Geochemical responses to a major transgression in giant African Lakes

Bulk organic matter (OM) in sediments that accumulated in Lakes Victoria, Albert and Tanganyika during the major transgression that transformed these basins from a desiccated or lowstand condition at the time of the Last Glacial Maximum to highstand with overflow in the terminal Pleistocene and early Holocene, show some striking compositional trends. Sediment OM content (TOC) and Rock-Eval Hydrogen Index (HI) both rise markedly, reflecting increasing primary productivity during the transgression and enhanced conditions for the preservation of labile OM, In Lake Victoria rising sedimentary sulphur content tracks the change in HI. Over the same time interval in all three lakes, delta C-13(OM) shows a falling trend of 5-8 degrees. Nitrogen isotopic and C/N trends are, in contrast, more variable. Relatively high delta N-15 values at an early stage in the transgression may reflect isotopic effects associated with ammonia loss from an alkaline waterbody, while high delta N-15 later in the transgression was probably due to periods when stratification-related denitrification had a particularly strong impact upon the lakes' N cycle. The observed changes are attributed to the influence of the landscape drowned during the transgression upon the nutrient budget of the lake, a rise in river-supplied nutrients as the climate became wetter, and an increasing tendency to stable chemical stratification as the water column deepened. In the case of delta C-13, additional contributory factors were changes in the amount and isotopic composition of river-supplied dissolved inorganic carbon (DIC) as runoff and soil-moisture turnover rates increased, and the replacement of a C4-grassland dominated flora by mixed woodland-grassland and forest with a significant presence of C3 plants. Distinct geochemical changes are also apparent in Lakes Victoria and Albert sediments at the end of the transgression which are consistent with a reduction in nutrient loading. In the case of N this resulted in biological N fixation becoming a major part of the lakes' N cycle, Some of the observed geochemical variations may be generally applicable as proxy indicators of major transgressions in large lakes.