A groundbreaking study published in Nature Geoscience has revealed the significant role of ancient groundwater reservoirs located beneath the ocean floor. Researchers from Stockholm University, in collaboration with international scientists from Norway, Poland, and Germany, have demonstrated how these subterranean freshwater sources were shaped by past ice sheet dynamics and subsequent sea level fluctuations.
The research, which focused on the seafloor off the northern Norwegian coast, involved collecting fluid samples at a depth of 760 meters. Analysis of these samples uncovered freshened groundwater emerging from the seabed, providing crucial evidence of its historical connection to glacial periods. By examining the radiocarbon content, a marker of its last contact with the atmosphere, scientists precisely dated the groundwater's origin to over 11,700 years ago, during the time the region was covered by the Fennoscandian ice sheet. During this glacial period, meltwater from the ice sheet is believed to have filled underground spaces. However, following the ice sheet's collapse and the subsequent rise in sea levels, this freshwater reservoir was gradually infiltrated and replaced by seawater. This process highlights the dynamic interplay between glacial cycles, sea level changes, and subterranean water systems.
This study provides the first detailed timeline illustrating how fossil groundwater flowed into the ocean and was influenced by glacial changes, even in offshore locations. It confirms the timing of shifts in groundwater composition and the rate at which these ancient freshwater sources became vulnerable to seawater intrusion after the supply of glacial meltwater ceased. The findings have far-reaching implications for understanding glacier stability, nutrient supply to marine ecosystems, and the ocean's carbon absorption capacity. In the context of current climate change, which is driving glacier retreat and impacting global sea levels, understanding these ancient groundwater systems is particularly relevant. The research underscores that fossil groundwater is not a static resource but is actively influenced by Earth's climatic history. Further research into submarine groundwater discharge (SGD) indicates its significant influence on coastal environments, potentially enhancing primary production or leading to eutrophication and harmful algal blooms if polluted. The composition of SGD can be altered by biogeochemical reactions within aquifers, making these subterranean estuaries critical sites for understanding nutrient and solute transport to the ocean. The global flux of nutrients, carbon, and metals to coastal waters via SGD is recognized as a significant component of the hydrological cycle.