ALBERT

Description: Concentrations of cosmogenic iodine, 129 I, in the pore fluid of marine sediments often indicate that the pore fluid is much older than the host sediment, even when vertical flow due to sediment compaction is taken into account. Old pore fluid has been used in previous studies to argue for pervasive upward fluid flow and a deep methane source for hydrate deposits. Alternatively, old pore fluid age may reflect more complex flow patterns. We use a two-dimensional numerical transport model to account for the effects of topography and fractures on pore fluid pathlines when sediment permeability is anisotropic. We find that fluid focusing can cause significant lateral migration as well as regions where downward flow reverses direction and returns toward the seafloor. Longer pathlines can produce pore fluid ages much older than that expected with a one-dimensional compaction model. For steady-state models with geometry representative of Blake Ridge (USA), a well-studied hydrate province, we find pore fluid ages beneath regions of topography and within fractured zones that are up to 70 Ma old. Our results suggest that the measurements of 129 I/ 127 I reflect a mixture of new and old pore fluid. However, old pore fluid need not originate at great depths. Methane within pore fluids can travel laterally several kilometers, implying an extensive source region around the deposit. This type of focusing should aid hydrate formation beneath topographic highs. © 2013 American Geophysical Union. All rights reserved

Description: 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 2D, multi-phase fluid flow model, using the Canadian Beaufort Shelf as an example. The numerical model includes freeze/thaw permafrost processes and predicts the unsteady, 2D 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 under-saturated in methane gas. While intra-permafrost hydrate inventory varies widely depending on permafrost extent, sub-permafrost hydrate stability remains largely intact across consecutive glacial cycles, allowing widespread sub-permafrost 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 mid-shelf to shelf edge and the upper slope, where a gap in hydrate stability allows free gas to accumulate and numerous PLFs have been observed.