VARVE FORMATION IN THE NORTHERN BASIN OF LAKE MALAWI, EAST AFRICA

Most of the sediment accumulating below 200 m depth in the north basin of Lake Malawi consists of alternating light laminae enriched in diatom frustules and dark laminae with high concentrations of terrigenous silty clay. A comparison of lamination counts and Pb-210 analyses demonstrated that these laminations are annual layers, or varves. The light, diatomaceous laminae have been interpreted by many investigators to reflect the dry, windy months of austral winter, when upwelling promotes high algal productivity, and the dark, clastic laminations to be deposited during the rainy months of austral summer, when river discharge is at a maximum. We examined the accumulation rate of terrigenous clastic sediment and the size distribution of "sortable silt" in the varved sequence. Sortable silt, making up about a quarter of the varved sediment, has a size range of 10 to 63 µm, and has been argued to respond in a dominantly non-cohesive manner that is particularly sensitive to bottom current strength. The mean size of sortable silt (MSS) deposited at the core site has been remarkably uniform (16.3 – 18.9 µm) over the past 700 years. We envision the annual load of coarse silt delivered by rivers during the rainy season to be deposited temporarily in a few tens of meters water depth within a kilometer of the river mouth, as observed off the Linthipe River delta by McCullough (2006). With the onset of high wind events at the beginning of the cool season, the seasonal river deposit is resuspended by surface wave activity and possibly internal wave action on the sharpened pycnocline, triggering a sediment gravity flow of the relatively cool surface and thermocline waters that descends basin-ward as a bottom nepheloid layer/very low velocity turbidity current. Calculations of bottom shear stress, density flow velocity and sediment suspension indicate that the terrigenous laminae in the varves are unlikely to have been deposited by single annual events during the rainy season, but rather by a few events each year. We conclude that the annual signature of the varves may be due solely to the annual pulse of diatoms to the lake floor.