THE vast reservoirs of organic carbon in marine sediments(1-3) have the potential to influence the properties of organic matter in the overlying water column, For example, it has been suggested that marine sediments are a possible source of the old, refractory dissolved organic carbon (DOC) found in deep water(3,4), Natural radiocarbon and stable carbon isotope ratios (Delta(14)C and delta(13)C) can be used to constrain the role of sediments in the ocean carbon cycle(5-8). Here we report the distributions of Delta(14)C and delta(13)C associated with dissolved organic and inorganic carbon in sediment pore water, together with those of the particulate sedimentary organic carbon, from two geochemically distinct marine environments, Concentration gradients of dissolved organic and inorganic carbon across the sediment-water interface imply significant diffusive fluxes of these solutes from the sediment to the water column. But the DOC fraction in the sediments is greatly enriched in C-14 compared with that in the overlying sea water (by as much as 370 parts per thousand), indicating that the DOC supplied by sediments to ocean waters must be relatively young, and that its remnant ages in the water column itself.