Gas seeps in Lake Baikal-detection, distribution, and implications for water column mixing

Echo sounders served to locate a large number of shallow- and deepwater gas seeps at the bottom of all three basins of Lake Baikal during the years 2005 to 2008. A substantial proportion of the shallow gas seeps was located near the delta of the Selenga River, and at the Posolskii uplift. Deepwater gas seeps were recorded at the lake bed both inside and outside of areas where a bottom-simulating reflector was identified in seismic profiles. By monitoring the activity of gas emissions at the gas seeps, times of episodic gas ebullition could be distinguished from times of persistent gas bubble streams. A maximum gas flare height of more than 950 m above the bottom was recorded at the St. Petersburg mud volcano located in the central basin of Lake Baikal. Based on calculations from echo sounder data, the ascent velocity of gas bubbles reached 40 cm/s. In the area of gas seepage, there was a thick near-bottom layer, in which the gradient of water temperature was equal to the adiabatic gradient. This implies complete mixing of the water close to the lake bed, resulting from ascending gas bubbles released at seep sites. Analyses of vertical temperature profiles indicate possibly localized upwelling up to the lake surface when gas emissions are intensive.