The Gulf of Lion is located in the north-western part of the western Mediterranean basin, between Cap Creus (Pyrenean chain) in the south-west (Spanish border) and Cap Sicié (near Toulon) in the north-east. It is characterised by a wide crescent-shaped continental shelf, 200 km long. The continental shelf reaches a maximum width of 70 km off Cap d'Agde and narrows towards the east and west as it approaches the Provencal and Pyrenean-Catalan margins. It extends between 0 and 120 to 180 m deep and has an average inclination of 0.5° (reaching up to 5° at the deltaic slopes). Numerous canyons notch the edge of the platform. The river Rhône is the main source of terrigenous inflows from the Gulf of Lion, up to about 90% (Bourrin et al., 2006), and the Rhône prodelta is an area of significant accumulation of fluvial solid inflows.

The current morphology of the Gulf of Lion is strongly linked to current river flows, but also to the legacy of past environmental conditions from the last low sea level 20,000 years ago.

In 2012, Ifremer had planned to produce a map of the seabed substrate in this sector based on a compilation of acoustic and sedimentary data. Indeed, the Gulf of Lion has been the subject of oceanographic studies and campaigns for many years. An inventory of the campaigns at sea had been carried out thanks to the SISMER (Scientific Information Systems for the Sea) community databases and Infoterre internet portal managed by the BRGM (the French Geological Survey). These databases had enabled the referencing of laboratories involved campaigns and samples collection in the Gulf of Lion since 1991:

- Ifremer,

- Shom

- University of Perpignan Via Domitia (Cefrem),

- Aix-Marseille University (CEREGE ; MIO Mediterranean Institute of Oceanology),

- CNRS (French National Centre for Scientific Research)

- Sorbonne University,

- IRSN (Institute for Radiological Protection and Nuclear Safety),

- the French Office for Biodiversity (OFB) (previously Marine Protected Areas Agency),

- the Conseil départmental de l’Hérault in the framework of the European projects Beachmed,

- the Rhone-Mediterranean-Corsica Water Agency.

Partnership agreements for the provision of samples and data had been signed with these research laboratories or local authorities.

If the production of a map had not been successful, a total of 2 110 samples (see xls-sheet1 file) had been recorded in the Gulf of Lion.

The sampling tools used were diverse and the techniques used to analyse the samples also varied from one campaign to another. To validate the results of existing analyses, we compared the samples preparation and analysis protocols. When we had no information on these protocols, the granulometric analysis was redone when the sample still existed, or the result was compared with the valid analysis of a geographically close sample. In the absence of these information, the analyses were rejected. The analyses provided by IRSN (150 samples) and CEREGE (321 samples) were acquired using a sample preparation and analysis protocol similar to that adopted for our study and were validated. Out of 144 samples taken by Ifremer, the analyses were redone in order to obtain homogeneity with the sample preparation and analysis protocol of CEREGE and IRSN.  Data from the University of Perpignan, resulting from the work of Jean-Claude Aloisi (between 1970 and 1986) which led to the first sediment distribution map of the Gulf of Lion dating from 1986, were analysed by sieving at 40 µm, determining the pelite/ sand limit at this size. Other classifications (Folk, ISO 13 320, Uden and Wentworth, ...) determine a pelite/sand limit at 63µm. It is therefore difficult to compare these data without recalibrating all the data of the 40µm laser granulometers.

In the end, in order to guarantee an intercomparison of the analyses, only 1659 samples were validated after harmonisation and standardisation and all the data from the so-called "Aloisi" campaign had to be discarded (see xls-sheet2 file).

The composition of the surficial sedimentary cover (of 10 cm thick; which corresponds to the capacity of penetration into the sediments of the acoustic waves emitted by the multibeam echosounders) of the Gulf of Lion shelf is relatively heterogeneous. It is strongly linked to current river inputs, but also to inherited environmental conditions from the last low sea level 20,000 years ago. At present, coarse river inflows feed the sandy coastline near the mouths, while fine inflows are transported by currents and waves over the entire platform.

Different sedimentary facies appear from the coast towards the slope, parallel to the shoreline:

- The inner shelf is subject to the action of swell and storms (Guizien, 2009) that cause sediment to resuspend. Richly sandy outside of the areas influenced by rivers, this area is relatively poor in clay (from 2 to 16%); clay deposits cannot be made under these conditions. It runs along the coastline, up to 30 m. It gets thinner, until it disappears, opposite the mouth of the Rhône (Gulf of Fos) in its eastern part, and opposite Cap Creus in its western part.

- The median shelf, richer in clay (from 16 to 26%) located between 30 and 100 m depth on average, is known as the "median mudflat". The slope break, observed between 40 and 50 m, corresponds to the transition zone between the silto-pelitic deposits and the offshore sediments (clay phase). The particle diameter gradient is quite marked, depending on the bathymetry: 50 to 80 µm between 30 and 50 m and 10 to 15 µm from the 50 m isobath. This tendency is rather marked near the mouths of rivers, particularly the Petit Rhône, the Orb and the Hérault.

- The outer shelf (located at the limit between the continental shelf and the slope) presents a sandy to sandy-muddy mixture (60% sand with a median diameter of between 80 and 100 µm and up to 500 µm). These so-called "relict" sands are derived from coastal deposits dated of Ante-Holocene age, when the continental shelf was nearly emerging.

At the mouth of the Grand Rhône, the most important source of particles in the Gulf of Lion, the internal plateau does not have the same appearance, as it is the site of significant sedimentation concentrated in the form of a prodelta. At the level of this prodelta, the distribution of sediments shows a significant granulometric gradient near the mouth, with a concentration of coarse sands (>450µm) at the top (mouth bar), followed by a rapid predominance of fine sediments from the 20 m isobath, which in the area corresponds to the depth of closure of the swells. The predominance of the fine fraction occurs much faster than on the rest of the plateau. The average slope of the prodeltaic zone has been evaluated at 5° and the 100 m isobath is about ten kilometres from the mouth of the Rhône, compared with more than 50 km for most of the French Mediterranean coast of the Gulf of Lion. The Rhone's fluvial inflows are also subject to a transport dynamic towards the west and south-west, which leads to a circulation of particles over the entire continental shelf of the Gulf of Lion and their potential transport to deeper area through the heads of submarine canyons (Durrieu de Madron et al., 1990).

The hydro-sedimentary functioning of the Gulf of Lion is thus characterised as follows: (i) a forced sedimentation of fine particles in the prodeltas, (ii) an advective transfer of the fine mineral from the continental sources and prodeltas to the sediments of the circalittoral mudflat, and (iii) the export of homogeneous material from the continental shelf, through the outfalls of the south-western shelf. Within this particulate dynamics, the prodelta plays an important role. By its characteristics as an area of decantation and regulation of continent/sea exchanges, the prodelta is apparented to the estuarine turbidity maximum of the macrotidal seas.

Finally, as the surface sediments analysed in the Gulf of Lion are mainly composed of fine sediments, the authors believe that the simplified Flemming classification best suited to this type of environment and should be preferred for mapping the distribution of sedimentary facies in the study area.

 

References :

Guizien (2009) - Spatial variability of wave conditions in the Gulf of Lions (NW Mediterranean sea). Life and Env. 59 (3-4):1-10.

Durrieu de Madron et al. (1990) - Hydrographic structure and nepheloid spatial distribution in the Gulf of Lions continental margin.Continental Shelf Research 10(9-11):915-929. DOI: 10.1016/0278-4343(90)90067-V