EPHEMERAL SEDIMENT DEPOSITION AND LONG TERM ACCUMULATION ON THE WESTERN LOUISIANA INNER SHELF

Recent time-series cores collected on the western Louisiana inner shelf show the region to be subject to transient fluid mud deposition, which leads to high long-term accumulation rates. Sediment cores were collected in March, May, and October 2002 from the inner shelf landward of the 10 m isobath, 100 km west of Atchafalaya Bay. The cores were analyzed using ⁷Be, ²¹⁰Pb and ¹³⁷Cs geochronology, x-radiography, porosity and granulometry. ⁷Be activities provide evidence of high porosity event layers 2-25 cm thick composed of clay with basal silt laminations. These event layers appear to be ephemeral features on a seasonal time scale, but tend to concentrate around a depocenter located primarily 95-110 km west of the Atchafalaya River, landward of the 7 m isobath. Preliminary results of ²¹⁰Pb and ¹³⁷Cs indicate that the short-term deposition of fluid mud layers leads to long-term accumulation rates ranging from 1-14 cm/yr.

Sediment cores were also collected after the passage of Hurricane Lili in October 2002. Seaward of Freshwater Bayou and Southwest Pass, the outlet from Vermillion Bay into the Gulf of Mexico, cores collected contain 1-5 cm of ⁷Be laden fluid mud. These deposits are controlled by bathymetry and proximity to their source. Cores collected throughout the rest of the region have an ~4 cm thick fluid mud layer that does not contain ⁷Be, indicating resuspension of old sediment, with little to no new deposition of fluid mud. This provides further evidence that fluid mud deposition on the western Louisiana inner shelf is associated with high spring discharge, but can subsequently be resuspended by storms and hurricanes.