Shallow Gas

Great amounts of methane are formed in marine sediments during the sub-surface degradation of buried organic matter. Methane accumulates in the seabed below the sulfate zone and, as methane is poorly soluble in seawater, methane bubbles may develop at a depth where the methane concentration exceeds saturation. The depth and extent of such shallow gas formation depend on the rate of methanogenesis, the thickness of the methanogenic deposit, and the hydrostatic pressure and thus the water depth. While most of the methane is oxidized by sediment microbial communities, methane bubbles occasionally also reach the sediment surface and escape from the sea floor in a diffuse or eruptive manner. Among the goals of our research is to understand what controls the distribution of gas and of methane emission.

Baltic Sea

As part of the EU funded BONUS program, the project BALTIC GAS studies shallow gas in the Baltic Sea with a focus on the effects of climate change and eutrophication on methane fluxes and gas storage. Eutrophication can accelerate seabed methane generation by enhancing sub-surface burial of organic matter. Shallow gas occurs widespread in the Baltic Sea, but the magnitude of gas storage and its potential instability are not known. BALTIC GAS aims to understand how climate change and long-term eutrophication affect the accumulation of shallow gas and the emission of methane and hydrogen sulfide from the seabed to the water column and atmosphere. Among the questions addressed are: what is the methane distribution in the seabed, what are the main sources of the methane, what is the rate of production and what controls the rate? An international group of BALTIC GAS scientists, including biogeochemists, geophysicists and chemists, addresses these questions. An overview poster presented at the 2010 AGU fall meeting can be found here.

The project terminated officially by December 31, 2011, and the results are now being published. Visit the BALTIC GAS webpage to learn more about the project and the results: www.balticgas.net.

Aarhus Bay

Seismic studies of the sea floor in Aarhus Bay have shown that gassy sediments are found only in areas where the organic-rich Holocene mud layer, deposited over the past 8000 years, exceeds a certain threshold thickness. The presence of free gas leads to a strong stimulation of methane fluxes compared to the gas free sediment. Coupled to the increased methane flux is an upwards shift of the sulfate-methane transition. The constraints on accumulation of shallow gas in the Holocene mud is widely observed but poorly understood: Why does free gas accumulate only at a certain thickness of the Holocene mud layer? Is the shift of the SMT by the appearance of free gas gradual or abrupt? What happens biogeochemically along the transition of a gas-free and gassy area? How do the fluxes of methane, sulfate, dissolved inorganic carbon and ammonium change along such a transition? We are trying to find answers to these questions by comparing pore water profiles and mineralization rates at multiple coring sites along a seismic line that showed a sudden appearance of free gas and a smooth increase of the Holocene mud layer.