Rapid deep-water renewal in Lake Issyk-Kul (Kyrgyzstan) indicated by transient tracers

Simultaneous profiles of the transient tracers sulfur hexafluoride (SF6), H-3-He-3, and the chlorofluorocarbons CFC-11 and CFC-12 were measured in Lake Issyk-Kul, a large, deep lake in Kyrayzstan. Apparent water ages derived from these measurements suggest rapid mixing, with a deep-water renewal rate >10% yr(-1) at 650 in depth. SF6 and H-3-He-3 ages agree reasonably well, whereas CFC ages are significantly greater. The discrepancy is explained by the nonlinear relationship between tracer age and tracer concentration and by the flattening of the atmospheric growth curves for CFCs. Novel to physical limnology is the application of SF, dating, which proves to be an excellent tool for the study of mixing in lakes, complementing H-3-He-3 and CFC dating techniques.