WEST AFRICAN MONSOON FLUCTUATIONS AS INFERRED FROM THERMOCLINE SHIFTS IN THE TROPICAL EAST ATLANTIC

Changes in the surface water stratification during the Late Holocene are evident in equatorial East Atlantic stable oxygen isotope records. This is based on two sample sites of about 5000 m water depth in the central Guinea Basin. The reconstruction of the thermocline in the Guinea Basin was based on the use of differences in d18O (d d18O) between shallow (Sphaeroidinella dehiscens), intermediate (Globorotalia truncatulinoides) and deep dwelling (Globorotalia crassaformis) foraminiferal species. An upward increasing d d18O trend between S. dehiscens and G. truncatulinoides points to an intensification of equatorial upwelling during the Late Holocene which is linked with a decreasing West African monsoon. This trend is interrupted by a sharp reduction of d d18O between 3000 and 2500 yr. BP which can be explained by increasing monsoon activity in West Africa. The following increasing d d18O trend correlates with the final aridification of the Sahara as we know it today. This supports the hypothesis of centennial to millennial fluctuations of the West African monsoon system. The monsoon is the main factor for agricultural production south of the Sahara which is in turn influencing human population dynamics (Weldeab et al., 2005). The rapid climate changes comprise regional, nonlinear feedback processes which could even be influenced by human activity (Zheng and Eltahir, 1997). This interplay is described in numerous studies that reported the weakening in the global monsoon system, interconnected with an aridification which led to increasing complexity of human communities. This human response to regional climate variations underlines that the historical record of the Late Holocene could be an important archive to unravel mode and duration of centennial to millennial climate fluctuations.

Brooks, N., 2006. Cultural responses to aridity in the Middle Holocene and increased social complexity. Quaternary International 151, 29-49. Weldeab, S., Schneider, R., R., Kölling, M., Wefer, G., 2005. Holocene African droughts relate to eastern equatorial Atlantic cooling. Geology 33, 981-984. Zheng, X., Eltahir, E., A., B., 1997. The response to deforestation and desertification in a model of West African monsoons. Geophysical Research Letters 24 (2), 155-158.