Organic carbon burial in marine sediments generates virtually all atmo
spheric oxygen, and provides a long-term sink for about 20% of all car
bon(1,2); it is therefore important to understand the mechanisms contr
olling organic carbon preservation. There is a fraction of organic mat
ter that is preserved under reducing conditions but which can be rapid
ly oxidized if exposed to molecular oxygen(3-6). It is not clear, howe
ver, how much of the total organic matter preserved in marine sediment
s 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 pres
ence of diffusively introduced oxygen, but were well preserved for at
least 100 kyr under anoxic conditions. We also present pollen data fro
m the Pacific Northwest continental shelf, which suggest that oxic deg
radation can explain the decrease in organic carbon with depth commonl
y observed in coastal sediments.