Sources and sinks of methane in Lake Baikal: A synthesis of measurements and modeling

We studied the methane (CH4) budget of Lake Baikal, the most voluminous lake in the world and the only freshwater body with known occurrences of methane hydrates in the sediments. CH4 concentrations were measured in water samples taken during six expeditions between October 2002 and June 2004; these expeditions covered the entire lake volume. A one- dimensional model was applied to (1) estimate the large- scale vertical CH4 fluxes within the South Basin of Lake Baikal, (2) determine the exchange with the atmosphere, and (3) constrain the CH4 inputs from seeps and mud volcanoes to the deep water. Fluxes were generally several orders of magnitude below previous estimates. The annual internal source of CH4 to the pelagic surface mixed layer was roughly estimated to be 40 Mg CH4. A large part of this input diffuses downwards and is consumed in the water column, with a CH4 residence time of about 4 yr. The input of CH4 from deep gas seeps and mud volcanoes is less than a few 10 Mg CH4 yr(-1), most of which is oxidized before reaching the surface. The net CH4 flux between the atmosphere and the main waterbody distant from shallow areas is negligible and not significantly different from zero. However, occasional high CH4 concentrations, both in the surface water and in the atmosphere, indicate that the region near the Selenga delta is a local CH4 source to the atmosphere. CH4 fluxes in the Central Basin are very similar to those in the South Basin, whereas in the North Basin, the shallow CH4 sources are weaker.