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

Simultaneous profiles of the transient tracers sulfur hexafluoride (SF6), 3H-3He, and the chlorofluorocarbons CFC-11 and CFC-12 were measured in Lake Issyk-Kul, a large, deep lake in Kyrgyzstan. Apparent water ages derived from these measurements suggest rapid mixing, with a deepwater renewal rate .10% yr21 at 650 m depth. SF6 and 3H3He 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 SF6 dating, which proves to be an excellent tool for the study of mixing in lakes, complementing 3H-3He and CFC dating techniques. Lake Issyk-Kul (Fig. 1) is located at an altitude of 1,606 m in the Tien Shan mountains of northeast Kyrgyzstan. With a water volume of 1,740 km3 and a maximum depth of 668 m, Lake Issyk-Kul is one of the largest and deepest lakes in the world (Adyshev et al. 1987; Herdendorf 1990). The lake has no outflow and a refill time of ;330 yr. Precipitation over the lake surface is 277 mm yr21, and surface inflow corresponds to 568 mm yr21 (Tsigelnaya 1995). The salinity (S) of the lake water is ;6‰ (the chemical composition is given by Tsigelnaya 1995 and Lyons et al. 2001). January mean air temperatures at the lake are below 228C (Adyshev et al. 1987), but Lake Issyk-Kul never freezes completely. This is reflected in the Kyrgyz meaning of the lake’s name— ‘‘warm lake.’’ The lake is situated in a predominantly rural area with no major industries. Because of its unspoiled setting and its unpolluted water, Lake Issyk-Kul was a tourist attraction during the period of Soviet rule, and the Kyrgyz government is planning to invest in infrastructure improvements around the lake to establish a tourist industry that will serve as an important economic factor in the region. Because of concerns about the ecological impact not only of the tourist industry but also of potential contamination resulting from gold ore processing (a cyanide spill occurred in 1998) and from the possible leakage of radioactive waste from an abandoned uranium mine located near the southern shore of the lake, several research projects have been initiated to investigate the ecology of Lake Issyk-Kul. The lake is also a promising site for studying the paleoclimate of central Asia using sediment cores (Ricketts et al. 2001), the interpretation of which requires a detailed understanding of the lake’s biogeochemistry and transport processes. From 1978 to 1989, the Hydrometeorological Survey of the Soviet Union/Kyrgyzstan measured temperature, major ions, and nutrients at numerous stations in the top 200 m of the water column of Lake Issyk-Kul at intervals of at least 1 month. Similar data, sampled less frequently, are also available for the deep water. To complement this information, we provide data herein on the transient tracers 3H, 3He, sulfur hexafluoride (SF6), and the chlorofluorocarbons CFC11 and CFC-12, which can be used to estimate deep-water residence times by use of dating techniques. In limnology, the application of 3H-3He dating (Torgersen et al. 1977, 1981; Aeschbach-Hertig et al. 1996; Hohmann et al. 1998) and dating by CFC-11 and CFC-12 (Weiss et al. 1991; Weiss and Volmer 1999; Peeters et al. 2000a; Aeschbach-Hertig et al. in press) is well established. The use of a combination of these tracers improves the reliability of the interpretation of the tracer concentrations in terms of mixing and deep-water residence times (Peeters et al. 1997, 2000a,b). On the basis of 3H concentrations from 1976 (Romanov et al. 1989) and dissolved oxygen and conductivitytemperature-depth (CTD) data, Kipfer and Peeters (in press) have suggested that the residence time of the deep water of Lake Issyk-Kul can be expected to be ,13 yr. Because the atmospheric input functions of CFC-11 and CFC-12 have increased only very slowly since 1996 (and, in the case of CFC-11, has even decreased over the past few years), errors in CFC dating applied to water masses younger than ;10 yr are large. Dating based on SF6 concentrations has therefore been suggested as an alternative technique for the study of mixing in Lake Issyk-Kul (Kipfer and Peeters in press). The atmospheric concentration of SF6 has increased rapidly since 1970, enabling reliable dating of waters with residence times between 0 and 30 yr. To our knowledge, this is the first study that has made use of SF6 dating to study mixing in large lakes. In this note, we present and discuss our transient tracer data set from Lake Issyk-Kul and provide an estimate of the deep-water residence time. Sampling—On 6 March 2001, the water column was sampled at the deepest point of Lake Issyk-Kul (42823.8469N, 77806.0419E), approximately at the center of the lake (Fig. 1). Water samples were collected at 50-m depth intervals with 5-liter Niskin bottles. Via a silicon tube, water was filled into copper tubes and sealed gas-tight for the analysis of 3H, 3He, 4He, Ne, CFC-11, and CFC-12. In addition, water was filled into 500-ml stainless-steel sampling cylinders (Whitey, 304L-HDF4-500; Arbor, Inc.) equipped with two plug valves (Nupro, SS-4P4T1; Arbor, Inc.) for the combined analysis of SF6 and CFC-12. At the same time, a CTD profile was measured with a Seabird SB9 probe. Analytical methods—Concentrations of 3H, 3He, 4He, and Ne were measured according to the method of Beyerle et al. (2000). Because the measured 3H concentrations did not vary significantly, only a subset of the samples was analyzed for 3H, and for the calculation of the 3H-3He water ages the 3H concentrations were interpolated. The water samples collected in copper tubes for CFC-11 and CFC-12 analysis were measured according to the method of Hofer and Imboden (1998). Because the samples were measured within 80 d of sampling, no corrections for anaerobic CFC-11 depletion in the copper tubes were necessary. The error in the concen-