Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

Investigations of the correlation between depth of free methane gas and geochemical pore water profiles in the Arcona and Bornholm Basins

The deposition of organic material on the sea floor and its burial below the sulfate zone is the basis for the microbiological production of vast amounts of methane. Methane is an aggressive greenhouse gas when emitted into the atmosphere. The microbiological key process of sub-surface methane oxidation accounts for perhaps 90% of the entire methane flux in the sea floor and therefore plays a critical role as a barrier against methane emission. In order to understand the efficiency of this methane oxidation and its environmental regulation it is important to understand how the thickness of the Holocene layer covering the Pleistocene sediment affects the production and consumption of methane and sulfate. As part of the METROL project research cruises were made to the Western Baltic to study sedimentary systems with diffusive methane flux in the holocene deposition environments of the Bornholm and Arkona basins.

Key results:
The combined analyses of field data and reactive transport modelling showed that methane production in these sediments depends strongly on the quantity of labile organic matter deposited during the Holocene at the sediment-water interface and on the Holocene thickness. In areas of large organic matter inputs, high methanogenesis rates result in supersaturation of methane in the porewater and the formation of methane gas bubbles.

Geophysical seismic surveys of the Western Baltic Sea revealed a wide range of
-Holocene sediment thickness,
-The depths of the sulfate methane transition zone (SMTZ) in the sediment,
-The upper limit of free gas.

Calibration of the Reaction-Transport Models allowed for the identification of the most important factors controlling the depth of free gas and methane turnover. The approach of combining improved high resolution hydroacoustic technology with spatially dense sampling for methane profiles in a selected key area of Aarhus Bay closed a gap between interpretations of seismic imaging of free gas and calculations of methane flux dynamics. The data showed linear correlation between the saturation depth of methane and the upward methane flux, which equals the net methane oxidation. The data further allowed calibration of the upper limit of free gas seen in the seismic images with methane saturation depth in the pore water.

These results enabled us to map net oxidation of methane in Aarhus Bay on the basis of hydroacoustic information alone and thus mark another milestone for the project. This work illustrates how geophysical, biogeochemical and modelling techniques can be synthesized to efficiently determine rates of methane turnover in shelf sediments.

Potential users
-Marine geologists and microbiologists may benefit from the overview of gas occurrences in the area

-Environmental agencies in charge of monitoring coastal areas.

Informazioni correlate

Reported by

Max Planck Institute for Marine Microbiology
Celsiusstr. 1
28359 Bremen
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