Sources of sedimentary organic matter in the Mississippi River and adjacent Gulf of Mexico as revealed by lipid biomarker and δ13CTOC analyses

Changes in the organic matter (OM) composition (C/N, δ13CTOC, and lipid biomarker compounds) of surface sediments (0–1 cm) were examined along the dispersal pathway from Mississippi River and marsh/estuarine end members to the adjacent shelf and canyon regions of the Gulf of Mexico. Organic carbon content decreased fourfold from the marsh to the offshore sites, with corresponding changes in organic matter sources. Biomarkers representing allochthonous (higher plant) and autochthonous (algal/plankton/bacteria) sources demonstrated regional differences in the sources of sediment organic matter (SOM). A two end member mixing model using δ13CTOC indicated that C3 vascular plant sources comprised 80% and 50% of the TOC at the river and marsh sites, respectively. However, sources of SOM differed in these regions, with contributions of soil organic matter/terrigenous plant sources dominant in the river (δ13CTOC = −25‰) while marsh plants likely contributed to the enriched signatures found in the marsh sediments (δ13CTOC = −18‰). Allochthonous OM contributions calculated from fatty acid and sterol biomarkers in the river and marsh regions (39–48% and 50–72%, respectively) differed from those determined using δ13CTOC. This was likely due to overlapping δ13CTOC values for vascular plants and freshwater microalgae and the higher lipid content of the autochthonous sources. Although biomarkers representing terrigenous sources decreased with distance offshore, they comprised approximately 17–34% of the fatty acid and sterols at even the most distal slope and canyon sites, suggesting that these deeper regions could be an important sink for terrigenous carbon. In contrast, the shelf sites were enriched in algal material (60–78% autochthonous OC), with biomarkers for diatoms dominating, suggesting that terrigenous carbon is either diluted in, or bypasses, this region. Results from this study suggest that processes including hydrodynamic sorting, diagenesis, and variations in river flow should be considered in future studies investigating the fate of terrigenous OM in coastal and shelf regions.