Methane venting associated with submarine groundwater discharge in Eckernförde Bucht, Baltic sea

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Dando, P. R. , Rees, E. I. R. , Dando, M. A. , Schlüter, M. and Sauter, E. (2000): Methane venting associated with submarine groundwater discharge in Eckernförde Bucht, Baltic sea , Proceedings of the 6th International Conference on Gas in Marine Sediments; 5.-9. 9. 2000; St. Petersburg, Russia .
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Abstract:Submarine groundwater discharges are responsible for pockmark depressions on the bed ofEckernförde Bucht, Western Baltic. In these depressions there is an interface between almostfluid sediment and sea water that is typically about 1.5 2m below the surrounding seabed.Transects, made with a underwater digital video camera sled in October 1998 and September1999, showed that near the rims of the depressions the clay aquaclude was not covered byrecent methane generating organic sediment. Cracks could be seen in the clay indicating thatedges of the aquaclude layer are slowly collapsing into the steep sided depressions. Somesharply angular cobble sized clay clasts were seen just outside the pockmark rims suggestingthat blow-outs may occur. Acoustic turbidity is widespread in the sediments of the deeperparts of Eckernförde Bucht due to methane gas bubble formation within the sediment. Themethane seems to derive mainly from the decomposition of recent organic matter. Becausethe Bucht forms an cast facing funnel, near bed compensatory flows for the prevailingwesterly wind driven surface drift, seem to bring in phyto-detritus and faecal pellets whichdeposit and decay. Methane bubble release, gas escape holes with sediment and bacterial matswere widely observed through the Bucht in late summer and minor sediment disturbancecaused by, for example, crab tracks show that sulphidic sediment lies at best only a fewmillimetres below the surface in the deeper parts of the Bucht. Methane release wasparticularly prevalent towards the edge of the pockmarks and the release of free gas could beestimated from the density of recent bubble release holes.

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