1. 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

2. Introduction: Global warming (Source: Climate Research Unit, Univ. of East Anglia, UK)

3. Introduction: Ocean warming Source: IPCC (2007) 0.1°C World Ocean, 0 – 700 m water depth

4. Tishchenko, Hensen, Wallmann & Wong (2005) Introduction: Stability of gas hydrates Hydrate Gas Water

5. 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.

6. Introduction: Important biogeochemical processes water column reduced sediment CH 4 Anaerobic oxidation of methane (AOM) Aerobic oxidation of methane atmosphere archaea sulfate- reducing bacteria Boetius et al. 2000

7. 7 Hydrate dissociation & methane release gas hydrates deep sea continental margin H 2 O + CH 4 free gas rising CH 4 SO 4 2- AO M

8. 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)?

9.  Model parameters Sediment column: 100 m Simulation time: 100 year AOM reaction rate constant: 10 -2 m 3 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

10.  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.

11. Simulation results: Temperature profile in the sediment column °C°C

12. Simulation Results: Gas hydrate volume fraction profile in the sediment column V gas Hydrage V sediment

13. Simulation results: gas volume fraction profile in the sediment column V gas V sediment

14. Simulation results: AOM rate profile in the sediment column mol m 3.yr

15. Simulation results 0 20 40 60 80 100 0. 4 1.0 2.0 3.0 0 10 20 30 40 50 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)

16. Conclusion: Methane mass distribution 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

17. 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.

18. 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.

19. Acknowledgements  Cluster of Excellence “The Future Ocean” funded by the German Research Foundation (DFG)  Dr. Giovanni Aloisi  All of my colleagues