A direct comparison between the historical record of lake level and the δ18 O signal in carbonate sediments from Lake Turkana, Kenya

Abstract

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).