Sediment infilling of Louisiana continental-shelf dredge pits: a record of sedimentary processes in the Northern Gulf of Mexico

Meg Cathlin O'Connor, Louisiana State University, Geology and Geophysics, Baton Rouge, LA, United States, Samuel J Bentley, Louisiana State University, Baton Rouge, LA, United States, Kehui Xu, Louisiana State University, Oceanography and Coastal Sciences, Baton Rouge, LA, United States, Jeffrey Obelcz, Coastal Studies Institute, Baton Rouge, LA, United States, Chunyan Li, Louisiana State University, Department of Oceanography and Coastal Sciences, Baton Rouge, LA, United States and MIke D Miner, BOEM, Gulf of Mexico Region, New Orleans, LA, United States
Abstract:
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 and analyzed for Beryllium-7. SP1, the northwestern site in the Sandy Point dredge pit, showed 26 cm of 7Be. In the Raccoon Island multicores, 7Be penetration depths varied from 36 cm in the center of the dredge pit (RI3) to all 50 cm of the northern-most site (RI1) core, 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 90 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.
Back to: Geological and Biogeochemical Dynamics in Major Deltaic Coasts II Posters