SOURCE AND CONTRIBUTION OF TERRIGENOUS ORGANIC-CARBON TO SURFACE SEDIMENTS IN THE GULF-OF-MEXICO

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 b etter understanding of the global carbon cycle(1). In particular, the nature and fraction of the terrestrial organic carbon preserved in mar ine sediments is poorly constrained. Here we use the chemical and stab le carbon isotope signatures of oxidation products from a macromolecul ar component (lignin)(2) of the terrigenous organic matter preserved i n offshore surface sediments in the Gulf of Mexico to complement simil ar data from an existing onshore transect(3) in this region. The compl ete onshore-offshore data set, along with radiocarbon dates of the bul k organic material at the same sites, allows the differentiation of ma terial originating from plants that photosynthesize using the C-4 mech anism 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 forest s and swamps, This distribution is probably due to the hydrodynamic so rting of the different source materials(4) during their seaward transp ort, These results suggest that previous studies(3,5) have significant ly underestimated the terrigenous fraction of organic matter in offsho re sediments by not recognizing the contribution of C-4 vegetation to the carbon-isotope composition. Such an underestimate may force revisi ons in the assessment of past marine primary productivity and associat ed organic carbon fluxes(6), and of organic matter preservation/remine ralization(7) and nutrient cycling(8) in marine sediments.