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

Abstract

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 87 Sr/ 86 Sr 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.

Microbial communities from Arctic marine sediments respond slowly to methane addition during ex situ enrichments

“…In contrast, sediments from an active seep showed increases in SR‐AOM rates that corresponded with higher abundances of mcrA genes and percentage abundances of ANME. Microbial community data may be of use when paired with geochemical markers to further elucidate methane seepage in areas with spatiotemporally heterogeneous methane fluxes (Hong et al ., ).…”

Geological Controls on Fluid Flow and Gas Hydrate Pingo Development on the Barents Sea Margin

“…Therefore, they are sensitive to even minor changes in the pressure‐temperature field. After the discovery of the Storfjordrenna GHP field in 2014, several studies have been published on sediment and pore‐water chemistry, seabed biology, and postglacial evolution (Hong et al, , ; Sen et al, ; Serov et al, ). According to coupled ice sheet and gas hydrate modeling, the gas hydrate system in the Storfjordrenna existed under subglacial conditions over a long time, that is, at least 33 kyr (Serov et al, ).…”

Anomalous Concentration of Methane Emissions at the Continental Shelf Edge of the Northern Cascadia Margin

“…Thus, it seems important to understand the processes responsible for the leakage of mobilized carbon from this large global reservoir. A well‐recognized but poorly understood process that transfers mobilized carbon from the interior of continental margin sediments is through the emission of methane, both as gas bubbles and dissolved within the modified pore fluid that escapes from the seafloor (Boetius & Wenzhöfer, ; Egger et al, ; Hong et al, ; Suess, ). A minor portion of this mobilized carbon is immediately captured and returned to the sediment reservoir as solid authigenic carbonate deposits that form directly at the surface of the seafloor by anaerobic oxidation of methane, while the remaining methane is directly transferred to seawater where it is further oxidized to carbon dioxide (Sample et al, ; Torres et al, ).This study focuses on the 250‐km length of the Washington State portion of the Cascadia margin, extending from the Strait of Juan de Fuca to the Columbia River (Figure ) and uses well‐established, acoustic‐based geophysical surveys to image methane bubble streams within the water column (Loher et al, ).…”

“…Thus, it seems important to understand the processes responsible for the leakage of mobilized carbon from this large global reservoir. A well-recognized but poorly understood process that transfers mobilized carbon from the interior of continental margin sediments is through the emission of methane, both as gas bubbles and dissolved within the modified pore fluid that escapes from the seafloor (Boetius & Wenzhöfer, 2013;Egger et al, 2018;Hong et al, 2018;Suess, 2014).…”