Radiolaria as indicators of hydrological changes since the last glacial maximum of Cap Blanc (Northwest Africa)

Radiolarian distributions are studied in four cores from a coastal upwelling region off the Mauritanian coast (Northwest Africa) near Cap Blanc (21 degrees N). The author attempts to establish the relationships between ecological factors and radiolarian faunas. Only the total abundance of Radiolaria and species that seem to be especially connected with the hydrologic characteristics of this region is studied. 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 degrees N, but its distribution coincides with the South Atlantic central water; for this reason, it may be considered 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 abundant only in plankton tows below 800 m. Around Cap Blanc, it is present only in the upwelling areas in which the sea is 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 westward. 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 in sea level and the intensity of the trade winds. 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; however, they can be explained satisfactorily 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 and ri