Fine-grained laminated lake beds are frequently interpreted as having developed under anoxic bottom-water conditions. The absence of pervasive bioturbation is considered to indicate O 2 levels too low to support burrowing metazoans. Evidence from Lake Turkana, Kenya, a large tropical Rift Valley lake, suggests that food-resource availability may be equally important in laminae preservation. Oxygen and temperature measurements from localities throughout the lake show bottom waters to be saturated or supersaturated with respect to O 2 even at great depths (> 100 m). Strong regional winds keep bottom waters well mixed, even at the prevailing water temperatures (25-30 degrees C) at which water is readily stratified. Sediments cored in offshore settings of Lake Turkana consist of finely laminated clays with very little interstitial organic material. Primary productivity is extremely high on the margin of the lake, but phytoplankton populations in offshore regions are markedly less productive. This study is intended to inject a cautionary note into potential misinterpretations of lacustrine paleoenvironments, particularly for areas which experience tropical climates at the time of deposition.--Modified journal abstract.

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