Relationships between carbonate sedimentation and hydrodynamics on the temperate shelf of the Strait of Bonifacio (western Mediterranean)

The sedimentary features of the inner-middle shelf of the strait of Bonifacio (western Mediterranean) were analyzed to evaluate the relationship between the production and transport of biogenic carbonate sediments and the basin morphology and hydrodynamics. A three-dimensional hydrodynamic modeling was performed in order to simulate the influence of waves and currents at the seabed level. Superficial sediments were collected at depths ranging from 5 to 80 m and were analyzed for grain size, mineralogical composition and skeletal carbonate composition. Shallow seismic data (3.5 kHz) were acquired to evaluate the spatial distribution and architecture of sedimentary facies. Posidonia oceanica seagrass meadows border the coasts in a narrow strip at both sides of the strait down to a depth of 40 m. In greater depths, the seabed is characterized by the presence of plateaus and ridges which are controlled by outcropping bedrock morphology. Waves and seabed currents are driven by the prevailing northwest and northeast winds. For both wind directions, higher values for the seabed current velocity, associated with windstorm events, were detected in shallower sectors and along an east–west-oriented belt that connects the western Mediterranean and the Tyrrhenian Seas. The sediments range from sand to gravel and show a mixed biogenic carbonate /siliciclastic composition. This is due to the carbonate production associated with benthic ecosystems and the mixing of modern carbonate with relict sediments. Biogenic gravelly sands were found in association with P. oceanica seagrass meadows and extended down to the circalittoral zone outside the deeper limit of the meadows. This sedimentary facies was derived from the modern carbonate production associated with the P. oceanica ecosystem. Sediments collected outside the deeper limit of the meadows were identified as relict sediments deposited during the stand of the meadows at a deeper level during lower sea-level conditions. Maërl (free-living calcareous red algae) beds are the main carbonate factory and are mainly located at the top of the rocky plateaus (at ~50-55 m) formed by the outcropping of the bedrock. Downward, the increased currents at the seabed level in the east–west-oriented belt, which connects the western Mediterranean and the Tyrrhenian Seas, limit the extension of this carbonate factory. This results in a mixed sedimentary facies composed of biogenic carbonate and relict siliciclastic sandy gravel. Compared to other Mediterranean shelves, the strait of Bonifacio is characterized by a distinct oceanographic setting. This is due to the connection between two basins. The currents at the seabed play a crucial role in controlling the distribution of the carbonate factories