The Late Pleistocene-Holocene palaeolimnology of Lake Victoria, East Africa, based upon elemental and isotopic analyses of sedimentary organic matter

Three piston cores from Lake Victoria (East Africa) have been analysed for organic carbon (TOC) and nitrogen (TN) content, stable isotopes (delta(13)C and delta(15)N), and Hydrogen Index (HI). These data are combined with pub lished biogenic silica and water content analyses to produce a detailed pal aeolimnological history of the lake over the past ca. 17.5 ka. Late Pleisto cene desiccation produced a lake-wide discontinuity marked by a vertisol. S ediments below the discontinuity are characterised by relatively low TOC an d HI values, and high C/N, delta(13)C and delta(15)N, reflecting the combin ed influence of abundant terrestrial plant material and generally unfavoura ble conditions for organic matter preservation. A thin muddy interval with lower delta(13)C and higher HI and water content indicates that dry conditi ons were interrupted by a humid period of a few hundred years duration when the lake was at least 35 m deep. The climate changed to significantly more humid conditions around 15.2 ka when the dry lake floor was rapidly floode d. Abundant macrophytic plant debris and high TOC and delta(13)C values at the upper vertisol surface probably reflect a marginal swamp. delta(13)C va lues decrease abruptly and HI begins to increase around 15 ka BP, marking a shift to deeper-water conditions and algal-dominated lake production. C/N values are relatively low during this period, suggesting a generally adequa te supply of nitrogen, but increasing delta(15)N values reflect intense uti lisation of the lake's DIN reservoir, probably due to a dramatic rise in pr oductivity as nutrients were released to the lake from the flooded land sur face. An abrupt drop in delta(13)C and delta(15)N values around 13.8-13.6 ka refl ects a period of deep mixing. Productivity increased due to more efficient nutrient recycling, and delta(13)C values fell as C-12-rich CO2 released by bacterial decomposition of the organic material was brought into the epili mnion. A weak drop in HI values suggests greater oxygen supply to the hypol imnion at this time. Better mixing was probably due to increased wind inten sity and may mark the onset of the Younger Dryas in the region. After the period of deep mixing, the water column became more stable. TOC, C/N, delta(13)C and HI values were at a maximum during the period between 1 0 and 4 ka, when the lake probably had a stratified water column with anoxi c bottom waters. A gradual decrease in values over the last 4000 yrs sugges t a change to a more seasonal climate, with periodic mixing of the water co lumn. Rising sediment accumulation rates and a trend to more uniform surfac e water conditions over the last 2000 yrs are probably a result of increase d anthropogenic impact on the lake and its catchment. Following a maximum at the time of the rapid lake-level rise during the ter minal Pleistocene, delta(15)N has remained relatively low and displays a gr adual but consistent trend to lower values from the end of the Pleistocene to the present. TN values have risen during the same period. The lack of co rrelation between delta(13)C and delta(15)N, and the absence of any evidenc e for isotopic reservoir effects despite the rise in TN, suggests that the atmosphere, rather than the lake's dissolved nitrogen pool has been the pri ncipal source of nitrogen throughout the Holocene. The importance of atmosp heric N fixation to Lake Victoria's nitrogen cycle thus predates by a very considerable margin any possible anthropogenic eutrophication of the lake.