THE IMPORTANCE OF EPISODIC EVENTS IN CONTROLLING THE FLUX OF METHANE FROM AN ANOXIC BASIN

Methane distributions and fluxes were determined in the more southerly of two permanently anoxic basins at the head of the otherwise shallow and tidal Pettaquamscutt River. Fluxes out of the sediment were appro ximately equal to fluxes across the air-sea interface as determined by flux chamber and were of the order of 3 - 6 mg CH4 m-2 d-1. Ebullitio n, observed during one set of flux chamber experiments, increased air- sea fluxes by several orders of magnitude. Methane oxidation rates wit hin the waters of the basin were measured by using (CH4)-C-14, and a m aximum in oxidation rate was observed at the oxic-anoxic interface. Ox idation rates were about a factor of 10 lower than either measured sed iment-water or air-sea fluxes. Fluxes across the pycnocline also were lower than sediment-water fluxes and fell at the low end of the range of calculated air-sea fluxes (which were lower than measured air-sea f luxes). Therefore, on the basis of a short term survey, the system app eared to be roughly at steady state. However, in spite of the appearan ce of steady state, methane profiles measured over a period of several years varied dramatically suggesting that episodic events (ebullition , tidal mixing, upwelling, and intrusion events) must be major factors in the methane budget not obvious from short-term experiments. Episod ic events are probably also important for other systems such as produc tive coastal environments, suboxic estuaries or any other systems in w hich reduced gases might accumulate within or below a pycnocline. When the pycnocline is tilted or breaks down, substantial amounts of gases may be released to the atmosphere.