A Numerical Investigation of Sediment Dynamics in the Northern Gulf of Mexico in Connection with Hurricanes, Fluid Mud, Climate Change, and Biogeochemical Cycling

Abstract

Sediment transport and deposition in marginal seas is jointly controlled by many factors including hydrodynamics, fluvial inputs, and the characteristics of sediment particles. This dissertation study employs the coupled ocean-atmosphere-wave-and-sediment transport modeling system (COAWST) to investigate the mechanism of sediment transport in the northern Gulf of Mexico (nGoM) on different temporal scales, as well as its interaction with biogeochemical processes. First of all, a three-way coupled (atmosphere-wave-ocean) hurricane model reproduced the hydro- and sediment dynamics during hurricane Gustav (2008). Intensive alongshore and offshore currents were simulated on the eastern/western sectors of hurricane track, respectively. High suspended sediment concentration (SSC) was confined to the inner shelf at the surface layer and extended to the 200 m isobaths at the bottom layer. Suspended sediment flux (SSF) was convergent to the hurricane center. The averaged post-hurricane deposition over the Louisiana shelf was 4.0 cm. Secondly, the 20-yr two-way coupled (ocean-wave) model simulated contrasting sediment dispersal in different seasons. Sedimentation rate varied from zero to more than 10 cm/yr. A phase shift in Mississippi River discharge was detected in 1999 and was associated with the El Niño-Southern Oscillation event. Sediment flux and sedimentation rate were largely reduced around the deltas owing to the decreased fluvial supply. Thirdly, wave-supported fluid mud (WSFM) processes were incorporated into the COAWST platform. The new model simulated a lutocline between the wave boundary layer (WBL) and the water column above as WSFM formed. Downslope WSFM transport resulted in thick offshore deposition. WSFM flux peaked along the Chenier Plain coast due to strong wave activities. Sensitivity tests suggested sediments were transported further offshore as WSFM due to reduced settling velocity. Lastly, a new algorithm was adapted to assess the impact of sediment-induced shading on primary production during hurricane Gustav. The performance of the model was improved with the new algorithm. The high SSC shifted the coastal ecosystem from nutrient-limited to light-limited. Nutrient accumulated during Gustav supported a post-hurricane algal bloom in the surface layer, while productivity in the lower layer was still light-limited due to the suspended sediment. An outer shelf bloom was stimulated by the lateral transported nutrient.