The assessment of climate change factors includes a constraint of methane sources and sinks. Although marine geological sources are recognized as significant, unfortunately, most submarine sources remain poorly quantified. Beside cold vents and coastal anoxic sediments, the large number of submarine mud volcanoes (SMV) may contribute significantly to the oceanic methane pool. Recent research suggests that methane primarily released diffusively from deep-sea SMVs is immediately oxidized and, thus, has little climatic impact. New hydro-acoustic, visual, and geochemical observations performed at the deep-sea mud volcano Håkon Mosby reveal the discharge of gas hydrate-coated methane bubbles and gas hydrate flakes forming huge methane plumes extending from the seabed in 1250 m depth up to 750 m high into the water column. This depth coincides with the upper limit of the temperature-pressure field of gas hydrate stability. Hydrographic evidence suggests bubble-induced upwelling within the plume and extending above the hydrate stability zone. Thus, we propose that a significant portion of the methane from discharged methane bubbles can reach the upper water column, which may be explained due to the formation of hydrate skins. As the water mass of the plume rises to shallow water depths, methane dissolved from hydrated bubbles may be transported towards the surface and released to the atmosphere. Repeated acoustic surveys performed in 2002 and 2003 suggest continuous methane emission to the ocean. From seafloor visual observations we estimated a gas flux of 0.2 (0.08-0.36) mol s−1 which translates to several hundred tons yr−1 under the assumption of a steady discharge. Besides, methane was observed to be released by diffusion from sediments as well as by focused outflow of methane-rich water. In contrast to the bubble discharge, emission rates of these two pathways are estimated to be in the range of several tons yr−1 and, thus, to be of minor importance. Very low water column methane oxidation rates derived from incubation experiments with tritiated methane suggest that methane is distributed by currents rather than oxidized rapidly.

Abstract.A methane surplus relative to the atmospheric equilibrium is a frequently observed feature of ocean surface water. Despite the common fact that biological processes are responsible for its origin, the formation of methane in aerobic surface water is still poorly understood. We report on methane production in the central Arctic Ocean, which was exclusively detected in Pacific derived water but not nearby in Atlantic derived water. The two water masses are distinguished by their different nitrate to phosphate ratios. We show that methane production occurs if nitrate is depleted but phosphate is available as a P source. Apparently the low N:P ratio enhances the ability of bacteria to compete for phosphate while the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) is utilized as a C source. This was verified by experimentally induced methane production in DMSP spiked Arctic sea water. Accordingly we propose that methylated compounds may serve as precursors for methane and thermodynamic calculations show that methylotrophic methanogenesis can provide energy in aerobic environments.

Abstract. The bacterially mediated aerobic methane oxidation (MO x ) is a key mechanism in controlling methane (CH 4 ) emissions from the world's oceans to the atmosphere. In this study, we investigated MO x in the Arctic fjord Storfjorden (Svalbard) by applying a combination of radio-tracerbased incubation assays ( 3 H-CH 4 and 14 C-CH 4 ), stable C-CH 4 isotope measurements, and molecular tools (16S rRNA gene Denaturing Gradient Gel Electrophoresis (DGGE) fingerprinting, pmoA-and mxaF gene analyses). Storfjorden is stratified in the summertime with melt water (MW) in the upper 60 m of the water column, Arctic water (ArW) between 60 and 100 m, and brine-enriched shelf water (BSW) down to 140 m. CH 4 concentrations were supersaturated with respect to the atmospheric equilibrium (about 3-4 nM) throughout the water column, increasing from ∼ 20 nM at the surface to a maximum of 72 nM at 60 m and decreasing below. MO x rate measurements at near in situ CH 4 concentrations (here measured with 3 H-CH 4 raising the ambient CH 4 pool by < 2 nM) showed a similar trend: low rates at the sea surface, increasing to a maximum of ∼ 2.3 nM day −1 at 60 m, followed by a decrease in the deeper ArW/BSW. In contrast, rate measurements with 14 C-CH 4 (incubations were spiked with ∼ 450 nM of 14 C-CH 4 , providing an estimate of the CH 4 oxidation at elevated concentration) showed comparably low turnover rates (< 1 nM day −1 ) at 60 m, and peak rates were found in ArW/BSW at ∼ 100 m water depth, concomitant with increasing 13 C values in the residual CH 4 pool. Our results indicate that the MO x community in the surface MW is adapted to relatively low CH 4 concentrations. In contrast, the activity of the deep-water MO x community is relatively low at the ambient, summertime CH 4 concentrations but has the potential to increase rapidly in response to CH 4 availability. A similar distinction between surface and deepwater MO x is also suggested by our molecular analyses. The DGGE banding patterns of 16S rRNA gene fragments of the surface MW and deep water were clearly different. A DGGE band related to the known type I MO x bacterium Methylosphaera was observed in deep BWS, but absent in surface MW. Furthermore, the Polymerase Chain Reaction (PCR) amplicons of the deep water with the two functional primers sets pmoA and mxaF showed, in contrast to those of the surface MW, additional products besides the expected one of 530 base pairs (bp). Apparently, different MO x communities have developed in the stratified water masses in Storfjorden, which is possibly related to the spatiotemporal variability in CH 4 supply to the distinct water masses.

Sea ice is an important transport vehicle for gaseous, dissolved and particulate matter in the Arctic Ocean. Due to the recently observed acceleration in sea ice drift, it has been assumed that more matter is advected by the Transpolar Drift from shallow shelf waters to the central Arctic Ocean and beyond. However, this study provides first evidence that intensified melt in the marginal zones of the Arctic Ocean interrupts the transarctic conveyor belt and has led to a reduction of the survival rates of sea ice exported from the shallow Siberian shelves (−15% per decade). As a consequence, less and less ice formed in shallow water areas (<30 m) has reached Fram Strait (−17% per decade), and more ice and ice-rafted material is released in the northern Laptev Sea and central Arctic Ocean. Decreasing survival rates of first-year ice are visible all along the Russian shelves, but significant only in the Kara Sea, East Siberian Sea and western Laptev Sea. Identified changes affect biogeochemical fluxes and ecological processes in the central Arctic: A reduced long-range transport of sea ice alters transport and redistribution of climate relevant gases, and increases accumulation of sediments and contaminates in the central Arctic Ocean, with consequences for primary production, and the biodiversity of the Arctic Ocean.

Numerous articles have recently reported on gas seepage offshore Svalbard, because the gas emission from these Arctic sediments was thought to result from gas hydrate dissociation, possibly triggered by anthropogenic ocean warming. We report on findings of a much broader seepage area, extending from 74° to 79°, where more than a thousand gas discharge sites were imaged as acoustic flares. The gas discharge occurs in water depths at and shallower than the upper edge of the gas hydrate stability zone and generates a dissolved methane plume that is hundreds of kilometer in length. Data collected in the summer of 2015 revealed that 0.02-7.7% of the dissolved methane was aerobically oxidized by microbes and a minor fraction (0.07%) was transferred to the atmosphere during periods of low wind speeds. Most flares were detected in the vicinity of the Hornsund Fracture Zone, leading us to postulate that the gas ascends along this fracture zone. The methane discharges on bathymetric highs characterized by sonic hard grounds, whereas glaciomarine and Holocene sediments in the troughs apparently limit seepage. The large scale seepage reported here is not caused by anthropogenic warming.Methane is, after water vapor and CO 2 , the most abundant greenhouse gas on Earth. When averaged over a 100 yr timescale, the warming effect of methane per unit mass is 28 times higher than that of CO 2 1 . Methane is produced in oceanic sediments either by methanogens at temperatures typically below ~80 °C, or through the breakdown of organic molecules at higher temperatures 2,3 . Buoyancy and pressure gradients can drive gas advection to shallower sediments where methane can be consumed via anaerobic oxidation of methane (AOM) 4 at the sulfate-methane transition zone and aerobic methane oxidation at the sediment surface 5 . Methane can also be sequestered within a cage of water molecules, in a gas hydrate structure, stable under the low temperature and high pressure conditions that define the gas hydrate stability zone 6 . If the upward methane flux is not fully exhausted by these processes, methane is emitted to the ocean either dissolved in the venting fluids or, in case of over-saturation, as gas bubbles 7 . As the bubbles ascend through the water column, a fraction of the methane gas dissolves 8 , generating patches of high methane concentration 9 . When the gas discharge is persistent and vigorous, it leads to the formation of large dissolved methane plumes. The dissolved methane is diluted by mixing with the surrounding ocean water and it is further oxidized by aerobic methanotrophs. Only in cases where dissolved methane reaches the surface-mixed layer in concentrations above saturation, can it be transferred to the atmosphere via sea-air gas exchange 10 . At present, the oceanic methane source to the atmosphere is very small (2-10%) 11 , as it is limited to emissions from vigorous and shallow seeps (<100 m) 1,7,8 . There is, however, an ongoing controversy regarding the methane discharge from sediments during warming events througho...