Articles | Volume 11, issue 5
Research article
11 May 2015
Research article |  | 11 May 2015

Northern Hemisphere control of deglacial vegetation changes in the Rufiji uplands (Tanzania)

I. Bouimetarhan, L. Dupont, H. Kuhlmann, J. Pätzold, M. Prange, E. Schefuß, and K. Zonneveld

Abstract. In tropical eastern Africa, vegetation distribution is largely controlled by regional hydrology, which has varied over the past 20 000 years. Therefore, accurate reconstructions of past vegetation and hydrological changes are crucial for a better understanding of climate variability in the tropical southeastern African region. We present high-resolution pollen records from a marine sediment core recovered offshore of the Rufiji River delta. Our data document significant shifts in pollen assemblages during the last deglaciation, identifying, through changes in both upland and lowland vegetation, specific responses of plant communities to atmospheric (precipitation) and coastal (coastal dynamics and sea-level changes) alterations. Specifically, arid conditions reflected by a maximum pollen representation of dry and open vegetation occurred during the Northern Hemisphere cold Heinrich event 1 (H1), suggesting that the expansion of drier upland vegetation was synchronous with cold Northern Hemisphere conditions. This arid period is followed by an interval in which forest and humid woodlands expanded, indicating a hydrologic shift towards more humid conditions. Droughts during H1 and the shift to humid conditions around 14.8 kyr BP in the uplands are consistent with latitudinal shifts of the intertropical convergence zone (ITCZ) driven by high-latitude Northern Hemisphere climatic fluctuations. Additionally, our results show that the lowland vegetation, consisting of well-developed salt marshes and mangroves in a successional pattern typical for vegetation occurring in intertidal habitats, has responded mainly to local coastal dynamics related to marine inundation frequencies and soil salinity in the Rufiji Delta as well as to the local moisture availability. Lowland vegetation shows a substantial expansion of mangrove trees after ~ 14.8 kyr BP, suggesting an increased moisture availability and river runoff in the coastal area. The results of this study highlight the decoupled climatic and environmental processes to which the vegetation in the uplands and the Rufiji Delta has responded during the last deglaciation.

Short summary
This study has great paleoclimatic and paleoecological significance, as it deals with the poorly documented tropical SE African ecosystem during the last deglaciation. Changes in the Rufiji upland vegetation evidenced the response of the regional hydrologic system to high-latitude climatic fluctuations associated with ITCZ shifts, while changes in sensitive tropical salt marshes and mangrove communities in the Rufiji lowland evidenced the impact of sea level changes on the intertidal ecosystem.