Origin and fate of organic matter in sediments along the Mississippi River-ocean transect

The burial of organic matter (OM) in marine sediments forms an important sink of atmospheric CO₂. Especially river-dominated coastal zones, where large amounts of terrestrial OM (TerrOM) is introduced by rivers and where marine production is high, are regions where large amounts of OM can be stored in the sediments. However, only a small portion of the OM ends up buried in the sediments, while the remainder is degraded or transported further offshore. Which part of the OM ends up within the sediments and which part is degraded is not fully understood, since most studies only consider bulk OM, without taking into account the different types and composition of the OM. In this thesis, the origin and fate of OM in sediments from river-dominated continental margins is studied. We focus on the Gulf of Mexico, which is fed by the Mississippi and Atchafalaya rivers. Using lipid biomarkers and palynology, OM is disentangled into plant-, freshwater-, soil-, and marine material. By looking at the origin and fate of the OM, we study how and if the Gulf of Mexico acts as a source or sink for carbon sequestration, and which conditions are most ideal for carbon burial. This research shows that plant material is transported furthest into the ocean, while other TerrOM sources are rapidly lost. The total burial efficiency of the Gulf of Mexico is rather low, however, as only 12% of the TerrOM introduced by the Mississippi and Atchafalaya rivers is ultimately buried in sediments of the Gulf of Mexico.