ALBERT

Description: Large quantities of methane are stored in hydrates and permafrost within shallow marine sediments in the Arctic Ocean. These reservoirs are highly sensitive to climate warming, but the fate of methane released from sediments is uncertain. Here, we review the principal physical and biogeochemical processes that regulate methane fluxes across the seabed, the fate of this methane in the water column, and potential for its release to the atmosphere. We find that, at present, fluxes of dissolved methane are significantly moderated by anaerobic and aerobic oxidation of methane. If methane fluxes increase then a greater proportion of methane will be transported by advection or in the gas phase, which reduces the efficiency of the methanotrophic sink. Higher freshwater discharge to Arctic shelf seas may increase stratification and inhibit transfer of methane gas to surface waters, although there is some evidence that increased stratification may lead to warming of sub-pycnocline waters, increasing the potential for hydrate dissociation. Loss of sea-ice is likely to increase wind speeds and seaair exchange of methane will consequently increase. Studies of the distribution and cycling of methane beneath and within sea ice are limited, but it seems likely that the sea-air methane flux is higher during melting in seasonally ice-covered regions. Our review reveals that increased observations around especially the anaerobic and aerobic oxidation of methane, bubble transport, and the effects of ice cover, are required to fully understand the linkages and feedback pathways between climate warming and release of methane from marine sediments.

Description: River estuaries are responsible for high rates of methane emissions to the atmosphere. The complexity and diversity of estuaries require detailed investigation of methane sources and sinks, as well as of their spatial and seasonal variations. The Elbe river estuary and the adjacent North Sea were chosen as the study site for this survey, which was conducted from October 2010 to June 2012. Using gas chromatography and radiotracer techniques, we measured methane concentrations and methane oxidation (MOX) rates along a 60 km long transect from Cuxhaven to Helgoland. Methane distribution was influenced by input from the methane-rich mouth of the Elbe and gradual dilution by methane-depleted sea water. Methane concentrations near the coast were on average 30 ± 13 nmol L−1, while in the open sea, they were 14 ± 6 nmol L−1. Interestingly, the highest methane concentrations were repeatedly detected near Cuxhaven, not in the Elbe River freshwater end-member as previously reported. Though, we did not find clear seasonality we observed temporal methane variations, which depended on temperature and presumably on water discharge from the Elbe River. The highest MOX rates generally coincided with the highest methane concentrations, and varied from 2.6 ± 2.7 near the coast to 0.417 ± 0.529 nmol L−1 d−1 in the open sea. Turnover times varied from 3 to 〉1000 days. MOX rates were strongly affected by methane concentration, temperature and salinity. We ruled out the supposition that MOX is not an important methane sink in most of the Elbe estuary and adjacent German Bight.

Description: In the eastern part of Lake Constance, the second largest pre-alpine Lake in Europe, about five hundred pockmarks (morphological depressions on the lake floor) were recently discovered of which ~ 40% release methane bubbles. The carbon isotopic composition of the escaping gas indicated that the methane is of biogenic origin. In our study, we investigated the fate of the released methane bubbles, i.e., the dissolution, oxidation or transport of the bubbles to the surface. At a littoral pockmark site (PM12, 12 m water depth) and a profundal pockmark (PM80, 80 m water depth), we analysed the dissolved methane concentrations and the methane isotopic carbon signature in the water column. At PM80, higher methane concentrations (up to 1523 nM), compared to the control site and the surface waters (225 ± 72 nM), were recorded only on some occasions and only in the bottom water, despite the fact that the released bubbles were dissolving within the hypolimnion based on bubble modeling. The isotope data suggest that most of the dissolved methane is oxidized below 40 m water depth. The isotopic signature of the methane in the surface water at PM80, however, differed from that of the methane in the hypolimnion; therefore, the surface methane at this profundal site is most likely an export product from the littoral zone. Assuming an initial bubble diameter of 5 mm, we calculated that these small bubbles would reach the surface, but approximately 96% of the methane would have dissolved from the bubble into the hypolimnion. At PM12, we observed higher concentrations of dissolved methane (312 ± 52 nM) with no significant differences between seasons or between control sites versus pockmark site. In the shallow water, divers estimated the bubble size to be 10 - 15 mm, which from a release depth of 12 m would barely dissolved in to the water column. The isotopic signature also indicated that there had been almost no methane oxidation in the shallow water column. Thus, the water depth of bubble release as well as the initial bubble size determine whether the methane enters the atmosphere largely unhindered (shallow site) or if the released methane is incorporated into the profundal water column.