Lake Victoria is the largest lake in Africa and harbors more than 300 endemic species of haplochromine cichlid fish. Seismic reflection profiles and piston cores show that the lake not only was at a low stand but dried up completely during the Late Pleistocene, before 12,400 carbon-14 years before the present. These results imply that the rate of speciation of cichlid fish in this tropical lake has been extremely rapid.

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Geochemical analyses of sedimentary barites (barium sulfates) in the geological record have yielded fundamental insights into the chemistry of the Archean environment and evolutionary origin of microbial metabolisms. However, the question of how barites were able to precipitate from a contemporary ocean that contained only trace amounts of sulfate remains controversial. Here we report dissolved and particulate multi-element and barium-isotopic data from Lake Superior that evidence pelagic barite precipitation at micromolar ambient sulfate. These pelagic barites likely precipitate within particle-associated microenvironments supplied with additional barium and sulfate ions derived from heterotrophic remineralization of organic matter. If active during the Archean, pelagic precipitation and subsequent sedimentation may account for the genesis of enigmatic barite deposits. Indeed, barium-isotopic analyses of barites from the Paleoarchean Dresser Formation are consistent with a pelagic mechanism of precipitation, which altogether offers a new paradigm for interpreting the temporal occurrence of barites in the geological record.

We compared oxygen isotopic profiles from authigenic calcite in nine freeze cores from Lake Turkana, Kenya, with the historical record of lake levels for the past 105 years. 210Pb dating of the cores indicates that sediment in two of the cores spans the complete historical record. We observed spatial variability in 6180 and 617C values between cores and an offset from expected isotopic equilibrium values in several of the cores. Both of these characteristics can be explained by proximity to river input. In model calculations of lake level, we muted the spatial variability of the S1*O dataset by using a normalized and stacked record. There is good agreement between the lake level history modeled using the normalized and stacked record and the historical record of lake level. The lake level record derived from the S180 data indicates that lake level fluctuated by 15 m during the last century. Our model calculations indicate that S'*O records can be used to generate quantitative lake level curves even using datasets with significant spatial variability.The isotopic composition of carbonates has been used in numerous lacustrine studies to determine paleoclimatic information. Fluctuations in the isotopic record preserved in carbonates are created mainly by variations in the temperature and composition of the water in which the carbonate formed. In turn, these variations can be caused by climatic changes in the lake basin. Carbonates are therefore potentially an archive of the paleoenvironment of a lake basin.In tropical or closed-basin settings, shifts in the oxygenstable isotopic composition (S180) of carbonates are often attributed to shifts in the evaporation-to-input (E : Z) ratio of a lake. Previous workers have used the relationship between changes in the S180 value of carbonates and the E: Z ratio of a lake basin to discuss qualitative changes in paleo-lake levels. Although the relationship between the &*O composition of lake water and the various inputs and outputs to a basin has been described quantitatively (Gat 1984; Phillips et al. 1986;Benson and White 1994), only one quantitative comparison between a historical lake level record and lake level determined from the aI80 composition of carbonates has been attempted (Li 1995). AcknowledgmentsWe thank Marty Fleisher, Paul Hogan, Ryan Johnson, Tom Johnson, Patrick Ng'ang'a, and Keith Sturgeon for collecting the 1994 cores with assistance from R.D.R. Yvonne Chan and Keith Sturgeon gave invaluable aid in the laboratory. We thank Tom Johnson and Michael Talbot for providing valuable comments, and two anonymous reviewers for helpful reviews.