Effect of Submarine Groundwater Discharge on Relict Arctic Submarine Permafrost and Gas Hydrate

Abstract:

Permafrost-associated gas hydrate deposits exist at shallow depths within the sediments of the circum-Arctic continental shelves. Degradation of this shallow water reservoir has the potential to release large quantities of methane gas directly to the atmosphere. Gas hydrate stability and the permeability of the shelf sediments to gas migration is closely linked with submarine permafrost. Submarine permafrost extent depends on several factors, such as the lithology, sea level variations, mean annual air temperature, ocean bottom water temperature, geothermal heat flux, and the salinity of the pore water. The salinity of the pore water is especially relevant because it partially controls the freezing point for both ice and gas hydrate. Measurements of deep pore water salinity are few and far between, but show that deep off-shore sediments are fresh. Deep freshening has been attributed to large-scale topographically-driven submarine groundwater discharge, which introduces fresh terrestrial groundwater into deep marine sediments.

We investigate the role of submarine ground water discharge on the salinity field and its effects on the seaward extent of relict submarine permafrost and gas hydrate stability on the Arctic shelf with a 2D shelf-scale model based on the finite volume method. The model tracks the evolution of the temperature, salinity, and pressure fields given imposed boundary conditions, with latent heat of water ice and hydrate formation included. The permeability structure of the sediments is coupled to changes in permafrost. Results show that pore fluid is strongly influenced by the permeability variations imposed by the overlying permafrost layer. Groundwater discharge tends to travel horizontally off-shore beneath the permafrost layer and the freshwater-saltwater interface location displays long timescale transient behavior that is dependent on the groundwater discharge strength. The seaward permafrost extent is in turn strongly influenced by the salinity field and location of the freshwater-saltwater transition. Our results suggest that the role of salt transport and its effect on permafrost evolution can provide context for the interpretation of recent permafrost maps and methane observations in the Arctic.