The burial of organic carbon is a major sink in the global carbon cycle. Lake sediments could account for about 50% of the marine carbon flux. In this project we analyze the effect of three major factors on carbon burial efficiencies: 1) Oxygen penetration has been shown to affect the fate of organic carbon in marine sediments. With O2 micro optodes applied to dated sediments we quantify the effect of oxygen exposure time for carbon mineralization. 2) Mineral surfaces have been shown to protect organic matter in soils and marine sediments against microbiological uptake. We use electron microscopy combined with gas adsorption to characterize the mineral surface area in samples of lake sediments. 3) The quality of organic matter determines its bioavailability, i.e. old organic matter from soils is mineralized only slowly while fresh algal remeins are turned over into carbon dioxide rather quickly. New methdos for organic carbon characterization based on nuclear magnetic resonance spectroscopy and on mass spectrometry are combined with other analytical techniques to link organic matter properties with mineralization rates. In order to provide general answers to the question what determines organic matter preservation we investigate a large number of lakes from the tropics to the arctic cycle.I a subproject we address the related question how much methane is released from lake sediment and which fraction reaches the atmosphere. In this case we combine analyses of methane fluxes with geophysical techniques like echo-sounding in order to detect rising CH4 bubbles and to quantify emission rates into the atmosphere. With such process-studies we try to gain enough general information on the controlling factors of methane release to extrapolate fluxes from freshwater lakes to the atmosphere.