2011 MISSISSIPPI RIVER FLOOD DEPOSITION ON THE LOUISIANA CONTINENTAL SHELF

Floods can dramatically increase the sediment load supplied to coastal margins, leading to greater and potentially geochemically unique deposition. The 2011 flooding of the Mississippi River provided an opportunity to examine if and how a large flood was received on the seabed along the adjacent continental margin. This was a geologically significant flood that occurred from May to June, 2011, surpassing historic 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 from this event has been evaluated using multi-cores collected at 68 sites along the Louisiana shelf. Cores were examined using x-radiography, the particle reactive radioisotope ⁷Be (t_1/2=53 days), polycyclic aromatic hydrocarbons (PAHs), and grain-size analysis.

Inventories of ⁷Be from post-flood cores varied across the shelf, ranging from 0 to7 dpm cm^-2. Greatest values were seen where deposits appeared thickest in x-radiography and were located down drift of river mouths. Near the Mississippi River, the highest inventories were 10-15 km away from the river mouth and at depths of 25-75 m. Near the Atchafalaya River, the highest inventories were farther from the river mouth but closer to the coast, at depths of 5-30 m. Based on maximum ⁷Be-penetration, the 2011 flood layer reached up to 9-cm thick. PAH levels were slightly higher at stations adjacent to the mouth of the Mississippi River (~400 ng g^-1) when compared to levels at the mouth of the Atchafalaya River (~200 ng g^-1). These PAH levels were lower than amounts previously quantified on Mississippi River suspended particles and lower than historical levels in the seabed of the Gulf of Mexico. Preliminary results suggest that the overall impact from the Mississippi River flooding may have been a dilution of particulate-phase PAH levels by particles lower in PAH concentration. Finally, flood deposit sediments showed an increased percentage of finer particles compared to previously deposited material, but only at a few sites was the flood deposit sedimentologically and stratigraphically distinct. Together, these data can help in the understanding of the event sedimentation to stratigraphic interpretation of this and other river-dominated shelves.