Variability of acoustic turbidity in Eckernfoerde Bay (southwest Baltic Sea) related to the annual temperature cycle

The sediments of Eckernfoerde Bay (southwest Baltic Sea) are known for their high contents of gas. It causes acoustic turbidity, i.e., the masking of sedimentary horizons by absorption and scattering of seismic energy due to free gas bubbles, also known as "basin effect". Repeated surveys with side-scan sonar and subbottom profiler along coincident profiles gave new insight into the variability of the acoustic turbidity. For the first time it is evidenced that its depth varies by several decimeters with an annual cycle. We explain this by the temperature-dependent solubility of methane. The depth of the acoustic turbidity, however, does not vary in phase with the annual surface temperature cycle. The delay is an effect of the poor heat conductivity of the overlying water and sediment.