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WG-B1: Biogeochemical processes in the shallow sub-seafloor and at the sediment-water interface

WG-B1: Biogeochemical processes in the shallow sub-seafloor and at the sediment-water interface. Helge Niemann. Major goals of WG-B. understand gas hydrate dissociation processes in warming Arctic sediments

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WG-B1: Biogeochemical processes in the shallow sub-seafloor and at the sediment-water interface

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  1. WG-B1:Biogeochemical processes in the shallow sub-seafloor and at the sediment-water interface Helge Niemann

  2. Major goals of WG-B • understand gas hydrate dissociation processes in warming Arctic sediments • what is the effect of ocean warming on CH4-flow rates and the physical state of CH4 (gaseous vs. dissolved)? • evaluate the capacity and limits of the microbial methane filter • how much of the liberated methane will be consumed in sediments and the water column? WG-B1:Biogeochemical processes in the shallow sub-seafloor and at the sediment-water interface

  3. WG-B1:Biogeochemical processes in the shallow sub-seafloor and at the sediment-water interface • Expectation • limited ability of anaerobic microbial community consuming CH4 in sediments to effectively compensate fast increases in CH4-advection (very, very slow growth rates) • limited ability of aerobic microbial community consuming CH4 at sediment surface (limitation of oxygen) and in the water column (low cell density & fast advection of CH4) • generally: methane rising in gas bubbles is usually inaccessible to methanotrophic organisms. Thus limited ability to consume CH4-flux such complex biogeochemical reactions can only be constrained by multidisciplinary approaches combining physical, geochemical, and microbiological methods

  4. CaCO3 H2S CH4 SO42- Microbial reactions at cold seeps atmosphere water column oxygenated sediment reduced sediment Anaerobic oxidation of methane (AOM)CH4 + SO42- HCO3- + HS- + H2O CH4 hydrate

  5. 5 µm Archaea CH4 + 2H2O → CO2 + 4H2 SO42- + 4H2 + H+ → HS- + 4H2OSulphate Reducing Bacteria (SRB) Boetius et al. 2000 Microbial consortium mediating the anaerobic oxidation of methane (AOM) CH4 + SO42-  HCO3- + HS- + H2O methane sulphate bicarbonate sulphide water

  6. CaCO3 O2 H2S CH4 NO3- SO42- Microbial reactions at cold seeps atmosphere water column oxygenated sediment Sulfide oxidationH2S + 2O2 SO42- + 2 H+5H2S + 8NO3- 5SO42- + 4N2 + 4H2O + 2H+ reduced sediment Anaerobic oxidation of methane (AOM)CH4 + SO42- HCO3- + HS- + H2O CH4 hydrate

  7. Chemosynthetic organisms at cold seeps Free-living filamentous sulphur bacteria: Beggiatoa tube worms: Lamellibrachia clams: Calyptogena mussels: Bathimodiolus mussels: Acharax / Solemya

  8. CaCO3 O2 H2S CH4 NO3- SO42- Microbial reactions at cold seeps atmosphere water column Aerobic oxidation of methaneCH4 + 2O2 CO2 + 2H2O oxygenated sediment Sulfide oxidationH2S + 2O2 SO42- + 2 H+5H2S + 8NO3- 5SO42- + 4N2 + 4H2O + 2H+ reduced sediment Anaerobic oxidation of methane (AOM)CH4 + SO42- HCO3- + HS- + H2O CH4 hydrate

  9. MARUM, Uni-Bremen AWI, IFREMER Bathymodiolus sp. Oligobrachia haakonmosbiensis Methanocella sp. Dedysh et al., 2000 Bacterial aerobic methane oxidation CH4 + 2O2 CO2 + 2H2O 9

  10. One major goal of the JRG "Seafloor Warming":Understanding the processes connected to gas hydrate dissociation The problem with (fast) gas hydrate dissociationIncrease in methane fluxes, release of gas bubbles How much methane will reach the hydrosphere? METHANE BUBBLES HEATFLOW HEATFLOW WATER SEDIMENT RISING FLUIDS + GASES DISSOCIATINGGAS HYDRATES

  11. What are the limits of the microbial methane filter?

  12. Data source: Michaelis et al. 2002; Treude et al. 2003, 2005 a/b/c adaptation of the microbial filter ? however... these seeps are old, AOM and community sizes are thus ‘steady state’ how quickly would the microbial methane filter adapt in a ‘new’ cold seep ? increase in methane flux

  13. Problem 1: slow growth of AOM-consortia AOM free energyΔG°= ca. -16 kJ mol-1 Doubling time: 7 months(it would take decades to colonize a seep...) Nauhaus et al. 2007

  14. Good news: Michaelis-Menten kinetics of enzymes Nauhaus et al. 2002 Enzymatic limit of AOM unknown

  15. CH4 CH4 Problem 2: No access to gas bubbles

  16. Questions of biogeochemists are: • How do gas hydrates dissociate in sediments? heat flow, dissociation, methane releases • How do gas bubbles behave when migrating through sediment? dissolution and gas exchange, availability to microbes • How does the microbial methane filter (both anaerobic and aerobic) respond to increasing methane fluxes? changes of turnover-rate and growth

  17. Methods we apply • Radiotracer measurements with 14C-methane and 35S-sulfat microbial turnover rates • Porewater/sediment dissolved and solid phase geochemistry (e.g sulphate, sulphide, methane, δ13C-methane, δ13C-DIC, alkalinity, δ18O, chlorinity) interpretations of microbial activity heat- & fluid flow and gas hydrate dissociation • Microsensor measurements characterization of microbial microniches, interpretations of microbial activity • Modeling of heat flow and biogeochemical reactions understanding processes correlated to gas hydrate dissociation • Flow-through systems with whole sediment cores change of fluxes/solutes and reaction of microbial filter • Fluorescence in situ hybridization (FISH) and lipid biomarker identification/quantification of microorganisms

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