Methane in ocean waters of the Bay of Bengal: its sources and exchange with the atmosphere

Three legs of cruise SO93 of the German research vessel R/V SONNE provided information on the methane distribution along different profiles of the Bay of Bengal during the NE monsoon in January 1994. A 650-km-long profile from the Sri Lankan coast to the Equator revealed maximum methane concentrations clearly associated with different water masses. Peak concentrations of 105 nl/l occur below 45 m water depth. A 2600-km-long profile from the Equator to the shelf of Bangladesh showed elevated concentrations in the surface waters (up to 800 nl/l on the shelf close to the Ganges/Brahmaputra mouth). Waters at 700 and 2100 m off Bangladesh are enriched in methane. Seismic profiles of the Parasound system point to the existence of a mud diapir at 2100 m, and a seismic wipe out at 700 m points to gas-charged sediments. Sediment gases are assumed to be the source of the methane in the deep water of this area. However, no exchange with the surface waters was observed. Methane contents of the surface waters are related to bacterial processes as shown by isotope data of methane. This newly generated methane only partly contributes to the atmospheric methane concentrations, especially on the shelf of Bangladesh close to the Ganges/Brahmaputra mouth with flux rates of 145 kg km−2 year−1. Large sections of the profiles, however, showed near-equilibrium conditions and even undersaturation of methane with respect to the atmosphere.