SEDIMENT INFILLING OF LOUISIANA CONTINENTAL-SHELF DREDGE PITS: A RECORD OF SEDIMENTARY PROCESSES IN THE NORTHERN GULF OF MEXICO

Many sand resources, including buried paleo river channels, have been used as dredge sites for coastal restoration; our knowledge of infill rates and dynamics with the seafloor are limited. Sediment cores were collected at continental-shelf sites of the Northern Gulf of Mexico to develop a better understanding of mud-capped dredge pit post-dredging evolution. The two pits studied were Sandy Point (SP) and Raccoon Island (RI) (both at shelf depths of ~10m), which were dredged in 2012 and 2013 respectively; vibracores and multicores were collected in 2015. The cores were processed using a multi-sensor whole core logger and analyzed for Beryllium-7 penetration. Whole-core logger data showed discrete packages of less dense sediment (mud) overlaying denser (sandy) sediment. The infill was as thick as 2.3 m. Radioisotope analysis confirmed the extent of recent sediment deposition. At SP, ⁷Be penetration depths ranged from 12-34 cm, with SP2, the most central site, showing greatest accumulation. In RI multicores, ⁷Be penetration depths varied from 36 - >50 cm, indicating that ⁷Be occurred deeper than the 50 cm long multicore. Surface sediment samples from outside the pit contained no ⁷Be. Of note is the surprising rate of deposition within the pits as well as the source of the infill material. Sediments laden with ⁷Be must have been deposited within ~1-3 half lives of ⁷Be (~50-150 days) prior to sampling, indicating recent sedimentation rates greater than 26-50 cm/year. The nearest major sources of river sediment are: 12.5 km offshore from Grand Pass of the Mississippi River for SP; and 147 km from the Mississippi and 65 km from the Atchafalaya River for RI. Because rivers are a major source of ⁷Be in coastal marine sediments, 26-50 cm of ⁷Be activity indicates rapid and long-distance transport of sediment from a likely fluvial source and suggests that the pits are efficient sediment traps. An alternative hypothesis would be that marine sediment accumulated and efficiently scavenged ⁷Be from marine and atmospheric sources. Regardless of source, the dredge pits are accumulating a record of sediment that would not be recorded on the shelf otherwise, and collapses of pit walls cannot explain the whole depositional process. Elucidating this record will be the focus of future study.