Methane production
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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.

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Methane production - Methanogenesis

Substrates / pathways

Isotopic studies

Hydrogen cycling

Methane consumption - Anaerobic methane oxidation

Methane hydrates

(Thermogenic methane)

(Hydrothermal vent methane)


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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.


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Two main methanogenic pathways:

CO2 reduction

Acetate fermentation

Both pathways found in both marine and freshwater systems

Many other substrates now recognized


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CO2 reduction

Acetate fermentation

Zinder, 1993


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Acetate fermentation

CO2 reduction


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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 build-up of hydrogen (which inhibits the acetogens)


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Zinder, 1993 common


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Obligate syntrophy is common 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.)


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Porewater sulfate and H common2 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


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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


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Dominant pathway for methanogenesis? H

Stable isotope approaches.

4H2 + HCO3- + H+ => CH4 + 3H2O

All H from water

Distinct dD (stable hydrogen isotope) values for CO2 reduction and acetate fermentation, based on source of the hydrogen atoms.

3 of 4 H from acetate

CH3COO- + H2O => CH4 + HCO3-

Whiticar et al., 1986


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Whiticar et al., 1986; H

but maybe not so simple

see Waldron et al., 1999

CO2 reduction - Slope near 1,

all H from water

Methanogenesis in freshwater systems dominated by acetate fermentation (larger fractionation);

in (sulfate-free) marine systems, by CO2 reduction (smaller fractionation)

Fermentation - Slope much lower, 1 of 4 H from water


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What happens to all this methane? H

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


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Alperin and Reeburgh, 1984 H

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.


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Alperin and Reeburgh, 1984 H

14C based CH4 oxidation rate profile

consistent with pore water methane profile; methane oxidation to CO2 in anoxic zone.



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Sulfate profiles and SR rate profiles match, too. oxidation to CO

Alperin and Reeburgh, 1985


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Anaerobic methane oxidation oxidation to CO 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)


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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


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H archaea (methanogens, red) and sulfate reducers (green) in sediments from Hydrate Ridge (OR) – observed very tight spatial coupling.2S production

CH4 consumption

Sediment incubations (Hydrate Ridge) demonstrating anaerobic methane oxidation, strong response to CH4 addition.

Nauhaus et al., 2002


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Anaerobic methane oxidation coupled with sulfate reduction archaea (methanogens, red) and sulfate reducers (green) in sediments from Hydrate Ridge (OR) – observed very tight spatial coupling.

CH4 + 2H2O => CO2 + 4H2

SO4-2 + 4H2 => S= + 4H20

DeLong 2000

(N&V to Boetius et al.)


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Low T + high P + adequate gas (methane, trace other HC, CO archaea (methanogens, red) and sulfate reducers (green) in sediments from Hydrate Ridge (OR) – observed very tight spatial coupling.2)

=> 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

Another fate for methane – gas hydrate formation


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Kvenvolden, ‘88 archaea (methanogens, red) and sulfate reducers (green) in sediments from Hydrate Ridge (OR) – observed very tight spatial coupling.

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!)


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Methane hydrate stability archaea (methanogens, red) and sulfate reducers (green) in sediments from Hydrate Ridge (OR) – observed very tight spatial coupling.

Methane gas

Methane hydrate


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Stable T and P, not enough methane archaea (methanogens, red) and sulfate reducers (green) in sediments from Hydrate Ridge (OR) – observed very tight spatial coupling.

permafrost

Continental margin


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Known global occurance of gas hydrates archaea (methanogens, red) and sulfate reducers (green) in sediments from Hydrate Ridge (OR) – observed very tight spatial coupling.

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


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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


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Porewater evidence of hydrate dissociation: “bottom simulating reflector” in seismic data, due to velocity contrast (hydrate / free gas).

low Cl- in zone of hydrate dissociation

(during core recovery; decompression, warming)


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Warming to LPTM – Late Paleocene thermal maximum “bottom simulating reflector” in seismic data, due to velocity contrast (hydrate / free gas).

Abrupt, global low-13C event in late Paleocene (benthic foraminifera, planktic foraminifera, terrestrial fossils): A gas hydrate release?


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Dickens et al., 1997 “bottom simulating reflector” in seismic data, due to velocity contrast (hydrate / free gas).

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)


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Simultaneous low- “bottom simulating reflector” in seismic data, due to velocity contrast (hydrate / free gas).d13C excursions in benthic and planktonic foraminifera consistent with release (and oxidation) of light methane, as a result of destabilization of clathrates – the .


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Marine clathrates “bottom simulating reflector” in seismic data, due to velocity contrast (hydrate / free gas).

Terrestrial wetlands

A constraint on hydrate release from the dD of methane in ice cores.

Sowers, 2006


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Clathrate release should result in lower “bottom simulating reflector” in seismic data, due to velocity contrast (hydrate / free gas).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


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