• carbon burial;
  • greenhouse gas;
  • reservoir;
  • sediment

[1] 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 (CH4) at high rates. Here we show that an old, temperate reservoir acts simultaneously as a strong OC sink and CH4 source, because the high sedimentation rate supplies reactive organic matter to deep, anoxic sediment strata, fuelling methanogenesis and gas bubble emission (ebullition) of CH4from 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 CO2 to atmospheric CH4. Similar high OC burial and CH4 ebullition is expected in other reservoirs and natural river deltas.