Geochemical Effects of Deep Permafrost Formation in the Canadian Shield and Mars

The tectonically inactive Canadian Shield provides a reasonable terrestrial analogue to modern Martian conditions. Because most Martian water is likely trapped within and beneath permafrost, understanding fluid-rock interactions in similar Earth environments can provide valuable insight for Mars exploration. The entire outcrop area of the Canadian Shield was affected by permafrost during Plio-Pleistocene glacial cycles, and permafrost remains prevalent across vast areas of the Canadian Shield today. The purpose of this study is to investigate the effects of deep permafrost formation and dissipation during the Pleistocene glacial/periglacial cycle to deep Canadian Shield groundwaters.

During permafrost development, ice formation can concentrate solutes and change fluids isotopic compositions. Empirical data suggest Na/Cl and Br/Cl ratios evolve differently during freezing of Ca- and Na- dominated fluids, but anionic dominance does not affect either ratio. Regardless of fluid composition, oxygen and hydrogen isotopes in water evolve along a line with a slope slightly less than the Global Meteoric Waterline. Similar effects are expected during methane hydrate formation.

Data from thirty-nine sites at twenty-four locations across the Canadian Shield were compiled; twelve sites are currently located within or near permafrost. Relationships between δ¹⁸O and Cl indicate the freeze-out process has affected some fresh, brackish, and saline fluids, with subsequent mixing with brines. However, the freezing affected fluids are often deeper than maximum paleo-permafrost depths. These trends may therefore be attributed to methane hydrate formation or deep glacial recharge. The most concentrated Canadian Shield fluids and brines do not appear to have been impacted by freezing processes, but evolved through other water-rock interaction processes. However, formation of more concentrated freezing-impacted fluids on Mars should not be discounted due to the lower subsurface temperatures. Presence of significant subsurface methane may result in methane hydrate formation extending the depths of freezing-impacted fluids significantly beyond the base of permafrost.