Submarine groundwater discharge as a possible formation mechanism for permafrost‐associated gas hydrate on the circum‐Arctic continental shelf

Abstract

Submarine groundwater discharge (SGD) is a large‐scale, buoyancy‐driven, offshore flow of terrestrial groundwater. If SGD occurs within the permafrost‐bearing sediments of the circum‐Arctic shelf, such fluid circulation may transport large amounts of dissolved methane to the circum‐Arctic shelf, aiding the formation of permafrost‐associated gas hydrate. We investigate the feasibility of this new permafrost‐associated gas hydrate formation mechanism with a 2‐D, multiphase fluid flow model, using the Canadian Beaufort Shelf as an example. The numerical model includes freeze/thaw permafrost processes and predicts the unsteady, 2‐D methane solubility field for hydrate inventory calculations. Model results show that widespread, low‐saturation hydrate deposits accumulate within and below submarine permafrost, even if offshore‐flowing groundwater is undersaturated in methane gas. While intrapermafrost hydrate inventory varies widely depending on permafrost extent, subpermafrost hydrate stability remains largely intact across consecutive glacial cycles, allowing widespread subpermafrost accumulation over time. Methane gas escape to the sediment surface (atmosphere) is predicted along the seaward permafrost boundary during the early to middle years of each glacial epoch; however, if free gas is trapped within the forming permafrost layer instead, venting may be delayed until ocean transgression deepens the permafrost table during interglacial periods, and may be related to the spatial distribution of observed pingo‐like features (PLFs) on the Canadian Beaufort Shelf. Shallow, gas‐charged sediments are predicted above the gas hydrate stability zone at the midshelf to shelf edge and the upper slope, where a gap in hydrate stability allows free gas to accumulate and numerous PLFs have been observed.

Groundwater Discharge in the Arctic: A Review of Studies and Implications for Biogeochemistry

“…The last finding of this study was that submarine groundwater discharge at this site was largely buoyancy-driven, as fresh groundwater is less dense than the corresponding seawater [10]. Frederick and Buffet built upon this work with a later paper, where they explored the ability of submarine groundwater discharge to aid in methane hydrate expansion by delivering methane to the cold continental shelf sediments using a numerical model of two-phase flow of pore fluids, based on the finite volume method [50]. They found that in subpermafrost areas, groundwater flow did assist in the development of methane hydrate formation, even if the groundwater was undersaturated with respect to methane [50].…”

“…Frederick and Buffet built upon this work with a later paper, where they explored the ability of submarine groundwater discharge to aid in methane hydrate expansion by delivering methane to the cold continental shelf sediments using a numerical model of two-phase flow of pore fluids, based on the finite volume method [50]. They found that in subpermafrost areas, groundwater flow did assist in the development of methane hydrate formation, even if the groundwater was undersaturated with respect to methane [50]. However, the results of hydrate formation were inconsistent within areas of continuous permafrost, and relied more on permafrost extent [50].…”

“…They found that in subpermafrost areas, groundwater flow did assist in the development of methane hydrate formation, even if the groundwater was undersaturated with respect to methane [50]. However, the results of hydrate formation were inconsistent within areas of continuous permafrost, and relied more on permafrost extent [50]. One of the first studies of submarine groundwater discharge occurred at Cambridge Fjord on Baffin Island (11 on Figure 2) [51].…”

“…Not only was submarine groundwater discharge found to be a minor source of methane to the Arctic Ocean on the Alaskan coast, but on the Canadian coast, freshwater discharge was shown to increase methane hydrate stability and trap methane in hydrates before it could reach the ocean in dissolved phase [9,10,50]. Therefore, methane transport in this environment represents a stopgap in the positive feedback loop.…”

“…Therefore, methane transport in this environment represents a stopgap in the positive feedback loop. Methane saturated or even undersaturated in groundwater precipitates out in the form of methane hydrates on the cold continental shelf system, essentially stripping the submarine groundwater discharge of methane, which would otherwise have eventually vented to the atmosphere [50]. Of course, this stopgap has a limit of effectiveness, as eventually enough warming on the shelf would destabilize the hydrates.…”