HR: 1340h
AN: OS23C-1325    [Abstracts]
TI: The Role of Cap de Creus Canyon in Off-Shelf Sediment Escape, Western Gulf of Lions, France
AU: * DeGeest, A L
EM: adegeest@ocean.tamu.edu
AF: Texas A&M University, Department of Oceanography, College Station, TX 77843-3146 United States
AU: Mullenbach, B L
EM: bmullenbach@ocean.tamu.edu
AF: Texas A&M University, Department of Oceanography, College Station, TX 77843-3146 United States
AU: Orange, D L
EM: Dan\_Orange@AOAGeophysics.com
AF: University of California, Santa Cruz, Department of Earth Sciences, Santa Cruz, CA 95064 United States
AB: Recent studies investigating across-margin sediment transport have identified submarine canyons as an important conduit for off-shelf sediment export during the present high-stand in sea level. The Gulf of Lions, France is a bathymetrically complex region with multiple submarine canyons and significant sediment input from the Rhone and many small rivers. It has been suggested that the western Gulf of Lions is a primary outlet for sediment due to narrowing of the shelf and the general east-to-west circulation pattern. This study focuses on Cap de Creus, the westernmost canyon, which is hypothesized to be an important factor in this escape because it incises a very narrow shelf and may intercept part of the sediment transport pathway. Box cores were collected in multiple locations within the canyon and on the shelf in November 2003 and March 2004 to determine sedimentation patterns. An along-shelf transect at 50 m water depth from March shows an increase in accumulation rates ($\sim$2 to $\sim$4 mm/yr) from north to south, indicating that significant amounts of sediment are reaching the seabed near the canyon head. Analysis of cores taken within the canyon also suggests that sediment is being transported down-canyon. Cores collected at shallow depths within the canyon (150 m) reveal a distinct sand layer with excess $^{210}$Pb activities (indicating deposition $<$100 years ago) that unconformably overlies a highly-consolidated mud with only supported levels of $^{210}$Pb (deposited $>$100 years ago). This coarse material was seen in both November and March and appears to extend as deep as 400 m. Deeper within the canyon (500 m), there is a blanket of unconsolidated mud (13 cm thick) that was deposited within the last 100 years; the canyon flanks are also observed to be accumulating at varying rates. This is significant because it indicates that both coarse and fine grained sediment are moving down the canyon during the present sea level high-stand. Most cores collected deeper in the canyon reflect a shift from old, highly consolidated basal muds or coarse material to unconsolidated, younger sediments near the surface, some with a very distinct interface. One such core (at 500 m in the canyon thalweg) shows similar excess $^{210}$Pb activities in both the upper and lower layers down to the base of the core (18 cm), indicating that both were emplaced in less than one half-life (22 years). This shift in characteristics may be due to (1) two phases of a single event or (2) a shift in source over a short timescale, although the exact timing and frequency are still under investigation. Regardless of the mechanism, these preliminary data suggest that Cap de Creus is an important outlet of sediment from the continental shelf in the western Gulf of Lions.
DE: 4219 Continental shelf processes
DE: 4243 Marginal and semienclosed seas
DE: 4558 Sediment transport
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3022 Marine sediments--processes and transport
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting