Deglaciation drove seawater infiltration and slowed submarine groundwater discharge

Submarine groundwater discharge—the flow of groundwater into the ocean—plays an important role in shaping coastal biogeochemical cycles. The absence of temporal constraints on offshore groundwater dynamics driven by proximal glacial loading hinders our assessment of how its circulation may vary in conceivable ice-free polar regions. Here we estimate residence times of saline groundwater at an active submarine groundwater discharge and methane seep site off the coast of northern Norway, near the continental shelf break. The subsurface hydrology in this area experienced drastic changes due to Fennoscandian Ice Sheet dynamics, offering insights into the consequences of glacial–interglacial transitions for offshore groundwater. Using radiocarbon dating of dissolved inorganic carbon in the upwards-advected groundwater, we determined saline groundwater residence times of 11.5 to 8.8 kyr and 4.8 to 2.6 kyr at two distinct discharge sites. The presence of a meteoric water component in sediment porewaters confirms offshore groundwater freshening driven by past glacial loading. This indicates that, as the ice sheet retreated and sea levels rose, seawater began to infiltrate the subsurface, replacing freshwater recharge. Our results provide observational evidence pinpointing the onset of seawater infiltration following deglaciation of the margin. These findings suggest that retreating marine-terminating glaciers will profoundly alter offshore groundwater composition and reduce discharge rates.