Shallow gas in the muddy sediments of Eckernforde Bay, Germany

The seafloor of central Eckernforde Bay is characterised by soft muddy sedi ments that contain free methane gas. Bubbles of free gas cause acoustic tur bidity which is observed with acoustic remote sensing systems. Repeated sur veys with subbottom profiler and side scan sonar revealed an annual period both of depth of the acoustic turbidity and backscatter strength. The effec ts are delayed by 3-4 months relative to the atmospheric temperature cycle. In addition, prominent pockmarks, partly related to gas seepage, were dete cted with the acoustic systems. In a direct approach gas concentrations wer e measured from cores using the gas chromatography technique. From differen t tests it is concluded that subsampling of a core should start at its base and should be completed as soon as possible, at least within 35 min after core recovery. Comparison of methane concentrations of summer and winter co res revealed no significant seasonal variation. Thus, it is concluded that the temperature and pressure influences upon solubility control the depth v ariability of acoustic turbidity which is observed with acoustic remote sen sing systems. The delay relative to the atmospheric temperature cycle is ca used by slow heat transfer through the water column. The atmospheric temper ature cycle as 'exiting function' for variable gas solubility offers an opp ortunity for modelling and predicting the depth of the acoustic turbidity. In practice, however, small-scale variations of, e.g., salinity, or gas con centration profile in the sediment impose limits to predictions. In additio n, oceanographic influences as mixing in the water column, variable water i nflow, etc. are further complications that reduce the reliability of predic tions. (C) 1998 Elsevier Science Ltd. All rights reserved.