Transport mechanism and paleoclimatic significance of terrigenous silt deposited in varved sediments of an African rift lake

We analyzed a varved sequence of sediment from a 350-m depth in the north basin of Lake Malawi, East Africa, for the size distribution of the sortable silt fraction (10–64 mm). Mean size of the sortable silt (SS) varies measurably in sediments spanning the past 650 yr and covaries with the mass accumulation rate of terrigenous silt and clay (TMAR) over much of the interval. Most of the silt and clay is delivered to the offshore basin in benthic nepheloid plumes of unknown duration and frequency. TMAR appears to be related to annual rainfall (which is related to the North Atlantic Oscillation) because it roughly tracks the historical record of lake level that extends back to 1860. SS should be related to density or thickness of underflow, thus related to resuspension intensity or river flood loading. It also tracks lake level and regional wind strength as determined by National Center for Environmental Prediction (NCEP) Reanalysis. The distribution of sediment sizes in lake basins is strongly bimodal; relatively coarse sand and gravel accumulate in the shallower depths where the oscillatory flow due to surface waves winnows away the fine-grained material and silty clay accumulates at greater depths, below the ‘‘mud-depositional boundary’’ (Rowan et al. 1992; Hakanson and Kallstrom 1978). The deep basins of the East African rift lakes, which can be several hundred meters deep and tens of kilometers across, provide immense repositories of organically rich mud up to several kilometers thick. The mud-depositional boundary is ,100 m in the large lakes of Tanganyika and Malawi, and ,30 m to 40 m in the smaller lakes such as Edward and Albert (Johnson 1996; cf. the ‘‘mud line’’ of the U.S. mid-Atlantic Bight slope at 250–300 m [Stanley and Wear 1978]). The mud is typically diatomaceous silty clay, usually deposited under anoxic conditions, with 5–30 wt% of organic carbon (Talbot 1988). Here we examine the accumulation rate of terrigenous clastic sediment and the size distribution of ‘‘sortable silt’’ in the north basin mud of Lake Malawi. Sortable silt (SS) is defined by a size range of 10 mm to 63 mm and has been argued to respond in a dominantly noncohesive manner that is particularly sensitive to bottom-current strength (McCave and Hall 2006; McCave et al. 2006). Fluctuations in the mean size of the SS (SS) have been found to reflect changes in ocean-bottom current intensity; vigorous flow inhibits the deposition of the slower settling (finer-grained) sediment. This results in a relatively coarse SS as well as a high percentage of SS (SS%) in the fine-grained deposits. Given the proximity of the shoreline and large rivers to the northern basin of Lake Malawi, the problem considered here is how the accumulating sediment is delivered: from direct fluvial input, aeolian fallout, or resuspension and dispersal in midwater or bottom plumes. We analyzed a sediment record spanning the past 650 yr, addressing both the mechanism by which the SS is transported to the offshore basin and the paleoclimatic significance of SS variability through time.