Shelf mud depocenters and storm-driven bottom transport controlling their formation

Mud depocenters on continental shelves are the proximal sink for continent-derived material during modern sea level conditions. They serve as habitat for benthic life and store large quantities of carbon, nutrients and contaminants. Most of them initiated to form during later Holocene. However, their growth dynamics and the particular oceanographic drivers which control deposition remain largely speculative. The project GALIOMAR uses dense-spaced subbottom profiles, numerous sediment surfaces and cores, and water-column monitoring data combined with numerical modeling to decipher the growth history and the environmental driving mechanisms.

The NW Iberian mud depocenter (Spain/Portugal) has a coast-parallel extent with an average thickness of 1 m (7 m in its core) holding 4,000 million tons of sediment. It started to form at 5.3 cal ka BP, probably responding to sea level stabilization (turnover from transgressive estuarine to exporting river mouths), but expanded not from the main river supplier but around detached accumulation nuclei. Compared to the natural fluvial input, about 35% of the continent-derived sediment is kept in this depocenter whilst about two thirds are released to the deep ocean.

To link the long-term formation with the modern system, bottom sediment transport was monitored during a storm event as representative for winter hydrodynamic conditions. Bottom sediment resuspension was nearly two orders of magnitude higher than during pre- and post-storm conditions. This data was fed into a 3D process-based coastal ocean model. The resuspension was induced by a short-lasting (1 h) coastal jet (bottom shear velocity of 2 cm/s) in combination with enhanced surface gravity waves (bottom shear velocity of 4 cm/s). A cross-shore horizontal density gradient of 0.32 g/m³ defined the boundary between two water masses (i.e., downwelling surface water and bottom water). Dense bottom nepheloid layers (several kg/m³ sediment concentration) were produced in those muddy shelf areas where sediment transport fluxes converged. Storm-driven nepheloid transport and post-storm deposition coincided with the mud depocenter location. The storm-generated oceanic density frontal zone matched the shoreward limit of the depocenter core (mud > 80%), suggesting it as main control for the location of mud deposition.