Les Radiolaires temoins de l'evolution hydrologique depuis le dernier maximum glaciaire au large du Cap Blanc (Afrique du Nord-Ouest)
Radiolarian distributions have been studied in four cores from a coastal upwelling region off the Mauritanian coast (Northwest Africa) near Cap Blanc (21°N). 
 
Our purpose is to establish the relationships between ecological factors and radiolarian faunas. We have only studied the total abundance of Radiolaria and species that seem to be specially connected with the hydrologic characteristics of this region. 
 
Results indicate that there are good correlations between the distribution of Radiolaria in the sediments and different aspects of upwelling. The abundance of Radiolaria in the sediments may be linked to the productivity of the surface waters. The quantitative distribution of cold-water species gives an indication of the influence of coastal upwelling. The distribution of two other species seems to have a marked relationship with the water masses in this region. Thus, Phormospyris stabilis scaphipes is considered very rare at latitude 21°N but its distribution coincides with the South Atlantic Central Water and for this reason it may be considered as a possible tracer-species of these waters. Theocalyptra davisiana can act as an indicator of nutrient concentrations for water masses of intermediate depths. Mondially, it is only abundant in plankton tows below 800 m. Around Cap Blanc it is only present in the upwelling areas in which the sea is the richest in nutritive salts. 
 
The stratigraphic data suggest that since the youngest glacial period, the marine environment off Northwest Africa has been variable. 
 
During the last glacial maximum radiolarian abundances indicate a higher and less strictly coastal productivity than at present. Coastal upwelling was stronger then and its maximal influence shifted westwards. During the Holocene the intensity of upwelling decreased and its influence may even have disappeared altogether before latest Holocene time. These processes are associated with fluctuations of sea level and the intensity of the trade winds. However, not all changes in the radiolarian distribution pattern during the transition from the last glaciation to the Holocene are completely explained by changes in intensity and a shift in the area of coastal upwelling. On the other hand, however, they can be satisfactorily explained by differences in the oceanic circulation. Thus, during the last glaciation, it may be assumed that there was a northerly increase in the subsurface southern Atlantic direction of water circulation which is more representative of cold waters, richer in nutrients than north Atlantic waters. These changes might have occurred at the same time as changes in the structure of deeper waters: for instance, the replacement of North Atlantic Deep Water by another water body with significantly different characteristics and a much higher production of Intermediate Water as has been suggested by various authors.