Geochemical and Grain Size Characterization of Surface Sediments on the Southern Continental Shelf of Morocco

On the continental shelf, the sediments are the only possibility to better understand the marine ecosystem, because of the material they contain (micropalaeontological tracers, i.e. pollen, microfossils, geochemical and mineralogical tracers, i.e. dust...). The physical, chemical, and biological characteristics of ocean sediments are not the only elements that determine their character; the source from which they originate also influences the nature of these deposits.

A missions aboard the scientific vessel "Amir Moulay Abdellah and Al Hassan Al Marrakchi, the oceanographic and environmental monitoring program (INRH)" and "Dr. Fridtjof Nansen, EAF-Nansen Programme/FAO" were carried out to collect a superficial sediment from continental shelf in the south Moroccan Atlantic coast.

This study investigates the mineralogical and geochemical composition of 38 surface sediment samples, between Cap Boujdour and Cap Blanc. Using a multi-pronged approach, the sedimentological characteristics were assessed through direct observation, grain size analysis, measurements of calcium carbonate (CaCO3) and organic carbon content, as well as geochemical and mineralogical composition analyzed using X-ray diffraction (XRD) and X-ray fluorescence (XRF).

The results indicate that the inner-shelf sediments are predominantly mud, characterized by abundant quartz grains and elevated concentrations of terrigenous elements such as Fe, Si, Rb, and K, likely originating from coastal erosion with minor contributions from aeolian dust. In contrast, the middle and outer shelf regions are dominated by biogenic carbonate, with calcium carbonate (CaCO3) levels approaching to 65%, and elevated Ca and Sr content. The mineralogy in these areas is primarily composed of calcite and aragonite. Slope sediments are enriched with mud and montmorillonite clay minerals.

The sediments in the middle-, outer-shelf, and slope regions are interpreted as relict deposits from previous glacial periods of lower sea levels. Aeolian contributions are more pronounced south of Dakhla and are particularly related to the Saharan Air Layer.