In marine sediments, anoxic degradation of organic matter proceeds through fermentation to volatile fatty acids, which are then oxidized to CO2 coupled to the reduction of respiratory electron acceptors such as nitrate, iron, manganese and sulfate. However, recent work has proposed this model may not always apply in dynamic permeable sediments that rapidly shift between oxic and anoxic conditions. It has been suggested that in environments with a highly variable oxygen regime, fermentation mediated by facultative aerobic bacteria (uncoupled to external terminal electron acceptors) becomes the dominant process. Here, we present the first direct evidence for this fermentation using a differentially-labelled glucose isotopologues assay (1-13C, 2-13C, 3-13C, 13C6) that distinguishes between CO2 produced from respiration and fermentation. Using this approach, we measured the relative contribution of respiration and fermentation to organic carbon metabolism in a range of permeable (sandy) and cohesive (muddy) sediments. We show that under anoxia, high energy sandy sites (larger median grain size) are dominated by fermentation via the Embden-Meyerhof-Parnas pathway that is largely uncoupled from anaerobic respiration. In contrast, anoxic muddy sediments (smaller median grain size) generally completely oxidised 13C glucose to 13CO2, consistent with the classical redox cascade model. These findings show that the classical biogeochemical model for carbon oxidation pathways cannot always be applied to sandy sediments. Future research should elucidate the exact fermentation pathways taking place during anoxia in sandy sediments.