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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

Helge Niemann


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


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


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 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


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


Chemosynthetic organisms at cold seeps

Free-living filamentous sulphur bacteria: Beggiatoa

tube worms: Lamellibrachia

clams: Calyptogena

mussels: Bathimodiolus

mussels: Acharax / Solemya


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


MARUM, Uni-Bremen

AWI, IFREMER

Bathymodiolus sp.

Oligobrachia haakonmosbiensis

Methanocella sp.

Dedysh et al., 2000

Bacterial aerobic methane oxidation

CH4 + 2O2 CO2 + 2H2O

9


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



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


Problem 1: a/b/c 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


Good news: a/b/c

Michaelis-Menten kinetics of enzymes

Nauhaus et al. 2002

Enzymatic limit of AOM unknown


CH a/b/c4

CH4

Problem 2: No access to gas bubbles


Questions of biogeochemists are: a/b/c

  • 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


Methods we apply a/b/c

  • 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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