West African monsoon variability during the last deglaciation and the Holocene: Evidence from fresh water algae, pollen and isotope data from core KW31, Gulf of Guinea

The study of aquatic and mangrove pollen grains and algae from freshwater environments from core KW31 recovered off the mouth of the Niger River (3°31′1″N-05°34′1″E; 1181 m water depth) documents large-scale changes in river discharge linked to variations of Atlantic Monsoon rainfall during the last glacial-interglacial transition and the Holocene. Two main taxa dominate the algal record: Pediastrum, which records the strongest river input from 14,500 to 7100 cal B.P. and Botryococcus, which indicates surficial runoff in more arid periods, particularly at the onset of the deglaciation and during the Younger Dryas. Several dry events punctuated the first and more intense phase of the so-called “African Humid Period” (14,500 cal B.P.-9800 cal B.P.): during the Younger Dryas (13,400-12,100 cal B.P.), then around 11,400-11,200 cal B.P., indicating major climate instability. Freshwater discharges decreased after 7100 cal B.P. and definitely dropped to modern values after 3500 cal B.P. in agreement with land data. The microfossil record of core KW31 shows that the increase in freshwater input to the ocean and correlative rainfall over western Tropical Africa were gradual in response to insolation forcing at the transition from glacial to interglacial situations.