Submarine groundwater discharge and methane seepage driven by Fennoscandian Ice Sheet dynamics offshore northern Norway

Abstract

Freshwater in sediment pore fluids and methane seepage from the seafloor have often been observed concurrently in Arctic regions where submarine groundwater discharge (SGD) occurs, with advective flows potentially reintroducing ancient carbon into the modern ocean. It is hypothesized that hydraulic loading by ice sheets enhances submarine groundwater discharge, and subsequently methane transport. In the presence of microbial communities, large amounts of methane can be consumed by the anaerobic oxidation of methane (AOM), inducing precipitation of authigenic carbonates that inherit unique carbon isotopic signatures from methane. At an SGD site offshore Lofoten Islands, northern Norway (ca. 800 meters water depth), in the vicinity of the maximum extent of the Fennoscandian Ice Sheet, we observed a downcore decreasing chlorinity profile and a linear relation between δ18O and δ2H of the porewater, known as the local meteoric water line. This demonstrates a meteoric water contribution to the porewater. We also found methane-derived authigenic carbonates (MDACs) with depleted δ13C values (< -30 ‰ VPDB), suggesting that microbial (or thermogenic) methane was incorporated during MDAC precipitation. Moreover, δ18O values (> 2.5 ‰ VPDB) of MDACs indicate precipitation in the presence of 18O-enriched water, possibly a result of past hydrate dissociation. To assess the carbon cycle and timing of the methane seepage at the Lofoten SGD site, we investigated the radiocarbon contents of Total Organic and Inorganic Carbon in sediments (TOC, TIC), as well as Dissolved Inorganic Carbon (DIC) in porewater from multiple sediment horizons. The radiocarbon contents of DIC have the lowest values among the three carbon pools, in the order of 10-30 percent modern carbon. Their radiocarbon ages (~ 17,000 years BP) are in the order of the Last Glacial Maximum. Consequently, the DIC must have been closed off from the atmosphere due to long groundwater retention times. Alternatively, a methane source low in radiocarbon could have contributed to the DIC pool through AOM. The TIC pool showed radiocarbon content half of that of the TOC in the same sediment horizons, which can be explained by carbonate precipitation from the radiocarbon-depleted DIC pool. To further constrain in situ carbon cycling and advection velocities, a reaction-transport model using mass balance calculations of 12C, 13C and 14C has been applied. Radiocarbon content profiles of the DIC indeed imply the advection of old groundwater into the marine sediment porous media.