In the Arctic Seas, the West Spitsbergen continental margin represents a prominent methane seep area. In this area, free gas formation and gas ebullition as a consequence of hydrate dissociation due to global warming are currently under debate. Recent studies revealed shallow gas accumulation and ebullition of methane into the water column at more than 250 sites in an area of 665 km 2 . We conducted a detailed study of a subregion of this area, which covers an active gas ebullition area of 175 km 2 characterized by 10 gas flares reaching from the seafloor at 245 m up to 50 m water depth to identify the fate of the released gas due to dissolution of methane from gas bubbles and subsequent mixing, transport and microbial oxidation. The oceanographic data indicated a salinity-controlled pycnocline situated 20 m above the seafloor. A high resolution sampling program at the pycnocline at the active gas ebullition flare area revealed that the methane concentration gradient is strongly controlled by the pycnocline. While high methane concentrations of up to 524 nmol L-1 were measured below the pycnocline, low methane concentrations of less than 20 nmol L-1 were observed in the water column above. Variations in the δ 13 C CH4 values point to a 13 C depleted methane source being mainly mixed with a background values of the ambient water. A gas bubble dissolution model indicates that 80% of the methane released from gas bubbles into the ambient water takes place below the pycnocline. This dissolved methane will be laterally transported with the current northwards and most likely microbially oxidized in between 50 and 100 days, since microbial CH 4 oxidation rates of 0.78 nmol d-1 were measured. Above the pycnocline, methane concentrations decrease to local background concentration o

A method is presented for the online measurement of methane in aquatic environments by application of membrane inlet mass spectrometry (MIMS). For this purpose, the underwater mass spectrometer Inspectr200-200 was applied. A simple and reliable volumetric calibration technique, based on the mixing of two end member concentrations, was used for the analysis of CH 4 by MIMS. To minimize interferences caused by the high water vapor content, permeating through the membrane inlet system into the vacuum section of the mass spectrometer, a cool-trap was designed. With the application of the cool-trap, the detection limit was lowered from 100 to 16 nmol/L CH 4 . This allows for measurements of methane concentrations in surface and bottom waters of coastal areas and lakes. Furthermore, in case of membrane rupture, the cool-trap acts as a security system, avoiding total damage of the mass spectrometer by flushing it with water. The Inspectr200-200 was applied for studies of methane and carbon dioxide concentrations in coastal areas of the Baltic Sea and Lake Constance. The low detection limit and fast response time of the MIMS allowed a detailed investigation of methane concentrations in the vicinity of gas seepages. aAm Soc Mass T he analysis of methane as well as other trace gases like carbon dioxide, nitrous oxide, or dimethyl sulphide in aquatic environments is a major objective of basic and applied research. This includes investigations of the air-sea exchange of these greenhouse gases as well as of the release of methane from gassy sediments, hydrocarbon reservoirs, and pipelines, or through dissociation of gas hydrates. Although the spatial extent of such discharge sites is often rather small, ranging from a few square meters to several square kilometers, they are considered major drivers for the marine methane cycle in the present and past [1][2][3][4].The localization of such discharge sites as well as the detection and quantification of trace gases in lakes or the ocean relies essentially on water sampling (e.g., by Rosette Water Samplers, Hydro-Bios, Kiel, Germany) and subsequent chemical analysis by gas chromatography (GC) or infrared-spectrometry (IR) onboard the research vessel. Application of GC as well as IR requires the phase transfer of gases from the dissolved to the gaseous phase. For this purpose, head-space techniques, vacuum-degassing, or spray chambers are applied [5].Head-space techniques and vacuum-degassing are very suitable for analysis of discrete water samples but or N 2 0 in surface waters sampled along transects by research vessels. Nevertheless, the time required for equilibration and subsequent GC or IR analysis is about several tens of minutes to hours [7]. Hence, highresolution investigations, in time and space, of trace gases at natural gas seepages like pockmarks (morphological depressions at the seafloor) or hydrocarbon leakages during surveys by research vessels are rather difficult and time consuming.Compared with such rather long measuring times, application of membrane inlet mass...

Abstract. We investigated dissolved methane distributions along a 6 km transect crossing active seep sites at 40 m water depth in the central North Sea. These investigations were done under conditions of thermal stratification in summer (July 2013) and homogenous water column in winter (January 2014). Dissolved methane accumulated below the seasonal thermocline in summer with a median concentration of 390 nM, whereas during winter, methane concentrations were typically much lower (median concentration of 22 nM). High-resolution methane analysis using an underwater mass-spectrometer confirmed our summer results and was used to document prevailing stratification over the tidal cycle. We contrast estimates of methane oxidation rates (from 0.1 to 4.0 nM day−1) using the traditional approach scaled to methane concentrations with microbial turnover time values and suggest that the scaling to concentration may obscure the ecosystem microbial activity when comparing systems with different methane concentrations. Our measured and averaged rate constants (k') were on the order of 0.01 day−1, equivalent to a turnover time of 100 days, even when summer stratification led to enhanced methane concentrations in the bottom water. Consistent with these observations, we could not detect known methanotrophs and pmoA genes in water samples collected during both seasons. Estimated methane fluxes indicate that horizontal transport is the dominant process dispersing the methane plume. During periods of high wind speed (winter), more methane is lost to the atmosphere than oxidized in the water. Microbial oxidation seems of minor importance throughout the year.

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Detailed organic geochemical and carbon isotopic (δ13C and Δ14C) analyses are performed on permafrost deposits affected by coastal erosion (Herschel Island, Canadian Beaufort Sea) and adjacent marine sediments (Herschel Basin) to understand the fate of organic carbon in Arctic nearshore environments. We use an end‐member model based on the carbon isotopic composition of bulk organic matter to identify sources of organic carbon. Monte Carlo simulations are applied to quantify the contribution of coastal permafrost erosion to the sedimentary carbon budget. The models suggest that ~40% of all carbon released by local coastal permafrost erosion is efficiently trapped and sequestered in the nearshore zone. This highlights the importance of sedimentary traps in environments such as basins, lagoons, troughs, and canyons for the carbon sequestration in previously poorly investigated, nearshore areas.