Mapping the abundance of 13C in leaf-wax components in surface sediments recovered from the seafloor off northwest Africa (0-35°N) reveals a clear pattern of δ 13C distribution, indicating systematic changes in the proportions of terrestrial C 3 and C 4 plant input. At 20°N latitude, we find that isotopically enriched products characteristic of C 4 plants account for more than 50% of the terrigenous inputs. This signal extends westward beneath the path of the dust-laden Sahara Air Layer (SAL). High C 4 contributions, apparently carried by January trade winds, also extend far into the Gulf of Guinea. Similar distributions are obtained if summed pollen counts for the Chenopodiaceae-Amaranthaceae and the Poaceae are used as an independent C 4 proxy. We conclude that the specificity of the latitudinal distribution of vegetation in North West Africa and the pathways of the wind systems (trade winds and SAL) are responsible for the observed isotopic patterns observed in the surface sediments. Molecular-isotopic maps on the marine-sedimentary time horizons (e.g., during the last glacial maximum) are thus a robust tool for assessing the phytogeographic changes on the tropical and sub-tropical continents, which have important implications for the changes in climatic and atmospheric conditions.