Dissociation of gas hydrates in marine sediments triggered by temperature
increase: a theoretical model
Lihua Liu, Klaus Wallmann, Tomas Feseker, Tina Treude Leibniz Institute of Marine Sciences (IFM-GEOMAR),
Germany
lliu@ifm-geomar.de
Introduction: Global warming
(Source: Climate Research Unit, Univ. of East Anglia, UK)
Introduction: Ocean warming
Source: IPCC (2007)
0.1°C
World Ocean, 0 – 700 m water depth
Tishchenko, Hensen, Wallmann & Wong (2005)
Introduction:
Stability of gas hydrates
Hydrate
Gas Water
Introduction: Consequence of ocean warming
Bottom waters increase 3ºC, 80% of the vast
methane reservoir along the continental margins might be destabilized.
(Buffett and Archer 2004).
The Arctic region is particularly sensitive to climate change. Arctic ocean is one the most rapidly
warming places on Earth and also a large reservoir
of methane.
Introduction: Important
biogeochemical processes
water column
reduced sediment CH4
Anaerobic oxidation of methane (AOM) Aerobic oxidation of methane
atmosphere
archaea sulfate- reducing bacteria
Boetius et al. 2000
7
Hydrate dissociation
& methane release
gas hydrates
deep sea continental margin
H2O + CH4
free gas rising
CH4 SO42-
AOM
Questions: Effect of seafloor warming on the stability of gas hydrates
How will heat be transferred from the water to the sediment column?
How fast will the gas hydrates dissociate under realistic environmental conditions?
How much methane will be released?
How much methane can be dissolved in porewater?
How much methane can be consumed by microorganism (eg., AOM)?
Model parameters
Sediment column: 100 m Simulation time: 100 year
AOM reaction rate constant: 10-2 m3 mol-1 yr-1
Initial conditions
no free gas in the sediment column
Boundary condition
Upper boundary: increase 3°C/100 yr
Lower boundary: a constant geothermal gradient
Simulation method: 1 D multiphase
reactive transport model
Simulation method: 1 D multiphase reactive transport model
Combination of:
- heat transfer from water column to sediment and
- mass balance of gas hydrate, methane (gas and dissolved), water, and sulfate.
Multiphase mass transfer and transport, coupled with diffusion and biogeochemical reaction.
(gas hydrate, dissolved methane, free gas, consumed by AOM)
- Heat transfer in each phase
- Gas transports in the sediment and into the water column, where gas bubbles rise and dissolve in water
synchronously.
Simulation results: Temperature profile in the sediment column
°C
Simulation Results: Gas hydrate volume fraction profile in the sediment column
Vgas Hydrage
Vsediment
Simulation results: gas volume
fraction profile in the sediment column
Vgas
Vsediment
Simulation results: AOM rate profile in the sediment column
mol m3.yr
020406080 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Simulation results
0
20
40
60
80
100
0.4 1.0 2.0 3.0
01020304050 0.000 0.005 0.010 0.015 0.020 0.025 0.030
0
10
20
30
40
50
0.01 0.02 0.03 0.01 0.02 0.03
0
1.0
2.0
3.0
0.0 1.0 2.0
Temperature °C Gas hydrate volume fraction
Free gas volume fraction
AOM rate (mol m-3 yr-1)
t=0 t=100 yr
t=50 yr
Depth (m)
Conclusion: Methane mass distribution
0 500 1000 1500 2000 2500
0 30 yr 50 yr 100 yr
Initial gas hydrate inventory
Melted gas hydrate Free gas escape
Free gas in sediment column Dissolved methane escape Consumed by AOM
20000
mol m-2
Conclusions: Answer the questions
The dissociation of gas hydrate slows down temperature increases in the seafloor.
Under simulation conditions, 10 % of the gas hydrates will melt in 100 yr. Of the released methane:
> 30 % rises into water column as gas bubbles
> 30 % remains in the sediment column as free gas.
~ 30 % dissolves into the sediment porewater ~ 3 % diffuses into the water column and
> 2 % is consumed by AOM.
Outlook
Future work will focus on the Arctic shelf, where gas hydrate destabilization caused by bottom water temperature increases
could become a major problem in the near
future.
Acknowledgements
Cluster of Excellence “The Future
Ocean” funded by the German Research Foundation (DFG)
Dr. Giovanni Aloisi
