Transcript Methane production - Methanogenesis Substrates / pathways Isotopic studies Hydrogen cycling Methane consumption - Anaerobic methane oxidation Methane hydrates (Thermogenic methane) (Hydrothermal vent methane)

Methane production - Methanogenesis
Substrates / pathways
Isotopic studies
Hydrogen cycling
Methane consumption - Anaerobic methane oxidation
Methane hydrates
(Thermogenic methane)
(Hydrothermal vent methane)
Methanogens (Zinder; Oremland)
Archaea.
Relatively few species (30-40), but highly diverse
(3 orders, 6 families, 12 genera).
Strict anaerobes.
Highly specialized in terms of food sources –
Can only use simple compounds (1 or 2 carbon atoms), and many
species can only use 1 or 2 of these simple compounds.
Therefore, dependent on other organisms for their substrates;
food web / consortium required to utilize sediment organic matter.
Two main methanogenic pathways:
CO2 reduction
Acetate fermentation
Both pathways found in both marine and freshwater systems
Many other substrates now recognized
CO2 reduction
Acetate fermentation
Zinder, 1993
Acetate fermentation
CO2 reduction
Obligate syntrophy between an acetogen and a methanogen is common
Each species (e.g., a methanogen and an acetogen) requires the other:
the acetogen provides the hydrogen; the methanogen prevents a buildup of hydrogen (which inhibits the acetogens)
Zinder, 1993
Obligate syntrophy is common
Both species (e.g., a methanogen and an acetogen) require the other:
the acetogen provides the hydrogen; the methanogen prevents a build-up
of hydrogen (which inhibits the acetogens)
In marine sediments, methanogens are competitive only after sulfate
is gone (< 0.2 mM sulfate). Sulfate reducers keep H2 partial pressure
too low for methanogens. (T. Hoehler et al.)
Porewater sulfate and
H2 in Cape Lookout
Bight sediments
Estimated porewater
H2 turnover times are
very short (0.1 to 5 s);
profile H2 gradients
don’t reflect transport,
but “local” production
rate variations.
Hoehler et al., 1998
Hoehler et al. – Microbial communities maintain porewater H2
concentrations at a minimum useful level (based on the energy
they require to form ATP from ADP). The bulk H2 may reflect
the geometry of the H2 producer / H2 consumer association.
H2 consumer –
sulfate reducer
H2 producer –
fermenter
Higher bulk H2
Lower bulk H2
Dominant pathway for methanogenesis?
Stable isotope approaches.
4H2 + HCO3- + H+ => CH4 + 3H2O
Distinct dD (stable
hydrogen isotope) values
for CO2 reduction and
acetate fermentation, based
on source of the hydrogen
atoms.
All H from water
3 of 4 H from acetate
CH3COO- + H2O => CH4 + HCO3-
Whiticar et al., 1986
CO2 reduction - Slope near 1,
all H from water
Fermentation - Slope much
lower, 1 of 4 H from water
Whiticar et al., 1986;
but maybe not so simple
see Waldron et al., 1999
Methanogenesis in freshwater
systems dominated by acetate
fermentation (larger fractionation);
in (sulfate-free) marine systems, by
CO2 reduction (smaller fractionation)
What happens to all this methane?
Diffusive transport up into oxic zone – aerobic methane oxidation
Bubble ebullition (in shallow seds, with strong temperature or
pressure cycles) followed by oxidation in atmosphere
Anaerobic methane oxidation coupled to sulfate reduction
Gas hydrate formation
Alperin and Reeburgh, 1984
Skan Bay, AK.
Seasonally anoxic bottom
water, sediments uniformly
black, with millimolar
hydrogen sulfide in p.w..
Oxygen penetration depth = 0
Anaerobic methane oxidation “controversial” (impossible)
– no AMO mechanism had been demonstrated,
no organism capable of AMO had ever been isolated.
Alperin and Reeburgh, 1984
14C
based CH4 oxidation rate profile
consistent with pore water methane profile;
methane oxidation to CO2 in anoxic zone.
Stable isotope profiles also consistent with
methane oxidation to CO2 in anoxic zone.
Alperin and Reeburgh, 1985
Sulfate profiles and SR rate profiles match, too.
Anaerobic methane oxidation by a consortium, made up of:
sulfate reducers (with H2 as electron acceptor)
SO4-2 + 4H2 => S= + 4H20
And
methanogens (running in reverse, due to low pH2)
CH4 + 2H2O => CO2 + 4H2
Together yielding
CH4 + SO4-2 => HS- + HCO3- + H2O
(Hoehler et al., ‘94)
Used fluorescent
probes to label, image
aggregates of archaea
(methanogens, red) and
sulfate reducers (green)
in sediments from
Hydrate Ridge (OR) –
observed very tight
spatial coupling.
Boetius et al., 2000
H2S production
CH4 consumption
Sediment
incubations (Hydrate
Ridge)
demonstrating
anaerobic methane
oxidation, strong
response to CH4
addition.
Nauhaus et al., 2002
Anaerobic methane oxidation coupled with sulfate reduction
CH4 + 2H2O => CO2 + 4H2
DeLong 2000
(N&V to Boetius et al.)
SO4-2 + 4H2 => S= + 4H20
Another fate for methane – gas hydrate formation
Low T + high P + adequate gas (methane, trace other HC, CO2)
=> gas hydrate
Why do we care about methane hydrates?
Resource potential
Fluid flow on margins
Slope destabilization / slope failure
Chemosynthetic biological communities
Climate impact potential
Kvenvolden, ‘88
1 m3 hydrate => 184 m3 gas + 0.8 m3 water
DIC = 980
Terr bio = 830
Peat = 500
Atm = 3.5
Mar bio = 3
Total fossil fuel =
5000 x 1015 gC
total hydrate =
10,000 x 1015
gC (a guess!)
Methane
hydrate stability
Methane gas
Methane hydrate
Stable T and P, not
enough methane
permafrost
Continental margin
Known global
occurance of
gas hydrates
Most marine gas hydrates
have d13C values lower than
–60 o/oo, and are of
microbial origin.
Hydrates with higher d13C
values (> - 40 o/oo) and
containing some higher
MW hydrocarbons are
thermogenic
Geophysical signature
of gas hydrates:
presence of a “bottom
simulating reflector” in
seismic data, due to
velocity contrast
(hydrate / free gas).
water
sediment
hydrate
free gas
Porewater evidence of hydrate dissociation:
low Cl- in zone of hydrate dissociation
(during core recovery; decompression, warming)
Warming to LPTM –
Late Paleocene
thermal maximum
Abrupt, global low-13C event in late Paleocene
(benthic foraminifera, planktic foraminifera,
terrestrial fossils): A gas hydrate release?
Dickens et al., 1997
High-resolution sampling
of the 13C event.
Magnitude, time-scales,
consistent with sudden
release of 1.1 x 1018 g CH4
with d13C of –60 o/oo, and
subsequent oxidation.
Did warming going into
LPTM drive hydrate
dissociation, and methane
release?
Did similar (smaller)
events occur during the last
glaciation? (Kennett)
Simultaneous low-d13C
excursions in benthic and
planktonic foraminifera
consistent with release (and
oxidation) of light methane,
as a result of destabilization
of clathrates – the .
Marine clathrates
Terrestrial wetlands
A constraint on hydrate release from
the dD of methane in ice cores.
Sowers, 2006
Clathrate release
should result in lower
dD values (black
model line); instead,
dD tends to increase
with CH4 increase.
Sower’s conclusion the glacial methane
increases were not
caused by clathrate
release.
Sowers, 2006