The sources and burial professes of organic matter in marine sediments are not well understood, yet they are important if we are to have a better understanding of the global carbon cycle(1). In particular, the nature and fraction of the terrestrial organic carbon preserved in marine sediments is poorly constrained. Here we use the chemical and stable carbon isotope signatures of oxidation products from a macromolecular component (lignin)(2) of the terrigenous organic matter preserved in offshore surface sediments in the Gulf of Mexico to complement similar data from an existing onshore transect(3) in this region. The complete onshore-offshore data set, along with radiocarbon dates of the bulk organic material at the same sites, allows the differentiation of material originating from plants that photosynthesize using the C-4 mechanism from those that undergo C-3 photosynthesis. We conclude that the offshore lignins derive from erosion of the extensive grassland (C-4) soils Of the Mississippi River drainage basin, and that the nearshore lignins originate largely from C-3 plant detritus from coastal forests and swamps, This distribution is probably due to the hydrodynamic sorting of the different source materials(4) during their seaward transport, These results suggest that previous studies(3,5) have significantly underestimated the terrigenous fraction of organic matter in offshore sediments by not recognizing the contribution of C-4 vegetation to the carbon-isotope composition. Such an underestimate may force revisions in the assessment of past marine primary productivity and associated organic carbon fluxes(6), and of organic matter preservation/remineralization(7) and nutrient cycling(8) in marine sediments.