaches
• 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.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Rate Measurements
Flux Measurements (chamber, eddy flux)
Sulfate Reduction
35SO -2
4
H235S (1.4 Ci mmole-1)
(carrier-free)
Methane Oxidation
Aerobic and Anaerobic
Carbon (14C)
14C-CH
4
Hydrogen (3H)
3H-CH
4
14CO
3H
2
20
(55 mCi mmol-1)
(3 Ci mmol-1)
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)
Methane Sources
Abiotic
“Serpentinization Reaction”
6[(Mg1.5Fe0.5)SiO4] + 7H2O
olivine
3[Mg3Si2O5(OH)4] + Fe3O4 + H2
serpentine
and
CO2 + 4H2
(300 C, 500bar)
magenetite
CH4 + 2H2O
“Thermal Cracking”, Pyrolysis
14CH
4
added by PWR’s
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
Methane Sinks
Photochemical Oxidation
(principal atmospheric sink)
O3 + h
O(1D) + H2O
CH4 + OH
O(1D) + O2
2OH
H20 + CH3
 = 315 nm
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 (CH4 & 13C-CH4)
Butler et al., 2005, JGR
Van der Werf et al., 2004, Science (wildfire contributions)
Bousquet et al., 2000, Nature
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
Aerobic Production
Aerobic Production
Aerobic
Production?
Aerobic Production?
Siberian thaw lakes/Yedoma soils
Siberian thaw lakes/Yedoma soils
Lost City Hydrothermal Field
Kelley et al. (2005)
Boetius (2005)
QuickT ime™ and a
TI FF (Uncompressed) decompressor
are needed to see this picture.
Treude et al., 2003
Michaelis et al. (2002)
3 - 4 m height
5 mm
Boetius et al. (2000)
Clathrate Hydrates
Mud Volcanoes
http://www.crimea-info.org
Fossil CH4 Additions
Cariaco Basin
Fossil CH4 Additions
Black Sea
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).
Resources
(2003) In Vol. 4 (The Atmosphere) Treatise on Geochemistry, Eds. Turekian and Holland,
Elsevier-Pergamon, Oxford. 2003 (2006 update for on-line version)
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
T
I
a
Q
F
F
r
e
u
ic
k
U
(
n
e
T
n
e
im
c
d
e
m
o
e
d
™
p
t
r
o
a
e
n
d
s
s
s
e
e
e
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Clathrate Hydrates
T
Q
F
I
a
r
u
ic
F
e
k
U
(
n
e
T
n
e
im
e
c
o
m
d
e
d
™
p
t
r
o
a
e
n
s
s
d
s
e
e
e
d
Approaches to Estimating the Clathrate-Derived
Methane Flux to the Ocean
Global Methane Budget Sinks
Aerobic oxidation of methane
2CH4 + O2
2CO2 + 2H20
(decreases pH, dissolves carbonates)
Anaerobic oxidation of methane (AOM or AMO)
CH4 + SO4-2
HCO3- + HS- + H20
(increases alkalinity; carbonates w/light
isotopic signature ppt.)