Cryospheric influences on offshore Arctic groundwater systems : Offshore freshened groundwater and submarine groundwater discharge in a polar context

Offshore freshened groundwater has been increasingly observed along passive continental margins across the globe. In many regions, the emplacement of offshore freshwater is attributed to paleo ice sheets that overrode continental margins during the Last Glacial Maximum. Beyond creating potential unconventional freshwater resources and influencing near-seafloor physicochemical conditions, these relic groundwater systems also provide a window into past and ongoing cryosphere–groundwater interactions. However, the absence of observational constraints on offshore groundwater residence times and submarine groundwater discharge rates over geological timescales limits understanding of their long-term evolution and solute flux variations to the ocean. This dissertation addresses this gap by improving residence time constraints at freshened groundwater sites offshore northern Norway and Svalbard through radiocarbon dating of dissolved inorganic carbon that is transported with the groundwater. In addition, the long-term evolution and flow dynamics of the freshened groundwater system offshore northern Norway under glacial advance and retreat were investigated using a coupled hydromechanical model. A meteoric water component was observed in porewaters as far as the continental shelf break offshore northern Norway and across multiple fjords in Svalbard. This freshwater is hypothesized to have been emplaced during time periods when ice sheets or glaciers advanced across the continental shelf or fjords, enabling subglacial meltwater recharge. Offshore northern Norway in particular, this mechanism is considered most probable, as water depths are sufficiently great such that the continental shelf was unlikely to have been exposed to the atmosphere during periods of sea-level lowstand during the Late Pleistocene. Residence time estimates of the saline groundwater component indicate that seawater infiltrated the Norwegian continental margin approximately 12,000 years after deglaciation of the outer continental shelf, replacing glacially driven freshwater recharge. This timing is corroborated by model results, which show that following a period of groundwater exfiltration after glacial retreat, seawater began infiltrating the subsurface of the outer continental shelf. The highest freshwater infiltration rates were modeled during periods of rapid glacial growth. Studies of the glacimarine fjords of Svalbard further highlight the role of sedimentation. High sedimentation rates inhibit submarine groundwater discharge and may effectively seal offshore freshened groundwater bodies. In conclusion, ice sheets can emplace large volumes of freshwater, resulting in subseafloor freshened groundwater bodies that still exist today. Moreover, mechanical loading from the ice influences groundwater flow, affecting submarine groundwater discharge rates alongside processes such as density-driven flow and sediment loading. Offshore freshened groundwater bodies emplaced by paleo ice sheets are expected to undergo progressive salinization, whereas the retreat of contemporary marine-terminating glaciers, ice tongues, and ice shelves is likely to cause a temporary increase in submarine groundwater discharge under low-sedimentation conditions, followed by a decline as the system adjusts to modern conditions.