After that, like micro-organisms living in the waters of the ETNP, they use nitrate until it is used up. These compounds must diffuse down to the micro-organisms through sediment pore waters, which allow much less contact with the overlying waters. Therefore the supply rate of these oxidants is greatly reduced in sediments. This has the effect of decreasing the isotopic fractionation within the sediments, because very little of what goes in diffuses back out to change overlying water isotopic values. For reference, in the water column both oxygen and nitrate isotopic values should follow the red .980 line on the graphs.
Oxygen shows a slight isotopicfractionation, while nitrate shows no fractionation whatsoever.