Master of Science (M.Sc.)
Virginia Institute of Marine Science
The development of accurate carbon budgets, as well as global climate models with predictive capabilities, requires an understanding of the delivery and fate of terrigenous carbon in the environment. Understanding the extent to which estuarine and continental shelf processes alter carbon transfer between marine and terrestrial systems, including estimates of organic matter accumulation in coastal sediments, is poorly known. Organic carbon and nutrients exported to the adjacent Gulf of Mexico by the Mississippi River are influenced by biological and physical processes including remineralization, hydrodynamic sorting, seabed mixing and bioturbation, and burial. These complex processes make it difficult to know the fate of this organic matter (OM). The objective for this study was to examine the sources and fate of OM along its dispersal pathway from the Mississippi River mouth to the adjacent shelf and canyon.
Changes in OM composition (C/N, δ13C, and lipid biomarker compounds) in surface sediment (0-1 cm) were examined along two transects. Using Factor Analysis, biomarkers representing allochthonous (plant) and autochthonous (algal) sources distinguished regional differences between the sites. The river, South West Pass (SW Pass), and marsh sites had greater contributions from higher plant sources. However, sources of OM delivered to these regions differed as evidenced by δ13C stable isotope signatures and C/N ratios. The river sites received inputs from soil organic matter and C3 terrestrial plants while the marsh region received inputs from marsh macrophytes. Despite offshore decreases in biomarkers representing terrigenous sources, signatures of terrigenous OM were present in surface sediments of the distal sites along each transect. The shelf sites had the greatest input of algal material, with biomarkers for diatoms dominating. Down-core analysis of sediment cores collected from three sites (SW Pass, 50 m Proximal, and canyon) further supported the surface sediment results. Plant / terrigenous materials were a major source of OM at the SW Pass site. The box cores from the 50 m Proximal and canyon sites received inputs from both autochthonous and allochthonous sources, with proportionately greater contributions from autochthonous OM.
The downcore profiles were influenced by diagenesis as indicated by differences in the ratio of terrestrial to aquatic fatty acids resulting from preferential losses of short-chained fatty acids. This study provides information about sources of OM within the Mississippi River / Gulf of Mexico, and builds upon previous studies in this region. Results from this study illustrate that complex processes influence the fate of terrestrial OM. Hydrodynamic sorting which delivers fine-grained organic rich material further offshore and dilution of terrigenous material by productive shelf sites affect distribution and fate of terrigenous OM.
This study also provides baseline information about carbon sources, important for understanding the influence of recent events such as Hurricanes Katrina and Rita, and further management / restoration efforts.
© The Author
Waterson, Elizabeth J., "Sources of Sedimentary Organic Matter in the Mississippi River and Adjacent Gulf of Mexico" (2005). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539617828.