Stratigraphic signatures due to flood deposition near the Rhône River: Gulf of Lions, northwest Mediterranean Sea

Abstract

Episodic flood delivery provides the bulk of the solid discharge for many small to moderate river systems, including the Rhône River in the northwestern Mediterranean. Several recent studies have demonstrated that the fate of this sediment depends on the coherence between river discharge and energetic ocean conditions. The deposition of flood sediment in the ocean can be confirmed by common signatures of episodic discharge events: presence of 7Be, physical stratification, and elevated clay content associated with low 210Pb activities. Previous research has indicated that the Rhône River discharge is episodic and generally independent of oceanic conditions. Sometimes the floods coincide with energetic storms and winds from the southeast, which facilitate the movement of sediment towards the southwestern Gulf of Lions. High-resolution coring near the mouth of the Rhône River provides a detailed record of sedimentation associated with past flood events. Cores were collected on two cruises, October 2004 and April 2005, in a study area seaward of the Rhône subaerial delta. Episodic sediment discharge from the Rhône River routinely deposits on the seabed in water depths shallower than 40 m. This is documented by the presence of 7Be in the surficial sediments of physically stratified cores. Through identification of a dilution signature in 210Pb profiles (i.e., increased clay content, decreased 210Pb activity), past flood events are recognized. Greater water depths and distances from the river mouth allow bioturbation to erase these signatures, except in the most extreme events where physical stratification is preserved. Excess 210Pb and 137Cs were ubiquitous in cores from this study, indicating apparent accumulation rates in the range of 2.5 to >10 cm/yr. This study confirms that although flood-event signatures provide a basis with which to examine recent flood deposits of all scales, only the thickest deposits are likely to be preserved over the long term (>100 yr).