Resumen de Sediment dynamics, geomorphology and inernal structure of the gulf of valencia continental margin (nw mediterranean)
This Thesis offers a general perspective of the hydrography, hydrodynamics and sediment transport of the Gulf of Valencia (GoV) continental margin (NW Mediterranean), focussing on the geomorphological and internal structure description of the bedforms observed over the continental slope.
The hydrography has been characterized evaluating the relation between currents, water masses and turbidity. An important detachment of particulate matter (nepheloid layer) was observed at shelf-break depths, extending seawards all across the Ibiza Channel, indicating a preferential off-shelf sediment export at the southern end of the GoV. On the continental slope, several nepheloid layers detachments were observed and it was suggested that the presence of internal waves could be maintaining particles in suspension in the mid-slope region. Measured sediment fluxes indicated that sediment transport is mainly southwestwards along the slope with a minor off-slope component. However, during storm events, maximum downward particle fluxes were larger inside the submarine canyons.
Temperature fluctuations between water masses were monitored and multiple increases of current magnitude were observed, associated with mesoscale eddies and topographic waves. The currents were mainly south-southwestwards along the slope, with alternating periods of low current velocities dominated by near-inertial changes in the current direction, concordant with temperature fluctuations also at near-inertial frequencies. Such fluctuations have been here associated to the presence of near-inertial internal waves present on the GoV. There is a complex relation between the hydrodynamics and sediment transport and it is suggested that bottom trawling might be the possible mechanism for sediment resuspension. The analysis of recently acquired swath bathymetry, seismic profiles and sediment samples, revealed the presence of large-scale fine-grained sediment waves over the Gulf of Valencia continental slope, which were previously attributed as a result of gravitational slope failure, and are here reinterpreted as sediment wave fields. These sediment waves on the lower part of the slope are vertically accreting becoming up-slope migrating on the mid- and upper part of the slope, indicating that regional slope angle is an important factor controlling their development and shape. Sediment cores showed that bottom sediments are uniform along the sediment waves, with a dominant silt and clay fraction. Slightly higher sediment accumulation rates were observed on the wave crests, indicating that such features are presently growing at 100-yr timescales.
The sediment waves formation process was inferred from the hydrodynamic observations, suggesting a link with the presence and the reflection conditions of near-inertial internal waves interacting with the seafloor. Internal waves' activity is suggested to be the most likely mechanism for the development and/or maintenance of the observed sediment waves fields through time. Smaller sediment waves are also observed over the outer continental shelf within several sediment depositional units, some of them being truncated by erosive surfaces, which are apparently linked to eustatic sea-level oscillations. Such erosional surfaces can be followed downslope into conformable strata of the sediment waves on the continental slope, where constant bedform growth is observed, without being affected by sea level changes. It has been observed that sediment waves over the continental slope have been continuously developed on the foreset region of the prograding margin clinoform, and preserved in the sedimentary record since the Lower/Pliocene. As internal wave activity is assumed to be the main formation mechanism of the sediment waves, it is here suggested that these oceanographic processes would have been present in the GoV margin almost since the Zanclean reflooding of the Mediterranean Basin, following the Messinian Salinity Crisis event ~ 5.6 Myr.
