Diatom productivity in Northern Lake Malawi during the past 25,000 years: implications for the Position of the Intertropical Convergence Zone at Millennial and Shorter Time Scales

Abstract

The large lakes of the East African Rift Valley provide a magnificent array of sedimentary basins that are actively recording the modern climate dynamics of tropical East Africa. The basins are perhaps 10 million or more years old (Cohen et al. 1993), and their deepest reaches have archived continuous records of past climate change that are unparalleled anywhere else in the tropics in terms of longevity coupled with resolution (Johnson 1993). These lakes are highly sensitive to climate variability, and their sediments carry rich signals of past environmental dynamics, inscribed in the assemblages of microfossils, the abundance and isotopic composition of endogenic minerals, bulk sediment texture and structure, and organic geochemistry. The International Decade for the East African Lakes (IDEAL) is a programme that has helped to steer and coordinate palaeoclimate studies on the rift lakes since the programme’s inception in 1993 (Johnson 1993). The IDEAL community of scientists from North America, Europe, and Africa is investigating not only the climatic history of the large rift lakes, but modern processes within their basins as well. The “decade” of IDEAL officially started in 1995 with its first field programme on Lake Victoria (Lehman 1998). Since then IDEAL scientists have continued to carry on investigations on Lakes Edward, Malawi and Tanganyika, as well as on smaller lakes in East Africa. The IDEAL palaeoclimate community is presently turning its focus to Lake Malawi, where a major drilling programme is planned for the last two months of 2004. Lake Malawi is the southern-most of the rift lakes, and it appears to be exceptionally promising as a target for long, continuous records of past climate change. It has responded somewhat differently from lakes farther north in Africa to global climate forcing associated with orbital (Milankovitch) cycles of insolation. In contrast to Lake Tanganyika and the lakes of North Africa, Lake Malawi experienced relatively dry conditions during much of the early Holocene (Finney et al. 1996; Johnson 1996). The history of Lake Malawi derived from drilling operations therefore may provide important insight into how the southern tropics of Africa have responded differently from the northern tropics on time-scales ranging from millennia to decades. This paper presents recent data derived from piston cores recovered from the north basin of Lake Malawi. It focuses on the abundance and mass accumulation rate of biogenic silica (BSi MAR) measured in the cores, supplemented by some trace-element data and the species composition of the diatom assemblages. These results imply linkages between the climate dynamics of tropical Southern Africa and extratropical regions as far away as Greenland.