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

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.