Sources and contribution of terrigenous organic carbon to surface sediments in the Gulf of Mexico

The sources and burial processes 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 cycle1. 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 transect3 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 C4 mechanism from those that undergo C3 photosynthesis. We conclude that the offshore lignins derive from erosion of the extensive grassland (C4) soils of the Mississippi River drainage basin, and that the nearshore lignins originate largely from C3 plant detritus from coastal forests and swamps. This distribution is probably due to the hydrodynamic sorting of the different source materials4 during their seaward transport. These results suggest that previous studies3,5 have significantly underestimated the terrigenous fraction of organic matter in offshore sediments by not recognizing the contribution of C4 vegetation to the carbon-isotope composition. Such an underestimate may force revisions in the assessment of past marine primary productivity and associated organic carbon fluxes6, and of organic matter preservation/remineralization7 and nutrient cycling8 in marine sediments.