A. Palanques

Recent influence of man on the Ebro margin sedimentation system, northwestern Mediterranean Sea

Abstract Mans recent activities, such as construction of reservoirs, dams and irrigation channels, have changed the natural dynamics of the Ebro margin sedimentation system. The amount of sediment supplied by the Ebro river is presently less than 5% of the amount supplied 50–60 years ago. Suspended sediment supplied by the river at the present time has a higher proportion of clay minerals than before the construction of the dams and reservoirs. At present, only the finer particles escape the dams, leaving the remaining sediment and apparently a large proportion of heavy metal inputs coming from inland cities and industries trapped behind them. In the lower part of the river, heavy metal pollution in the suspended matter is relatively low. Heavy metal anomalies in the shelf muds suggest that present-day sedimentation is restricted mainly to the proximal mud-belt area, near the mouth of the river. Abnormally high concentrations of heavy metals in suspended matter have been detected at the shelf break and on the upper continental slope of the Ebro system. These polluted particles come from rivers and cities located north of the Ebro mouth and are transported by the general southward flow along the slope. Some pollutants can be used as tracers for particle dynamics on the Ebro margin. The present supplies and fluxes of suspended sediment are significantly different from those which occurred before management of the river and coast began.

Distribution and dispersal of suspended particulate matter on the Ebro continental shelf, northwestern Mediterranean Sea

Abstract Hydrographic data, water and bottom-sediment samples, and a GEOPROBE tripod experiment were used to examine the distribution and dynamics of suspended particulate matter on the Ebro shelf in the northwestern Mediterranean Sea. In the absence of strong winds and storms, primary sediment supply from the Ebro River is dispersed along the shelf by a general southward flow. In such calm conditions, suspended-matter concentrations on the shelf are lower than 3 mg/l and transfer of material from the shelf to the slope takes place principally over the shelf edge north of the Columbretes Islands. Very fine sediment deposited in a mid-shelf mud belt (30–80 m deep) is cohesive and resistant to erosion. Only relatively rare, strong storms are able to resuspend particles from the deeper, central region of this cohesive deposit. When resuspension takes place, suspended-particulate-matter concentration increases and the general dispersal pattern of suspended matter is altered. Near the seafloor, distribution of suspended matter is greatly influenced by the distribution of the mid-shelf muds from which particles are resuspended. Resuspension occurs more intensively and frequently along the shallower (20–40 m) edge of the cohesive deposit and near the delta.

Erosional and depositional patterns in the Valencia Channel mouth: An example of a modern channel-lobe transition zone

Recent deep-towed, high resolution sidescan sonar records and seismic profiles have been collected on the lower Valencia Fan (Northwestern Mediterranean). Three morphological zones, channelled, transition and unchannelled, have been recognized in the Valencia Channel mouth. Sonographs from the transition zone show a progresive transversal gradation from depositional to erosional bedforms. This asymmetry may be due to the lateral inputs of sediment flows from the rhone deep-sea fan and to the effect of the Coriolis force, which could have diverted the flows to the southwest. Bedforms recorded in the study area include trains of starved ripples and dunes, sand ribbons, and fields of elongated scours. Most morphological features, bedforms and seismic characteristics of the Valencia Channel mouth are typical of channel-lobe transition zones.

A Deep-sea Channel in the Northwestern Mediterranean Sea: Morphology and seismic structure of the Valencia Channel and its surroundings

The 400 km long Valencia Channel occupies the axis of the Valencia Trough in the Northwestern Mediterranean. Four different types of seismic reflection profiles were used to analyze the morphology and structure of the Valencia Channel with regard to the role played by both margins, Balearic and Iberian, of the Valencia Trough. From a detailed morphoseismic analysis of the Valencia Channel, its upper, middle, and lower courses can be characterized as follows: (1) in the upper course, tributaries are short and only slightly incised, with recent mass-transport deposits occurring on the adjacent continental slopes; (2) in the middle course, the channel deepens, and tributary valleys merge into it; and (3) the lower course begins after a sudden change in the direction of the channel, has a meandering path, is flanked by levees, and is fed by some valleys.During the Pliocene and Quaternary, at least four erosional and filling phases are observed in seismic profiles of the lower course of the Valencia Channel. The varying intensity of mass-transport processes and associated retrogressive slumping, which are related with fluctuations in sediment supply and relative sea-level changes, have played a major role in the formation, maintenance and deepening of the Valencia Channel. In addition to these sedimentary processes, a basement tectonic control and some morphostructural features affect the direction of the Valencia Channel locally.

Resting stages in a submarine canyon : a component of shallow-deep-sea coupling?

The ecological importance of resting stages in shallow waters is well known, but their presence in the deep sea is practically unrecorded. Samples of sinking particles were collected from April 1993 to May 1994 in and around the Foix Canyon (northwest Mediterranean Sea) using PPS3 sediment traps located between 600 m and 1180 m. Dead and viable organisms were collected, and inorganic empty shells constituted most of the biologically-derived matter. Resting stages, considered as POM, had a flux of up to 70 000 items m 2 d 1 . They were the second most abundant fraction of total POM after tintinnids (mainly represented by empty, chitinous loricas), and first of the viable POM fraction. Most remained unidentified, but 58 morphotypes were referable to coastal species of Dinophyta, Tintinnina and Calanoida. Resting stages were rare in samples collected from the open slope adjacent to the canyon. These preliminary data suggest an important role of submarine canyons in concentrating POM and transferring it from shallow to deep-sea habitats. Due to their resistance to degradation processes, resting stages are probably the only POM component that can return to shallow areas by upwelling currents occurring in the canyon.

Sand Ridges on the Mid-Outer Valencia Continental Shelf

A sand ridge field with crests oriented NE–SW (52°) located between 55 and 85 m water depth on the Valencia continental shelf (Spain) was mapped with multibeam swath bathymetry and characterized with high-resolution seismic reflection profiling and sediment sampling. Boundary layer hydrodynamic and suspended sediment concentration measurements conducted over a sand ridge at 66 m depth showed evidence of wave and current sediment resuspension during a major storm event. It is argued that these sand ridges developed in a shallow environment and there is no evidence of present-day ridge migration or morphological degradation.