Organic carbon burial in marine sediments generates virtually all atmospheric oxygen, and provides a long-term sink for about 20% of all carbon(1,2); it is therefore important to understand the mechanisms controlling organic carbon preservation. There is a fraction of organic matter that is preserved under reducing conditions but which can be rapidly oxidized if exposed to molecular oxygen(3-6). It is not clear, however, how much of the total organic matter preserved in marine sediments is oxygen-sensitive. Here we present results from a relict turbidite in the Madeira abyssal plain which suggest that pollen grains can be used as a sensitive tracer of oxygen-sensitive organic carbon. We find that pollen grains were completely degraded within 10 kyr in the presence of diffusively introduced oxygen, but were well preserved for at least 100 kyr under anoxic conditions. We also present pollen data from the Pacific Northwest continental shelf, which suggest that oxic degradation can explain the decrease in organic carbon with depth commonly observed in coastal sediments.