The fate of fluvially discharged organic carbon in the northern Gulf of Mexico and its influence on marine productivity

Rivers form a crucial part in the global carbon cycle by transferring terrestrial organic carbon (TerrOC) from land to the coastal zone. Upon burial in marine sediments, TerrOC may be a significant long-term carbon sink, depending on the composition and properties of this organic material. However, much remains unknown about the dispersal of the different types of TerrOC in the marine realm and the influence on coastal marine productivity, as the commonly used bulk OC parameters do not reach the required level of source- and process-specific information. Here, we characterize TerrOC in marine surface sediments along transects offshore of the Mississippi and Atchafalaya rivers using sediment bulk properties, pollen and lipid biomarkers. The latter allow us to disentangle contributions of higher plants (long-chain odd numbered n-alkanes, plant sterols) and soil, fluvial and marine (branched and isoprenoid GDGTs, long-chain diols, alkenones) produced organic matter. Additionally, dinoflagellate cysts are used to assess the marine productivity in the coastal zone. Our data show that soil-derived OC and fluvially produced diols are most abundant near the Mississippi river mouth and decrease rapidly further offshore, while plant material remains abundant along the shelf and also in deeper waters. Dinocysts indicate higher abundances of autotrophic species in the open ocean, while heterotrophic species are dominant near shore, reflecting enhanced nutrient input from land and higher marine productivity close to shore, as supported by C/N ratios of the bulk sediment and terrestrial biomarkers. Next, this information will be applied to downcore sediment records taken from the continental shelf to study the variability in the contribution and the composition of TerrOC in relation to climatic and anthropogenic changes, as well as its effect on marine productivity in the coastal zone over the past few hundreds of years.