Variations in Gas and Water Pulses at an Arctic Seep: Fluid Sources and Methane Transport

Methane fluxes into the oceans are largely dependent on the methane phase as it migrates upward through the sediments. Here we document decoupled methane transport by gaseous and aqueous phases in Storfjordrenna (offshore Svalbard) and propose a three‐stage evolution model for active seepage in the region where gas hydrates are present in the shallow subsurface. In a preactive seepage stage, solute diffusion is the primary transport mechanism for methane in the dissolved phase. Fluids containing dissolved methane have high 87Sr/86Sr ratios due to silicate weathering in the microbial methanogenesis zone. During the active seepage stage, migration of gaseous methane results in near‐seafloor gas hydrate formation and vigorous seafloor gas discharge with a thermogenic fingerprint. In the postactive seepage stage, the high concentration of dissolved lithium points to the contribution of a deeper‐sourced aqueous fluid, which we postulate advects upward following cessation of gas discharge. Plain Language Summary How methane moves in the marine sediment, as a gas or a dissolved component, determines the environmental impact of this important greenhouse gas. In contrast to observations of biosphere activity beeing supported by dissolved methane, free gas methane cannot be used by microorganisms and can escape to the ocean more easily. Here we report the different ways methane moves in the sediments of an Arctic methane seep. We show that methane moves as free gas during the most active stage and as a dissolved component in the pore water before and after the most active period. Our results show that the supply of free gas methane in the sediments can explain why some of the seafloor features in our study area are more active than the others. Key Points We document the decoupled gaseous and aqueous fluid phases from Storfjordrenna, a cold seep south of Svalbard We propose a three‐stage evolution model with different modes of methane transport in each stage The evolution of the seepage is controlled by the availability of gaseous methane and/or the development of fluid conduits