Magnetic proxies for reconstruction of 'mega'-drought and palaeo-wind directions for Lake Challa, East Africa

Long, Quaternary climate records from the tropical and sub-tropical regions are essential for understanding low latitude influences on heat fluxes to higher latitudes, and, in the case of sub-equatorial Africa, the climatic context of human evolution. East African lakes in the eastern Rift Valley potentially provide key sedimentary records of African monsoonal climate. In this region, the inter-tropical convergence zone (ITCZ) migrates across the widest latitudinal range; tropical convective activity creates the inter-tropical, bimodal seasonal regime of two wet seasons and two dry seasons. The NE monsoon brings rainfall during the NH autumn-winter, and the SE monsoon brings rainfall during NH spring-summer. Major uncertainty surrounds the paleoclimate of the last interglacial period, Marine Isotope Stage (MIS) 5. Tantalising evidence, further west, from Lakes Tanganyika, Malawi and Bosumtwi, suggests that early hominids were subjected to millennial-scale droughts, more severe even than at the Last Glacial Maximum (LGM) drought. Such evidence for these MIS5 'mega'-droughts has been linked to evolution and expansion of Homo sapiens within and beyond Africa.

Here, using magnetic analyses as proxies for influx and provenance of windblown dust, and lake productivity, we examine the high-resolution sedimentary records of Lake Challa, on the Kenya/Tanzania border, where a core of 250m covers the last 220 kyrs. Lake Challa is groundwater-fed; its magnetic minerals include terrigenous dust (dominated by haematite), tephra and/or bacterial magnetosomes formed in situ. We use the variations in windblown haematite to identify intervals characterised by intense northerly winds, and possible failure of the rainfall-bearing monsoon winds.