Ellen Damm

Methane discharge from a deep-sea submarine mud volcano into the upper water column by gas hydrate-coated methane bubbles

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 Hakon 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.

Benthic oxygen uptake, hydrolytic potentials and microbial biomass at the Arctic continental slope

Oxygen (O 2 ) uptake and microbial activity in sediments of the eastern Arctic continental slope were investigated in both ice-covered and ice-free areas of the Laptev Sea. Total O 2 flux (J) decreased markedly from 2 mmol m~2 d~1 at the shelf edge (50 m) to 0.07 mmol m~2 d~1 at the bottom of the slope (3500 m), matched by the more than tenfold decline in chlorophyll pigments (CPE), protein and dissolved amino acids (DFAA). Furthermore, concentrations of these labile organic compounds were strongly correlated with extracellular enzyme potentials (EEA) in the sediments as well as with microbial biomass. The concentrations of labile substances and total microbial biomass (TMB) as well as the rates of O 2 uptake and EEA were independent of the distribution of TOC, probably due to the dominance of nonlabile terrigenous compounds. Di⁄erences in O 2 uptake and microbial EEA between ice-covered and ice-free transects were relatively small. Values of O 2 uptake, CPE, EEA and TMB at the Laptev Sea slope were considerably lower than at temperate continental slopes but nevertheless higher than in the central Arctic deep-sea basin. Considering newly published data on primary productivity in the central Arctic, our results indicate that the benthic respiratory demand at the Laptev Sea slope and in the Arctic basin could be satisfied by the vertical flux of POC and does not necessarily depend on lateral advection of POC from the shelf seas as previously anticipated. ( 1998 Elsevier Science Ltd. All rights reserved.

Fate of vent-derived methane in seawater above the Håkon Mosby mud volcano (Norwegian Sea)

Abstract The Hakon Mosby mud volcano (HMMV) is a cold methane-venting seep situated at the Norwegian–Barents–Spitsbergen continental margin. Methane discharged by the vent creates a plume in the ambient seawater at 1200 m water depth. Here, we study the hydrographic regime to evaluate its influence on the distribution pattern of methane as it is shown by the concentration gradients. The stable carbon isotopic signature of methane is used to trace the vent methane in the lateral and vertical direction and to trace its fate in the hydrosphere. Direct methane release into the bottom water occurs in the central zone of the HMMV. Methane included in the subseafloor reservoir and in the plume above the vent has the same carbon isotopic ratios, which means that the released methane is not oxidized. The shape of the methane plume is determined by spreading predominantly along its original isopycnal. However, vent methane is traceable in seawater up to 800 m above the HMMV by methane values, which exceed the background and high carbon isotopic signature heterogeneity. The fate of vent methane in the hydrosphere is dominated by dilution and mixing with background methane within the bottom water rather than by oxidation as it is shown by the carbon isotopic ratios. Methane released by the HMMV moves northward with deep intermediate waters, which are detached from the ocean surface and may enter the polar Arctic Ocean. Consequently, methane discharged at the HMMV results in a direct input of fossil methane into the Recent methane reservoir of the deeper ocean and reduces the capacity of the deep ocean as a sink for atmospheric methane.

Methane excess in Arctic surface water- triggered by sea ice formation and melting

Arctic amplification of global warming has led to increased summer sea ice retreat, which influences gas exchange between the Arctic Ocean and the atmosphere where sea ice previously acted as a physical barrier. Indeed, recently observed enhanced atmospheric methane concentrations in Arctic regions with fractional sea-ice cover point to unexpected feedbacks in cycling of methane. We report on methane excess in sea ice-influenced water masses in the interior Arctic Ocean and provide evidence that sea ice is a potential source. We show that methane release from sea ice into the ocean occurs via brine drainage during freezing and melting i.e. in winter and spring. In summer under a fractional sea ice cover, reduced turbulence restricts gas transfer, then seawater acts as buffer in which methane remains entrained. However, in autumn and winter surface convection initiates pronounced efflux of methane from the ice covered ocean to the atmosphere. Our results demonstrate that sea ice-sourced methane cycles seasonally between sea ice, sea-ice-influenced seawater and the atmosphere, while the deeper ocean remains decoupled. Freshening due to summer sea ice retreat will enhance this decoupling, which restricts the capacity of the deeper Arctic Ocean to act as a sink for this greenhouse gas.

Widespread methane seepage along the continental margin off Svalbard - from Bjørnøya to Kongsfjorden

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.

The Transpolar Drift conveys methane from the Siberian Shelf to the central Arctic Ocean

Methane sources and sinks in the Arctic are poorly quantified. In particular, methane emissions from the Arctic Ocean and the potential sink capacity are still under debate. In this context sea ice impact on and the intense cycling of methane between sea ice and Polar surface water (PSW) becomes pivotal. We report on methane super- and under-saturation in PSW in the Eurasian Basin (EB), strongly linked to sea ice-ocean interactions. In the southern EB under-saturation in PSW is caused by both inflow of warm Atlantic water and short-time contact with sea ice. By comparison in the northern EB long-time sea ice-PSW contact triggered by freezing and melting events induces a methane excess. We reveal the Ttranspolar Drift Stream as crucial for methane transport and show that inter-annual shifts in sea ice drift patterns generate inter-annually patchy methane excess in PSW. Using backward trajectories combined with δ18O signatures of sea ice cores we determine the sea ice source regions to be in the Laptev Sea Polynyas and the off shelf regime in 2011 and 2015, respectively. We denote the Transpolar Drift regime as decisive for the fate of methane released on the Siberian shelves.

Acoustic detection of methane plumes

To understand the complexity of the global methane cycle with its relation to climate change, it is necessary to identify relevant methane sources and to assess their environmental functions. The contribution of the various marine methane sources to the global cycle is only poorly understood so far. Methods are needed to survey efficiently methane release from the sea floor. Remote techniques are favoured since they allow a survey of larger areas than, for example, sediment sampling. Low frequency echosounders, as used in fishery, were recognized to be appropriate tools for plume detection and are hoped to be the key for efficient plume mapping and gas quantification. Here, we present methane plumes from three different marine environments.