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EME-09 Risks, Resources, and Record of the Past on the Continental Shelf
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Occurrence and significance of gas-bearing sediments in gdansk deep of the Baltic Sea
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Vadim Sivkov, Atlantic branch of Shirshov Institute of Oceanology, Russian Academy of Sciences (Russian Federation)
Marina Ulyanova, Atlantic branch of Shirshov Institute of Oceanology, Russian Academy of Sciences (Russian Federation)
Nikolay Pimenov, Vinogradsky Institute of Microbiology, Russian Academy of Sciences (Russian Federation)
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Gas-bearing sediments (GBS) were defined in different parts of world inner continental shelf including the Baltic Sea. GBS have to be considered as a risk for marine ecosystem because affect the "normal" passing of biogeochemical processes. Exit of degassing products to the near-bottom layer of the sea sets up areas with anomaly conditions which influence the composition and development of plankton, fish and benthos. Sediment methane has to be taken into account while studying global warming. Its contribution into biogeochemical cycles (including carbon cycle) is studied not enough. CMethane is an effective green house gas with a thermal molecular absorption coefficient 25-fold higher than that of CO2. Heightened concentration of gases in sediments makes them unstable for constructions on the sea bottom so they have to be considered as a risk in planning of such installations as drilling platforms, wind parks, pipelines, navigational signals and others.
Gas existence in bottom sediments sharply changes physical characteristics of geological conditions and abruptly influences the acoustic wave dissipation. As a result there are specific anomalies at the acoustic records. The sediments became acoustically transparent (acoustic "windows" or "holes"). This phenomena interrupts the real view of bottom surface so it's a risk for fishing and navigation in the deep of sea.
Hydrocarbon gases in sediments migrate up from the deep deposits of oil and gas and can serve as exploration indicators of these resources. It is actually for South-Eastern Baltic because exploration and production of oilfield are carried out there. The first studies on GBS in the South-Eastern part of the Baltic Sea were carried out in 1984-1987. Few years ago we began a new modern study.
We had found by echosounder and mapped several areas of muddy GBS and pockmarks which are attached to the eastern slope of the Gdansk Deep (90-100 m). Microbiological and biogeochemical investigations showed increasing in CH4 concentration in the near-bottom water and surface sediments in pockmark and higher microbial activity. The total area covered by investigations is divided to three parts: 1) shallow sandy sediments with low methane concentrations and microorganisms abundance (20 l/dm3 and 0.1x108 cells/ml, accordingly), 2) muddy sediments with medium values (160-375 l/dm3 and 3.0 x108 cells/ml), and 3) pockmark area with high values (617 l/dm3 and 7.4 x108 cells/ml).
Results of the measurement of anaerobic CH4 oxidation and sulfate reduction rate in the surface sediments of crater area are in agreement with a stoichiometry of 1:1 between these values. CH4 had mixed origin (with prevalence of biogenic methane), as indicated by δ13C values of ? 53.2 and -56.6 ‰.
Study was carried out under financial support of RFBR (07-04-00160-a, 07-04-100051-k).
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