| Summary |
The purpose of this study was to (1) evaluate seasonal sediment dynamics of the Mississippi River Deltaic region; (2) to assess the magnitude and extent of event driven sediment deposition associated with three tropical cyclones; and (3) contrasting seasonal and tropical cyclone-related sedimentation on the Mississippi River continental margin. Multiple box cores were collected in October, November, and December of 2003 and March, April, and May of 2004 at three on the continental shelf in the Mississippi Deltaic Region. Sediment samples were analyzed for particle reactive radionuclides ²³⁴Th and ⁷Be to examine seasonal sediment dynamics associated with variations of river discharge and hydrodynamics. High frequency sampling (~monthly) relative to the short half life of the radiotracers (²³⁴Th t1/2=24.1 d; ⁷Be t1/2=53.3) enabled us to isolate the relative influence forcing agents (river discharge, waves, currents) had on sediment inventories of ⁷Be and ²³⁴Th. During years with minimal tropical system activity, sediments initially deposited from late spring to early fall are remobilized by wind-driven currents and wave energy during extra-tropical (winter) weather systems. However, the amount of material delivered to and advected off-shelf as a result of winter storm systems may be insignificant when compared to event-driven sediment transport associated with the crossing of tropical cyclones. In the fall of 2004, the river dominated shelf of Louisiana was impacted by three tropical systems in less than a month including Hurricane Ivan, the eighth most intense Atlantic hurricane on record at the time. In order to assess the impact these tropical systems had on the continental margin west of the Mississippi delta, seabed samples were collected from box and kasten cores in October 2004 and analyzed for particle reactive radionuclides ⁷Be, and ²³⁴Th. Radiochemical data and observations from x-radiographs indicated that event-driven sediment deposits exhibit distinct radiochemical signatures and differ visually, texturally, and lithologicly from the underlying sediment. When compared to seasonal depositional rates, event deposits are up to an order of magnitude greater. With higher than average hurricane activity predicted by the National Hurricane Center over the next decade, hurricane-driven sediment transport is likely to be the major mechanism controlling sediment transport and deposition on the continental shelf and sediment accumulation in the Mississippi Canyon. |
