SEASONAL SIGNALS IN LAKE MALAWI SEDIMENTS

The sedimentary record of Lake Malawi, one of the large rift valley lakes of east Africa, contains thick sequences of biologically undisturbed, finely laminated muds and silts. Laminated sequences were recovered in cores from the extensive diatom ooze and silty diatom clay facies of the lake basin that is permanently anoxic below 250 m. The sequences consist of light-dark laminae couplets with average thickness of 1 mm. The light laminae are completely dominated by the diatom genus Melosira with a relatively minor contribution from the genus Stephanodiscus. The dark laminae consists of 50% diatoms (equal proportions of Melosira and Stephanodiscus, few Nitzschia, and occasional littoral diatom genera) and 50% terrestrial plant debris, mineral grains, and fine clay and organic material. Based on a Pb-210 and C-14 calculated sedimentation rate of 1 mm yr-1 for Lake Malawi basin sediments and the well-defined seasonal variations in fluvial input and diatom productivity levels, our data suggest that the lamination couplets are annual varves, representing biannual sedimentation sequences. The light laminae result from diatom blooms occurring during the dry, windy season (April-October) when the upper 250 m of the lake is well-mixed, localized upwelling occurs, and the epilimnion becomes enriched with nutrients. The dark laminae represent sedimentation in the rainy, low-wind velocity season (November-March) when runoff and allochthonous terrestrial input are at a maximum and primary productivity is at a minimum.