Extreme organic carbon burial fuels intense methane bubbling in a temperate reservoir

Organic carbon (OC) burial and greenhouse gas emission of inland waters plays an increasingly evident role in the carbon balance of the continents, and particularly young reservoirs in the tropics emit methane (CH 4 ) at high rates. Here we show that an old, temperate reservoir acts simultaneously as a strong OC sink and CH 4 source, because the high sedimentation rate supplies reactive organic matter to deep, anoxic sediment strata, fuelling methanogenesis and gas bubble emission (ebullition) of CH 4 from the sediment. Damming of the river has resulted in the build‐up of highly methanogenic sediments under a shallow water column, facilitating the transformation of fixed CO 2 to atmospheric CH 4 . Similar high OC burial and CH 4 ebullition is expected in other reservoirs and natural river deltas.