Fluid sources and water–rock interactions of cryosphere-associated offshore fossil groundwater in high-latitude regions

Groundwater is an important, though highly system-dependent, regulator of cryosphere mobility and stability. However, current understanding of these processes is constrained by a scarcity of direct observations due to logistical challenges. Much of the evidence for groundwater–cryosphere coupling therefore derives from numerical modeling and conceptual frameworks. The occurence of offshore groundwater systems along present/past glaciated continental shelves & slopes provides a unique opportunity to constrain the boundary conditions governing the coupling between groundwater and cryospheric processes. This is because the recharge of these offshore groundwater bodies, located several tens to hundreds of meters below seafloor, requires steep hydraulic gradients allowing for robust attribution of flow drivers to changes in cryospheric conditions. In addition, offshore groundwater systems are generally located far from their fluid sources, and thus may respond the first when fluid recharge—for example, ice-sheet basal melt—weakens.Six high-latitude offshore groundwater sites were investigated for sediment and fluid geochemistry: three sites proximal to past glaciation (Lofoten–Vesterålen from the Norwegian Sea, Fifång Bay close to the Stockholm archipelago, and the Gulf of Finland) and three others in the vincinity of modern glaciers/ice caps, submarine permafrost, or mud volcanoes (Tempelfjorden and Hornsund fjords in Svalbard, Victoria and Petermann fjords in northwest Greenland, and Beaufort Sea shelf and slope). Radiocarbon dating of the offshore groundwater suggest recharge events from early Holocene to pre-Holocene. The mixing of other radiocarbon sources in the sediments, such as carbon derived from degradation of particulate organic matter and dissolution of carbonates, complicates the interpretation of the groundwater signal. By comparing radiocarbon results from overlying seawater, organic matter, carbonate, and adjacent meteoric fluid sources (rivers and glacial ice) at these six locations, we discuss the limitations and potential for constraining the residence time of cryosphere-associated offshore groundwater.