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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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slide1

Methane production - Methanogenesis

Substrates / pathways

Isotopic studies

Hydrogen cycling

Methane consumption - Anaerobic methane oxidation

Methane hydrates

(Thermogenic methane)

(Hydrothermal vent methane)

slide2

Methanogens (Zinder; Oremland)

Archaea.

Relatively few species (30-40), but highly diverse

(3 orders, 6 families, 12 genera).

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.

Strict anaerobes.

slide3

Two main methanogenic pathways:

CO2 reduction

Acetate fermentation

Both pathways found in both marine and freshwater systems

Many other substrates now recognized

slide4

CO2 reduction

Acetate fermentation

Zinder, 1993

slide6

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.

slide10

Dominant pathway for methanogenesis?

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.

slide11

CO2 reduction - Slope near 1

Overlap in d13C; separation in dD

Fermentation - Slope much lower

slide12

CO2 reduction - Slope near 1

Methanogenesis in freshwater systems dominated by acetate fermentation; in (sulfate-free) marine systems, by CO2 reduction

Fermentation - Slope much lower

slide13

What controls the d13C of biogenic methane?

(strongly depleted, with a wide range)

N. Blair – link to organic C flux?

-100

-50

Alperin et al., 1992:

120 day sediment incubations

Measure concentrations and rates;

Infer pathways and fractionations

slide14

120 day sediment incubations

Alperin et al., 1992

Gas leak

SO4-2 < 0.2 mM

Acetogenesis

Methanogenesis

slide15

Total CH4 production

d13C DIC

Increase due to CO2 reduction

d13C CH4

Shifting pathways, and source d13C

Fraction from acetate

slide16

d13C of CH4 production (CO2 red., acetate ferment.)

d13C of CH4 from DIC,

E = –50 to – 70

d13C of CH4 in incubations (instantaneous, and integrated) reflects variation of pathways, and substrate d13C

slide18

Hoehler et al., 1998

Controls on H2 in anoxic sediments.

7 – 14 day slurry incubations to estimate steady-state H2 concentrations. For SR, methanogenesis, and acetogenesis, the observed [H2] levels are low enough to limit the next process.

slide19

Greater energy yield (more negative DG) allows sulfate reducers to outcompete methanogens for H2.

Zinder, 1993

slide20

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

slide21

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

slide23

What happens to all this methane?

Diffusion (transport) up into oxic zone – aerobic methane oxidation

Bubble ebullition followed by oxidation in atmosphere

shallow seds, with strong temperature or pressure cycles

Anaerobic methane oxidation coupled to sulfate reduction

Gas hydrate formation

slide24

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)

slide25

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

slide26

H2S production

CH4 consumption

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

Nauhaus et al., 2002

slide27

Anaerobic methane oxidation coupled with sulfate reduction

CH4 + 2H2O => CO2 + 4H2

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

DeLong 2000

(N&V to Boetius et al.)

slide29

Low T + high P + adequate gas (methane, trace other HC, CO2)

=> gas hydrate formation

Why do we care about methane hydrates?

Resource potential

Fluid flow on margins

Slope destabilization / slope failure

Chemosynthetic biological communities

Climate impact potential

slide30

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!)

slide31

Methane hydrate stability

Methane gas

Methane hydrate

slide32

permafrost

Continental margin

slide34

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

slide35

Porewater evidence of hydrate dissociation:

low Cl- in zone of hydrate dissociation

(during core recovery; decompression, warming)

slide37

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?

slide38

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 (MIS 3)?

(Kennett)

slide52

What controls the d13C of biogenic methane?

(strongly depleted, with a wide range)

-100

-50

N. Blair – link to organic C flux?

slide53

Porewater sulfate and H2 in Cape Lookout Bight sediments

Estimated porewater H2 turnover times are very short (0.1 to 5 s); profile gradients don’t reflect transport, but “local” production rate variations.

Hoehler et al., 1998

slide55

Can imagine a Redfield-type

sulfate reduction stoichiometry:

(CH2O)106(NH3)16(H3PO4) + 53SO4-2 =>

106(HCO3-) + 16NH3 + H3PO4 + 53(H2S)

Or even just:

2(CH2O) + SO4-2 => 2(HCO3-)+ H2S

Production of ammonia, H2S, and alkalinity at the depth of SR.

If NH3 and H2S diffuse up and are reoxidized;

consume O2, release H+ close to sediment-water interface

If H2S reacts with Fe++, reduced sulfur and Fe are buried.

slide56

But sulfate reducers can only oxidize a limited suite of simple organic

substrates. They typically function as part of a community that

includes fermenters, acetogens, and methanogens, as well as sulfate

reducers.

(Fenchel and Finlay, Ecology and Evolution in Anoxic Worlds)

slide57

Jorgensen, 1983

The pathways are complex and variable, the processes are tightly linked,

production and consumption of intermediates are rapid and in balance, …

slide58

Alperin, AMO-SR

Skan Bay AK