Date of Award
Master of Science in Coastal Marine and Wetland Studies
Coastal and Marine Systems Science
Kehui (Kevin) Xu
Susan M. Libes
Jenna C. Hill
Annual hypoxic events have been found to occur over the past several decades in the northern Gulf of Mexico (nGOM) and have prompted researchers to begin studying the mechanisms that control hypoxia formation so they may advise policy makers on the appropriate mitigating responses. This has led to the development of 3-dimensional modeling systems that incorporate marine physical, biological, geological, and chemical processes that may impact the formation and duration of hypoxic regimes in the nGOM. This study used field, laboratory, and modeling techniques to examine how sediment may be eroded from the seabed and where/how it is transported across the nGOM. Analysis of sediment texture, composition, and erodibility through field studies and Regional Ocean Model System (ROMS) simulations have shown that spatial variability in sediment grain size and erodibility relates mostly to the proximity to the major river deltas (Mississippi and Atchafalaya) and to the remnants of historic shifts in the Mississippi deltaic lobe system. Temporal variability stems mostly from changes in seasonal weather patterns, with more energetic weather in winter and spring setting up an active bottom boundary layer (BBL) which agitates seabed sediment and therefore increases its erodibility, compared to summer quiescent periods that can allow for seabed consolidation due to a low-energy BBL. This study has also found evidence that there is an organically enriched flocculent layer of material at the water-sediment surface that is highly erodible. Based on comparisons of model simulations and experiments, the shear stress levels during the quiescent periods may be strong enough to resuspend this material and reintroduce it into the lower water column where it may be decomposed by bacteria. Modeling studies have illustrated that understanding the interactions among physical, chemical, biological, and geological dynamics is a challenging but necessary in order to determine what mitigating practices will be most beneficial. This study only examines a part of the complex hypoxic water system, but it provides a stepping stone for future studies that examine the complex interaction of the different regimes that influence hypoxia formation.
Mickey, Rangley Claude, "Sediment Impact on the Formation of Hypoxic Waters in the Northern Gulf of Mexico: A Synthesis of Sediment Texture, Composition, Erodibility and Transport" (2013). Electronic Theses and Dissertations. 36.