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: CO 2 reduction Acetate fermentation Both pathways found in both marine and freshwater systems Many other substrates now recognized

CO 2 reduction Acetate fermentation Zinder, 1993

CO 2 reduction Acetate fermentation

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)

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 H 2 partial pressure too low for methanogens. (T. Hoehler et al.)

Hoehler et al., 1998 Porewater sulfate and H 2 in Cape Lookout Bight sediments Estimated porewater H 2 turnover times are very short (0.1 to 5 s); profile H 2 gradients don’t reflect transport, but “local” production rate variations.

Hoehler et al. – Microbial communities maintain porewater H 2 concentrations at a minimum useful level (based on the energy they require to form ATP from ADP). The bulk H 2 may reflect the geometry of the H 2 producer / H 2 consumer association. Higher bulk H 2 Lower bulk H 2 H 2 consumer – sulfate reducer H 2 producer – fermenter

Whiticar et al., 1986 Dominant pathway for methanogenesis? Stable isotope approaches. Distinct  D (stable hydrogen isotope) values for CO 2 reduction and acetate fermentation, based on source of the hydrogen atoms. All H from water 3 of 4 H from acetate CH 3 COO - + H 2 O => CH 4 + HCO 3 - 4H 2 + HCO H + => CH 4 + 3H 2 O

CO 2 reduction - Slope near 1, all H from water Fermentation - Slope much lower, 1 of 4 H from water Methanogenesis in freshwater systems dominated by acetate fermentation (larger fractionation); in (sulfate-free) marine systems, by CO 2 reduction (smaller fractionation) Whiticar et al., 1986; but maybe not so simple see Waldron et al., 1999

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 Anaerobic methane oxidation “controversial” (impossible) – no AMO mechanism had been demonstrated, no organism capable of AMO had ever been isolated. Skan Bay, AK. Seasonally anoxic bottom water, sediments uniformly black, with millimolar hydrogen sulfide in p.w.. Oxygen penetration depth = 0

Alperin and Reeburgh, C based CH 4 oxidation rate profile consistent with pore water methane profile; methane oxidation to CO 2 in anoxic zone.

Stable isotope profiles also consistent with methane oxidation to CO 2 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 H 2 as electron acceptor) SO H 2 => S = + 4H 2 0 And methanogens (running in reverse, due to low pH 2 ) CH 4 + 2H 2 O => CO 2 + 4H 2 Together yielding CH 4 + SO 4 -2 => HS - + HCO H 2 O (Hoehler et al., ‘94)

Boetius et al., 2000 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.

Nauhaus et al., 2002 Sediment incubations (Hydrate Ridge) demonstrating anaerobic methane oxidation, strong response to CH 4 addition. CH 4 consumption H 2 S production

DeLong 2000 (N&V to Boetius et al.) SO H 2 => S = + 4H 2 0 CH 4 + 2H 2 O => CO 2 + 4H 2 Anaerobic methane oxidation coupled with sulfate reduction

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

Kvenvolden, ‘88 1 m 3 hydrate => 184 m 3 gas m 3 water total hydrate = 10,000 x gC (a guess!) Total fossil fuel = 5000 x gC DIC = 980 Terr bio = 830 Peat = 500 Atm = 3.5 Mar bio = 3

Methane hydrate stability Methane hydrate Methane gas

permafrostContinental margin Stable T and P, not enough methane

Known global occurance of gas hydrates Most marine gas hydrates have  13 C values lower than –60 o/oo, and are of microbial origin. Hydrates with higher  13 C 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)

Abrupt, global low- 13 C event in late Paleocene (benthic foraminifera, planktic foraminifera, terrestrial fossils): A gas hydrate release? Warming to LPTM – Late Paleocene thermal maximum

Dickens et al., 1997 High-resolution sampling of the 13 C event. Magnitude, time-scales, consistent with sudden release of 1.1 x g CH 4 with  13 C 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-  13 C excursions in benthic and planktonic foraminifera consistent with release (and oxidation) of light methane, as a result of destabilization of clathrates – the.

Sowers, 2006 Marine clathrates Terrestrial wetlands A constraint on hydrate release from the  D of methane in ice cores.

Sowers, 2006 Clathrate release should result in lower  D values (black model line); instead,  D tends to increase with CH 4 increase. Sower’s conclusion - the glacial methane increases were not caused by clathrate release.