Pumping methane out of aquatic sediments – ebullition forcing mechanisms in an impounded river

[摘要] Freshwater systems contributesignificantly to the global atmospheric methane budget. A large fraction ofthe methane emitted from freshwaters is transported via ebullition. However,due to its strong variability in space and time, accurate measurements ofebullition rates are difficult; hence, the uncertainty regarding itscontribution to global budgets is large. Here, we analyze measurements madeby continuously recording automated bubble traps in an impounded river incentral Europe and investigate the mechanisms affecting the temporal dynamicsof bubble release from cohesive sediments. Our results show that the maintriggers of bubble release were pressure changes, originating from thepassage of ship lock-induced surges and ship passages. The response tophysical forcing was also affected by previous outgassing. Ebullition ratesvaried strongly over all relevant timescales from minutes to days; therefore,representative ebullition estimates could only be inferred with continuoussampling over long periods. Since ebullition was found to be episodic,short-term measurement periods of a few hours or days will likelyunderestimate ebullition rates. Our results thus indicate that flux estimatescould be grossly underestimated (by up to ~50%) if thecorrect temporal resolution is not used during data collection.

[学科分类] 地球化学与岩石