Temporal variability of fluxes of eolian‐transported freshwater diatoms, phytoliths, and pollen grains off Cape Blanc as reflection of land‐atmosphere‐ocean interactions in northwest Africa

Abstract

Fluxes of airborne freshwater diatoms (FD), phytoliths (PH), and pollen grains (PO) collected with sediment traps off Cape Blanc, northwest Africa, from 1988 till 1991 are presented. Both continental rainfall variations and wind mean strength and direction play a key role in the temporal fluctuations of the fluxes of eolian traces in the pelagic realm. Drier conditions in Northern Africa in 1987 could have preceded the high lithogenic input and moderate FD flux in 1988. The PH peak in summer 1988 was probably caused by increased wind velocity. Wetter rainy seasons of 1988/89 might have promoted a significant pollen production in summer 1989, and FD in late 1989 and early 1990, as well as contributed to the reduction of the lithogenic flux in 1989/90. Decreased fluxes of FD, PH and PO, and higher contribution of the 6–11 μm lithogenic fraction in 1991 would mainly reflect minor intensity and decreased amount of continental trade winds. Air‐mass backward trajectories confirm that the Saharan Air Layer is predominantly involved in the spring/summer transport. Trade winds play a decisive role in the fall/winter months, but also contribute to the transport during late spring/summer. Origin of wind trajectories does not support a direct relationship between transporting wind‐layers and material source areas in Northern Africa. High winter fluxes of eolian tracers and high amount of trade winds with continental origin in summer warn against a simplistic interpretation of the seasonal eolian signal preserved in the sediments off Cape Blanc, and the wind layer involved in its transport.

Mineral ballast and particle settling rates in the coastal upwelling system off NW Africa and the South Atlantic

“…A five-order polynomial fit highlights the long-term decrease of biogenic opal (b). In contrast, alkenones and organic carbon fluxes increased in 1991, when alkenone temperatures were relatively low of fine-grained lithogenic material (6-11 lm; Ratmeyer et al 1999) reflect minor intensity of the trade winds and probably reduced coastal upwelling (Romero et al 2003).…”

The nature and tropospheric formation of iberulites: Pinkish mineral microspherulites

“…North African dust contains appreciable quantities of iron (Jickells et al, 2005): the pinkish color can be attributed to these iron coatings. Pokras (1991) and Romero et al (2003) reported remains of silica diatom shells in dust samples. Their presence in the interior of iberulites establishes different source areas, like the Central and Western Sahara and Sahel, especially the northern region in the Bodelé Depression (Washington et al, 2006).…”

“…Model estimates of the amount of desert dust generated by the Sahara range from 270 to 670 Tg year À1 , and Saharan dust transport to Europe ranges from 80 to 120 Tg year À1 , reaching distant regions, like South America (Swap et al, 1996), the North Atlantic-Caribbean area (Prospero, 1999), Northern Europe (Franzen et al, 1994) and perhaps as far as the Himalaya (Carrico et al, 2003). Guerzoni and Chester (1996), Romero et al (2003) and Jaenicke (2005) reported the presence of marine nanoplankton, pollen, diatoms and bacteria in aerosols from these areas. Great importance is attached to the role of mineral aerosols as a reactive surface in the global troposphere (Wurzler et al, 2000;Usher et al, 2003), where heterogeneous nucleation phenomena can occur with the creation of new phases (neoformations), mainly of sulfates and nitrates.…”

Application of source-receptor models to determine source areas of biological components (pollen and butterflies)

“…These families are present in the territory (African Flowering Plants Database, 2009). Paleopalinologic studies of the pollen in the marine sediments off the coast of Africa have also reported the transport of Chenopodiaceae/Amaranthaceae from the area of the Sahara (Hooghiemstra et al, 2006) and the transport of Cyperaceae from Sahara-Sahel (Romero et al, 2003). Although these observations show the presence of pollen in the past, in which the distribution of plants might have been different from the current distribution, some recent works about the pollen contents in West Africa, such as the study of Calleja et al (1993), coincide with the results of the sediment, confirming the transport of these kinds of pollen from Africa.…”