Holocene Monsoon variability in East Africa: a marine perspective

The processes that control past monsoon \nvariability in the East African Tropics during the Holocene are poorly \nunderstood. Especially the role of Sea Surface Temperatures (SST) \ncontrolling East African Rainfall on millennial timescales, as it is \nobserved on decadal timescales, is currently intensely debated. In \naddition, it has been suggested recently that the longitudinal \nmigration of the Congo Air Boundary (CAB) modulates East African precipitation on a regional scale as well [Tierney et al., 2011]. \nHere, we present a high-resolution marine sediment record for the \npast 12 kyrs from offshore Tanzania, close to the Rufiji River delta, \nto contribute to the current debate from a marine point of view. We \nreconstructed past SST and !18Oseawater, derived from planktic \nforaminiferal Mg/Ca and !18O, and past Sea Surface Salinity (SSS) \nvariations, derived from planktic foraminiferal Ba/Ca-ratios. In the \nvicinity of river deltas, Ba/Ca-ratios have potential to record \nprecipitation changes in the rivers’ catchment area. \nOur records show that East African precipitation, derived from \nBa/Ca-ratios, roughly varies in concert with Indian Ocean SST, \nsuggesting higher Indian Ocean SST to be an important prerequisite \nfor stronger precipitation, and hence an intense monsoon episode in \nEast Africa. We calculated the difference ("SST) between our \nrecord of Indian Ocean SST and SST of the tropical Atlantic \n[Weldeab et al., 2005], showing that "SST variability resembles the \nisotopic pattern of the Kilimanjaro ice core record [Thompson et al., \n2002]. We suggest this to be the consequence of a longitudinal \nmovement of the CAB over the African Continent, changing the \ntrajectory of Indian Ocean moisture into the continent and therefore \naffecting the !18O of the East African rainout.