The depth of oxygen penetration into marine sediments differs considerably from one region to another1, 2. In areas with high rates of microbial respiration, O2 penetrates only millimetres to centimetres into the sediments3, but active anaerobic microbial communities are present in sediments hundreds of metres or more below the sea floor4, 5, 6, 7. In areas with low sedimentary respiration, O2 penetrates much deeper8, 9, 10, 11, 12 but the depth to which microbial communities persist was previously unknown9, 10, 13. The sediments underlying the South Pacific Gyre exhibit extremely low areal rates of respiration9. Here we show that, in this region, microbial cells and aerobic respiration persist through the entire sediment sequence to depths of at least 75 metres below sea floor. Based on the Redfield stoichiometry of dissolved O2 and nitrate, we suggest that net aerobic respiration in these sediments is coupled to oxidation of marine organic matter. We identify a relationship of O2 penetration depth to sedimentation rate and sediment thickness. Extrapolating this relationship, we suggest that oxygen and aerobic communities may occur throughout the entire sediment sequence in 15–44% of the Pacific and 9–37% of the global sea floor. Subduction of the sediment and basalt from these regions is a source of oxidized material to the mantle.