The distribution and relationship between palynomorphs in aerosols and deep-sea sediments off the coast of Northwest Africa

Abstract

Palynomorphs in eolian dust and bottom sediments from the Northeastern Atlantic, off the coast of Northwest Africa, were studied. Primary examination was for terrestrial plant detritus (pollen, spores, opal phytoliths and freshwater diatoms) but observations about marine entities were also made. Three distinct geographic palynofloras were detected: a northern palynoflora consisting of pollen derived primarily from the Mediterranean Basin, a central or Saharan palynoflora characterized by pollen derived from desert plants and a southern (Tropical-Equatorial) palynoflora composed of pollen derived from tropical plants but including up to 50% allochthonous pollen from the drier interior of West Africa. Palynomorph distributions are related closely to both source vegetation and to atmospheric and oceanic transport mechanisms and in many cases distribution patterns in bottom sediments and aerosols are complementary. The quantity of pollen and spores per gram of bottom sediment ranges from greater than 2000 off the Saharan coast in Mauritania to less than 50 in deep-ocean basins. Pollen and spores in Mediterranean aerosols may exceed 40 per cubic meter of air during the summer and range between 4 to 6 for tropical aerosols during the winter. Tropical aerosols and bottom sediments contained the greatest abundance of fungal spores. The distribution of opal phytoliths and freshwater diatoms in both aerosols and bottom sediments indicates that dust storms are a major transporting agent for these entities from the interior of West Africa to the Gulf of Guinea. These storms are also an important agent for the transport of pollen to the tropical atmosphere. Distances of transport may exceed 5000 km for Mediterranean pollen and even greater distances for palynomorphs in eolian dust from interior West Africa. The abundance of dinoflagellates and microforaminifera in bottom sediments is directly related to the area of upwelling off the West African coast.