Radiocarbon measurements on fresh-water carbonates have been used to determine the absolute chronology of the two largest fossil lakes in the Great Basin. The possibility of systematic errors due to exchange and to low initial C14 concentration has been considered with the conclusion that most of the measurements reported have not been affected by more than 10 per cent.

The results of the study suggest a high-water period from 25,000 to about 14,000 years ago. This period was preceded by an interval of moderately low water level extending back to at least 34,000 years before present. Following a recession to a moderately low water level close to 13,000 years ago Lake Lahontan and possibly Lake Bonneville rose to their maximum levels close to 11,700 years ago. This rapid rise was followed by an equally rapid fall close to 11,000 years ago. This latter decline is recorded by terrestrial deposits in many of the wave-cut caves on the shore lines of the ancient lakes. There is some evidence for another maximum close to 10,000 years ago. The lakes have probably remained low since 9000 years ago.

Consideration of the factors influencing the response of the lakes to climate change suggests that response is sufficiently rapid that the lake levels can be used as direct estimates of the relative climates. The lake-level chronology is hence a climate chronology for the Great Basin.

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