Studies On The Distribution Of Dissolved Methane And Its Biogeochemistry In Canadian Arctic And Sub-Arctic Seas

Methane is the second most important greenhouse gas(after carbon dioxide)in the atmosphere.The atmospheric concentration of methane has drastically increased since the industrial times.Global warming became one of the major environmental issues facing the present world.As the most sensitive region to global warming,the mean near-surface air temperature over the Arctic has risen approximately 3 °C over the last decades,which is twice the global average value.The Arctic Ocean contains huge amounts of methane,particularly on its shelves where extensive subsea permafrost and gas hydrates are located.Arctic warming may lead to permafrost thawing and destabilization of gas hydrates,thereby releasing the stored methane to the overlying water column.Moreover,increased river runoff due to climate warming brings more terrestrial organic matter to the Arctic Ocean,which enhances microbial activity and potentially methanogenesis as well in both seawater and sediments.These processes can significantly increase the atmospheric methane concentration through air-sea gas exchange,producing a potential positive feedback to climate warming.Therefore,understanding the distribution of dissolved methane and its biogeochemistry in the Arctic Ocean is essential for improving our knowledge about the role of methane in regulating the Arctic climate.This thesis focuses on methane biogeochemical cycling in the little explored Canadian Arctic and sub-Arctic seas.The first part of this thesis describes the spatiotemporal distributions of dissolved methane,discusses its potential sources,and estimates its air-sea flux.The second part presents results obtained from laboratory incubations aiming at determining the net microbial methane consumption rates in deep water on the shelf of Baffin Bay.The final part assesses the possibility of photodecomposition of dissolved organic matter as a significant source of oceanic methane through laboratory irradiation of Arctic water samples and model organic compounds.The main conclusions are as follows:1.Methane in surface water: Most surface waters were oversaturated(up to 539%)with methane compared to the atmosphere in the study areas,particularly partially sea-ice covered zones in the Canadian Arctic Archipelago,the Labrador Sea shelf,the Mackenzie River plume,and the west of Davis Strait,whereas methane concentrations were close to atmospheric equilibrium in offshore waters.It is likely that the decomposition of organic matter in brine channel of sea ice,river input,in-situ production and seabed seepages could contribute to the excess methane in surface waters.2.Methane in water column: High concentrations(up to 30.71 nmol/L)of methane were centered in the Bering Sea Water(BSW)on the shelf of the northeastern Chukchi Sea.It is highly likely that this methane anomaly originated from the degradation of organic matter in the underlying shelf sediments.Methane was trapped within the pycnocline of the BSW and transported into the Canadian Basin through the Barrow Canyon by the BSW current,leading to its spatial heterogeneity.In the southeastern Beaufort Sea,the vertical distribution showed a subsurface methane maximum in the Canadian Basin and the western part of the Parry Channel.In contrast,methane concentration was lower and its vertical distribution was almost homogeneous in the eastern part of the Parry Channel.Excess methane was mainly present in the Pacific Summer Water,which could arise from methane formed as a by-product of bacterial metabolism of dimethylsulfoniopropionate(DMSP)and possibly from other sources as well,such as demethylation of dimethylsulfide(DMS)and dimethylsulphoxide(DMSO),anaerobic microbial activity in sea-ice brine channels,and photodegradation of organic matters.Active gas seepages were identified by remotely controlled vehicle diving in and near the Scott Inlet on the western shelf of the Baffin Bay.Extremely high methane concentrations(up to 280.25 nmol/L)were observed in the bottom water of the seepage area.Farther south,high concentrations of methane were frequently found in subsurface water with salinity of 33-34‰ due plausibly to southward transport of the seepage-derived methane by the Baffin Island Current.In addition,the western shelf of Davis Strait may possess potentially active seabed methane seepages.In fjords in the western Labrador Sea,river input could explain the high surface water methane oversaturation.3.Net methane consumption rate: The net methane consumption rate was low in the subsurface water on the shelf of western Baffin Bay.The mean net consumption rate constant was 0.00782 d-1,yielding a turnover time of 128 days,which provides indirect evidence of that the seabed methane seepage near Scott Inlet could lead to the subsurface methane anomalies on the entire shelves of the western Baffin Bay and Labrador Sea.4.Photoproduction of methane: Dissolved organic matter and DMS in surface waters of the Canadian Arctic and sub-Arctic seas produced methane under solar-simulated irradiation.The production rate depended on the irradiation time and the concentrations of these compounds,meanwhile it might also be influenced by the temperature and salinity of seawater.