Seasonal variability of shallow biogenic gas in Chesapeake Bay

Methane gas in Chesapeake Bay sediments occurs in buried paleodrainage channels incised during the last glacial sealevel lowstand. Repeat acoustic surveys of two Chesapeake Bay sites during the past year (with a 2–15 kHz chirp sonar profiler and 100 kHz sidescan sonar) show that the top of the zone of acoustically opaque gas bubbles in the sediments fluctuates vertically by up to 0.5 m, apparently in response to seasonal temperature changes. The vertical movement of the gas layer is most apparent in the main channel of the bay, where the methane gas is very close to the seafloor. In areas where the gas is seen to be very near the seafloor on the chirp profiler records, the sidescan sonar data show corresponding high backscatter. This does not appear to be the result of differing sediment types. It is not likely that the sidescan signal is penetrating the seafloor at the shallow outer grazing angles of the sidescan swath, therefore, the gas must extend to the water–sediment interface. The areas of high backscatter caused by the gas are observed to change seasonally, expanding during the summer high-temperature period and shrinking during the winter low-temperature period.