Origin and stability of a permafrost methane hydrate occurrence in the Canadian Shield

Relatively little attention has been given to the stability of methane hydrates, formed during periods of past climate change, currently in areas of continuous permafrost. Although a large portion of the Canadian arctic is underlain by crystalline rocks, the occurrence, phase, and origin of alkanes in crystalline rocks under thick permafrost conditions (> 500 m) have not been reported. For the first time, composition and isotopic data for gases from a crystalline shield environment currently under permafrost conditions are presented. Gas and water samples were collected from exploration boreholes and seeps between 890 and 1130 m depths in the Lupin gold mine, Nunavut, Canada. Gases were methane-dominate (64–87%), with nitrogen (10–37%) the next largest component, and smaller amounts of ethane, propane, and carbon dioxide. Pressure and temperature measurements indicated gas hydrates were stable at the site prior to mining operations, a conclusion supported by noble gas and salinity determinations. Gas hydrate stability over the last 120 kyr glacial cycle was demonstrated by calculating transient subsurface pressure and temperature conditions utilizing the Memorial University/University of Toronto Glacial Systems Model (MUN/UofT GSM) and the Hydrogeosphere groundwater flow model. Model results also indicated glacial loading increased subsurface pressures, resulting in increased hydrate stability fields during glacial periods. Subglacial groundwater recharge would be limited by any significant formation of gas hydrates. Gas composition, combined with carbon and hydrogen isotopic determinations on methane (− 56 to − 42‰ VPDB and − 349 to − 181‰ VSMOW), carbon dioxide (− 55 to − 15‰ VPDB), ethane (− 37 to − 27‰ VPDB and − 330 to − 228‰ VSMOW), and propane (− 34 to − 27‰ VPDB and − 196 to − 172‰ VSMOW), indicated formation of natural gases by thermogenic processes, mixed with bacteriogenic gas, reasonable, given site geologic history. Methane hydrate formation affected gas composition and gas δ2H values, complicating the interpretation. Gas production is not a modern process at this location, and the overall contribution to the global carbon budget is small. Glacial groundwater recharge models need to account for methane hydrate formation and dissipation due to changes in subsurface stability fields during glaciation and the effect of methane hydrate on hydraulic conductivity.