Sediment Infilling of Louisiana Continental-Shelf Dredge Pits: A Record of Sedimentary Processes in the Northern Gulf of Mexico

Abstract

Sediment cores were collected at continental-shelf sites of the Northern Gulf of Mexico off the coast of Louisiana to develop a better understanding of post-dredging evolution of mud-capped dredge pits. The two pits studied were Sandy Point (SP) and Raccoon Island (RI) (both with water depths of ~10m and dredged to ~ 15-20 m below water surface), which were dredged in 2012 and 2013 respectively. Vibracores and multicores were collected in 2015. Preliminary radiochemistry analyses identified Beryllium-7 distribution in multicore samples from both sites. SP1, the northwestern site in the Sandy Point dredge pit, showed 26 cm of 7Be penetration. In the Raccoon Island multicores, 7Be penetration depths varied from 36 cm in the center of the dredge pit (RI3) to throughout the entire length of a 50 cm core from the northern-most site (RI1), indicating that 7Be occurred deeper than the 50 cm multicore. Because rivers are a major source of 7Be in coastal marine sediments (concentrated from the atmosphere), 26-50 cm of 7Be activity indicates rapid and long-distance transport of sediment from likely fluvial sources (here the Atchafalaya and Mississippi Rivers) and suggests that the pits are efficient sediment traps.