A Numerical Study of Pore Fluid and Gas Migration Patterns Within Arctic Shelf Sediments Associated With Relict Off-Shore Permafrost
Abstract
Permafrost-associated methane hydrate deposits along the shallow Arctic continental shelf are thought to be a relict of glacial periods, when a large volume of Earth's water was locked up in polar ice and sea levels were lower, exposing the continental shelves to sub-freezing temperatures. Because of the cold surface temperatures, hydrate deposits are potentially stable here at unusually shallow depths, creating an extensive near-surface carbon reservoir. However, re-submergence of the shelf due to rising sea levels since the last glacial maximum 18 kyr ago has brought a temperature change of roughly +18C to the surface sediments. The evolution of permafrost-associated methane hydrate deposits is potentially complex, and an understanding of the temperature field alone is not sufficient. Salt, which is concentrated in pore fluids when permafrost forms, substantially changes the growth and decay of both permafrost and methane hydrate. The permafrost, in particular, has a strong influence on the mobility of gas within the shelf sediments. In order to quantify these complex interactions we have developed a two-dimensional, finite-volume model for two-phase flow of pore fluid and methane gas within Arctic shelf sediments. We track the evolution of temperature, salinity, and pressure fields with prescribed boundary conditions, and account for latent heat of water ice formation during growth or decay of permafrost. The permeability structure of the sediments is coupled to changes in permafrost. The model can be run over several glacial cycles to simulate the natural environment in which Arctic hydrate deposits form, while also allowing us to explore the consequences of addition warming due to anthropogenic forcing. Preliminary results show that pore fluid and gas migration is strongly influenced by the permeability variations imposed by the overlying permafrost. When permafrost grows, high salinity pore fluids form as salt is excluded from ice. Increasing salinity progressively lowers the freezing point of the local pore fluid, thus formation is limited. In regions of high salinity, the permafrost is `leaky,' or relatively permeable to fluid and gas due to incomplete freezing. At the same time, salty pore fluids sink due to negative buoyancy, inducing a salt-driven convection current which transports salty fluid away from the freezing front, allowing permafrost growth to continue. When permafrost melts, fresh water is released into the existing pore fluid, slowing salt-driven convection. These secondary buoyancy-driven flows can redistribute methane and other trace components in the fluid, thus altering hydrate stability. Our preliminary results are used as a basis for interpreting recent methane flux field data in the Arctic.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMOS43B1803F
- Keywords:
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- 0545 COMPUTATIONAL GEOPHYSICS / Modeling;
- 0702 CRYOSPHERE / Permafrost;
- 1621 GLOBAL CHANGE / Cryospheric change;
- 3004 MARINE GEOLOGY AND GEOPHYSICS / Gas and hydrate systems