Reconstruction of Benguela Current Ocean Productivity and West African Vegetation During Oxygen Isotope Stages 100 and 101.

The Intensification of the Northern Hemisphere Glaciation (3.2 to 2.5 Ma) is a key climatic transition in Earth History. Deep-sea sediments recovered from ODP Leg 175 Site 1083 cover this important time period. Site 1083 has the advantage that it monitors both changes in the Benguela Current Upwelling system as well as the adjacent African continent. In this study we have focused on the interglacial (OIS 101) and glacial (OIS 100) periods that make up the final intensification step in this climatic transition. With a resolution of approximately 1 ka, we have reconstructed the following climatic parameters: Global ice volume (benthic foraminifera oxygen isotopes), wind strength and land aridity (HIRM, MS, Al/Ti ratios), upwelling intensity (UK37'-SSTs), surface water productivity (TOC, Chaetoceros resting spores, alkenone abundance, pigments, Ba), surface water nutrient availability (organic nitrogen isotopes), nutrient source (diatom species abundance), and land vegetation type (n-alkane abundance and carbon isotopes). Two conclusions have been drawn from this unique data set: 1. Surface water productivity peaks during glaciation, but is in fact lower during the full glacial than the previous interglacial. It is suggested that this is caused by the antagonistic effects of upwelling intensity and nutrient supply to this area. 2. The carbon isotopic record of n-alkanes, which monitors the relative abundance of C3 (tress and higher plants) vs C4 (tropical grasses) plants, clearly shows that the vegetation of South West Africa during this time period co-varies with precession and is independent of glacial-interglacial cycles. We suggest that this is due to the strong influence of precession on the penetration of the ITCZ-driven North-Central African monsoons (and resultant moisture availability) into the Congo Basin and the rest of SW Africa.