Examination of the 2011 Mississippi River Flood Deposit on the Louisiana Continental Shelf

Abstract

Floods can dramatically increase the sediment load supplied to continental margins, leading to greater and potentially geochemically unique deposition. The 2011 flooding of the Mississippi River and its discharge into coastal areas of the northern Gulf of Mexico provided an opportunity to examine how a large flood was received on the seabed along the adjacent continental margin and influenced stratigraphic development. This was a geologically significant flood that occurred from May to July, 2011, surpassing historic water levels at Vicksburg, MS and necessitating the opening of the Morganza Spillway for the first time in 37 years. For this study, the stratigraphic and geochemical nature of the deposition associated with this event was evaluated using multi-cores collected at 68 sites along the Louisiana shelf. Cores were examined using x-radiography, particle reactive radioisotopes, ²³⁴Th and ⁷Be, as well as analysis of grain-size distributions and organic matter content. Inventories of ⁷Be from post-flood cores varied across the shelf, ranging from 0 to 7 dpm/cm². Greatest ⁷Be values were seen where deposits appeared thickest in x-radiography and were located down-drift of the Mississippi and Atchafalaya River mouths. Near the Mississippi River, the highest inventories were 10-15 km seaward of the river mouth and at water depths of 25-75 m. Near the Atchafalaya River, the highest inventories were farther from the river mouth (~130 km), and on the inner shelf at depths of 5-30 m. Based on geochemical and sedimentological data, the 2011 flood deposit reached up to 8 cm thick. Flood deposit sediments showed a higher percentage of clays and lower dry bulk densities compared to pre-2011 deposited material, making the flood deposit relatively sedimentologically and stratigraphically distinct on the shelf. An estimated flood sediment budget suggests that the flood deposited approximately 80 ± 30 Mt (million tons) of sediment on the shelf during a three month span, which accounts for 47 ± 17% of the average annual sediment load of the Mississippi River, but is much less than the shelf sediments remobilized by Hurricanes Katrina and Rita in 2005 (1200 Mt). Physical oceanographic and fluvial conditions, and shelf morphology significantly influenced the spatial distribution and magnitude of flood sedimentation strata. This research demonstrates the identification and interpretation of flood sequences and provides further insight on the source, deposition, and transport of terrestrially-derived material on the Louisiana shelf.