Provenance of the Saharan Winter Dust Plume through the African Humid Period

Abstract:

North Africa is home to millions of people living in nation states considered to be particularly vulnerable to anthropogenically-driven climate change. The region is very likely to warm over the 21^st Century but there is fundamental disagreement among model projections about the magnitude, and even sign, of the rainfall climate response. Geological records of wind-blown Saharan dust accumulating in marine sediment cores in the North Atlantic Ocean provide a way to assess the response of North African rainfall climate to past intervals of global warmth.

Dust is transported to the North Atlantic Ocean from North Africa via two main routes, a summer (northern) route and a winter (southern) route. Virtually everything we have learnt so far from marine sediment cores about North African hydroclimate comes from drill sites located beneath the summer (northern) dust plume. Here we report geochemical records (radiogenic isotope (⁸⁷Sr/⁸⁶Sr and ε_Nd) and XRF core scanning) from ODP Site 662 in the eastern equatorial Atlantic spanning the last 25,000 years. We show that ODP Site 662 is ideally situated to study the palaeo-history of the winter dust route and present the first record of winter dust provenance spanning the most recent African humid period (AHP). Comparison with North African preferential dust source area (PSA) isotopic fingerprints shows that the primary source of terrigenous material to Site 662 is palaeolake Megachad, in the central PSA. Large downcore excursions in radiogenic isotope data indicate that dust provenance varies on orbital timescales with supply from Megachad reaching a minimum during the AHP, coincident with lake Megachad high-stand. Our results shed new light on the timing of the onset and termination of the AHP in the central Sahelian region, and on the question of the linearity of the response of African hydroclimate to insolation forcing.