Lake Baikal (Siberia) is the deepest (1630 m) and most voluminous (23000 km3) lake on earth. It holds about one fifth of the global inventory of unfrozen fresh surface water. The lake is located in the great Baikal Rift zone of east Siberia. It is divided by underwater sills into three main basins: the southern basin (max. depth 1420 m), the middle basin (1630 m) and the northern basin (900 m). Main inflows are the Selenga, the Turka and the Upper Angara Rivers. The only outlet is the Angara.
Lake Baikal is the oldest of all existing lakes. The Baikal tectonic depression is thought to have formed as early as the Oligocene (Golubev et al., 1993). Basin sediments, which are several kilometres deep, record more than 15 Myr of the lake's history (Members, 1992). During this time, a rich endemic flora and fauna have evolved.
Despite the great depth of the lake, oxygen concentrations are large throughout the water column. Bottom-water oxygen concentrations averag over 80 % of saturation (Weiss et al., 1991).
The high oxygen content implies a low oxygen consumption rate and efficient deep water ventilation. The analysis of tritium-helium-3 (T-3He) water ages provides a powerful tool for determining the rate of deep water renewal.
Light noble gases (He and Ne) and T were measured using static mass spectrometer techniques. Water ages were calculated from T- and tritiogenic 3He concentrations (Torgersen et al., 1977).
The reported results summarise the analysis of 290 water samples collected during four expeditions between 1992 and 1995.
In all three basins, mean water ages generally increase from values close to zero at the surface to maximum values at intermediate depth. Maximum water ages of approximately 11 years are reached at about 1100 m in the southern basin and at about 1400 m in the middle basin. In the northern basin, the maximum water age of 8-9 years is reached at 800 m. In the lowermost 100 to 200 m, water ages decrease significantly.
Excess 4He due to degassing of crustal He is present in the bottom water of all basins.
Ne concentrations, which are extremely constant, correspond to a water temperature at the time of deep water formation of approximately 3.5 °C (Mathieu, 1995).
- The T-3He water ages are similar to the values determined by Weiss et al. (1991) from CFC12 concentrations.
- The annual rate of deep water renewal determined from the mean water ages is 10 - 15 % yr-1 in the southern and middle basins and about 20 % yr-1 in the northern basin.
- The decrease in water age in the bottom water can only be explained by the advective flow of surface water to the very bottom.
- According to the Ne concentrations, deep water renewal takes place in late fall or early spring when the surface water is colder than 3.5 °C.
Golubev, V.A., Klerkx, J. & Kipfer, R., Bull. Centres Rech. Explor.-Prod. Elf Aquitaine 17, 53-65 (1993).
Mathieu, A., Tritium und Helium als Tracer für die Tiefenwassererneuerung im Baikalsee (Diplomarbeit, Abteilung für Umweltnaturwissenschaften, ETH, 1995).
Members, L.B.P.P., EOS Trans. AGU 73, 457-462 (1992).
Torgersen, T., Top, Z., Clarke, W.B., Jenkins, W.J. & Broecker, W.S., Limnol. Oceanogr. 22, 181-193 (1977).
Weiss, R.F., Carmack, E.C. & Koropalov, V.M., Nature 349, 665-669 (1991).