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  1. 1. Methane production - Methanogenesis Substrates / pathways Isotopic studies Hydrogen cycling Methane consumption - Anaerobic methane oxidation Methane hydrates (Thermogenic methane) (Hydrothermal vent methane)
  2. 2. 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.
  3. 3. Two main methanogenic pathways: CO2 reduction Acetate fermentation Both pathways found in both marine and freshwater systems Many other substrates now recognized
  4. 4. CO2 reduction Acetate fermentation Zinder, 1993
  5. 5. CO2 reduction Acetate fermentation
  6. 6. 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.
  7. 7. Syntrophy between an acetogen and a methanogen
  8. 8. Zinder, 1993
  9. 9. Whiticar et al. Dominant pathway for methanogenesis? Stable isotope approaches. 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 - 4H2 + HCO3 - + H+ => CH4 + 3H2O
  10. 10. CO2 reduction - Slope near 1 Fermentation - Slope much lower Overlap in d13C; separation in dD
  11. 11. CO2 reduction - Slope near 1 Fermentation - Slope much lower Methanogenesis in freshwater systems dominated by acetate fermentation; in (sulfate-free) marine systems, by CO2 reduction
  12. 12. What controls the d13C of biogenic methane? (strongly depleted, with a wide range) -50 -100 N. Blair – link to organic C flux? Alperin et al., 1992: 120 day sediment incubations Measure concentrations and rates; Infer pathways and fractionations
  13. 13. Acetogenesis SO4 -2 < 0.2 mM 120 day sediment incubations Methanogenesis Gas leak Alperin et al., 1992
  14. 14. Fraction from acetate Total CH4 production d13C DIC Increase due to CO2 reduction d13C CH4 Shifting pathways, and source d13C
  15. 15. d13C of CH4 from DIC, E = –50 to – 70 d13C of CH4 production (CO2 red., acetate ferment.) d13C of CH4 in incubations (instantaneous, and integrated) reflects variation of pathways, and substrate d13C
  16. 16. Controls on H2 in anoxic sediments. Hoehler et al., 1998 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.
  17. 17. Zinder, 1993 Greater energy yield (more negative DG) allows sulfate reducers to outcompete methanogens for H2.
  18. 18. Hoehler et al., 1998 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.
  19. 19. 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. Higher bulk H2 Lower bulk H2 H2 consumer – sulfate reducer H2 producer – fermenter
  20. 20. 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
  21. 21. 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)
  22. 22. 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.
  23. 23. Nauhaus et al., 2002 Sediment incubations (Hydrate Ridge) demonstrating anaerobic methane oxidation, strong response to CH4 addition. CH4 consumption H2S production
  24. 24. DeLong 2000 (N&V to Boetius et al.) SO4 -2 + 4H2 => S= + 4H20 CH4 + 2H2O => CO2 + 4H2 Anaerobic methane oxidation coupled with sulfate reduction
  25. 25. 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
  26. 26. Kvenvolden, ‘88 1 m3 hydrate => 184 m3 gas + 0.8 m3 water total hydrate = 10,000 x 1015 gC (a guess!) Total fossil fuel = 5000 x 1015 gC DIC = 980 Terr bio = 830 Peat = 500 Atm = 3.5 Mar bio = 3
  27. 27. Methane hydrate stability Methane hydrate Methane gas
  28. 28. permafrost Continental margin
  29. 29. Known global occurance of gas hydrates
  30. 30. 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
  31. 31. Porewater evidence of hydrate dissociation: low Cl- in zone of hydrate dissociation (during core recovery; decompression, warming)
  32. 32. Abrupt, global low-13C event in late Paleocene (benthic foraminifera, planktic foraminifera, terrestrial fossils): A gas hydrate release? Warming to LPTM – Late Paleocene thermal maximum
  33. 33. 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)
  34. 34. Blair – aerobic methane oxidation in CLB
  35. 35. What controls the d13C of biogenic methane? (strongly depleted, with a wide range) N. Blair – link to organic C flux? -100 -50
  36. 36. Hoehler et al., 1998 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.
  37. 37. 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.
  38. 38. 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)
  39. 39. The pathways are complex and variable, the processes are tightly linked, production and consumption of intermediates are rapid and in balance, … Jorgensen, 1983
  40. 40. Alperin, AMO-SR Skan Bay AK