Investigation of submarine groundwater discharge into the Baltic Sea through varved glacial clays

Submarine groundwater discharge (SGD) is an important process responsible for transporting terrestrial dissolved chemical substances into the coastal ocean, thereby impacting the marine ecosystem. Despites its significance, there are few studies addressing SGD in the northern Baltic Sea. Here we investigate the potential occurrence of SGD in an area characterized by seafloor terraces formed in varved glacial clay located around Fifång Island, Southern Stockholm Archipelago. We analyzed 222 Rn activity and porewater geochemistry in both marine and terrestrial sediment cores retrieved from Fifång Island and its surrounding offshore areas. Results from 222 Rn mass-balance calculations, water isotopes, salinity, chloride concentration, and dating (including 14 C and helium-tritium dating) indicate that modern groundwater flows through varved glacial clay layers and fractured rocks on Fifång Island and discharges into Fifång Bay. Additionally, the offshore cores reveal a saline groundwater source that, dating of the dissolved inorganic carbon, appears systematically younger than the hosting clay varves dated using the Swedish clay varve chronology. Acoustic blanking in our acquired sub-bottom profiles may be related to this fluid migration. The occurrence of this saline groundwater seems to be independent from the distance to the submarine terraces. Collectively, our study confirms the occurrence of submarine groundwater in the varved glacial clay close to Fifång Island and further offshore. Our findings help establish the significance of submarine groundwater discharge in influencing the past and present coastal environment in the Baltic Sea region. • Groundwater pathways are investigated in onshore and offshore varved glacial clay. • Seawater invasion and modern fresh groundwater discharges into sea occur onshore. • Younger saline groundwater intrudes into 11k-year-old varved glacial clay offshore. • Paleo submarine groundwater may impact salinity records on the northern Baltic Sea coast.