Tropical and sub-tropical West Africa: marine and by P. Giresse

By P. Giresse

West Africa and the japanese Atlantic stretching from Mauritania within the north to Namibia within the south supply a wide latitudinal stretch incorporating approximately symmetrical climatic gradients from the Equator. at the time scale of Quaternary Glacial and Interglacial cycles, at the present time, we own well-documented and lately released marine sedimentary documents exhibiting alterations in oceanic and atmospheric circulations and terrestrial fluxes. Deep-sea sediment documents comprise quite a lot of palaeoenvironmental signs like oxygen and carbon isotopes, alkenones, foraminiferal and different planktonic assemblages through the years classes as much as and larger than 125,000 years. those are signs of temperature and circulate shifts and make allowance Interglacial and Glacial comparisons on a local and inter-hemispheric scale. despite the fact that, this attempt to synthesize the prevailing wisdom can't but aspire to a world modelling. Linking with terrestrial files, albeit spatially patchy and usually missing an organization chronology, this e-book issues to shorter time scale chronologies from lakes, marshes and river deposits. diversified and never very trendy literature, either French and English, is pronounced right here. finally, the booklet files contemporary wisdom of the 1st steps of human career of usually adversarial environments and considers the environmental impression of historical and sleek societies. * Covers the new experiences approximately marine Quaternary environments off West Africa, in addition to continental Quaternary environments of tropical and sub-tropical West Africa (over 10,000 to 100,000 years)* Compares the parallel among palaeoenvironmental traits based on latitudinal gradients

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1989). Consequently, any weather or climate change would be liable to modify the atmospheric dust generation. In this way, Middleton (1985) considers that the increased dryness in the Sudan-Sahel area during the 1970-1980 decade coincides with an increase of dust storms (x6 in Mauritania and x5 in Sudan). Prospero and Nees (1977) observed that the mineral dust concentration was multiplied with a factor 3 during the same time in the western tropical Atlantic. Wind strength is responsible for dust storms, but in this sense, the Sahara and Namibia are two contrasting types of desert.

These air masses leave Africa and move westward within a persistent wind system towards the Caribbean. The vertical distribution of insoluble aerosol from the Bodele depression area of the Chad basin (McTainsh, 1984). However, Bertrand et al. (1975) have suggested on the basis of the visibility records that the Tenere Desert area may also act as a dust source area. 2. Vertical and Areal Atmospheric Distribution of Saharan and Namibian Dust Aeolian dust has been air sampled in the major wind systems along the eastern margin of the Atlantic between ca.

_1. 0 ' ' ' " " " ~ " " . , _ 3 4 . -- ,, : ' " 3 4 . 8 ~ , . 9 / \ 3 4 lwsow il ;34. ,:. ~ :. 70~ L 60 ~ ! _~ . . . 50 ~ 40 ~ 30 ~ , 20 ~ 10~ 0~ 10 ~ 20 ~ 30 ~ 40 ~ 50 ~ 60~ South North i i ~ ' ~ , . l z t .. / " ................ :~'":::, ~':i. ii ......... i. 'ii::i;i:~. i :5;::; 54-i-~. ,"1:! &:~ ,, r ": . ~2 9 Lower . -: . . . I, Discovery Seamount 70 60 50 40 30 20 10 0 Latitude [~ Fig. 1. Present day stratification of the main water masses and flow across the western Atlantic as displayed (a) by salinity contour lines (%o) and (b) by ~518Cvalues of dissolved inorganic carbon (adapted from Berger and Wefer, 1996).

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