Processes of deep-water renewal in Lake Baikal

Deep-water renewal in Lake Baikal (Siberia), the world’s deepest lake and largest lake by volume, is relatively fast. Water age calculated from tritium and helium as well as from chlorofluorocarbons does not exceed 19 yr. Relative saturation of dissolved oxygen typically exceeds 80%. The equation of state of Baikal water was determined including the effect-of dissolved ions and silicic acid. Based on nearly 600 CTD profiles taken between 1993 and 1995, two mechanisms of deep-water mixing were identified. (1) In spring, cold and relatively saline water from the Selenga, the major inflow to the lake, forms a density plume that reaches the bottom of the central basin during April and early May. Due to entrainment of lake water the plume transports about 125 km? of water per year to the deepest part of the basin. Later in spring, the river water forms the thermal bar observed along the eastern shore, There are indications that parts of the Selenga are also plunging to the deep part of the southern basin. (2) At Academician Ridge, separating the cold and “saline” water of the central basin from the warmer and slightly less saline water of the northern basin, horizontal mixing results in a water mass that can sink on either side of the sill. Whereas in the central basin the water mass stays at intcrmediatc depth, in the northern basin it sinks to the decpcst part. More detailed data are needed to quantify this density flux. No indication of a wind-induced thermobaric instability was found. Lake Baikal (Siberia) is the deepest (1632 m) and largest lake (volume 23,015 km”) on earth (Shimaraev et al. 1994). It holds -20% of the global fresh liquid surface water. The lake is located in the great Baikal Rift Zone of eastern Siberia. It is divided by underwater sills into three main basins (Fig. 1): the southern basin (max. depth 1,432 m), the central basin (1,632 m), and the northern basin (897 m). Main inflows are the Selenga, the Upper Angara, and the Barguzin. The only outlet is the Angara. Despite its great depth, relative saturation of dissolved oxygen exceeds 80% in the entire water column. Based on the vertical distribution of chlorofluorocarbons (CFCs), Weiss et al. (1991) have shown that the water age is nowhere greater than 16 yr. Water age is defined here as the time since a given water parcel was last exposed at the water surface. Both observations raise the question of how the bottom water is renewed in such a deep lake. In most lakes, partial or complete turnover of the water Acknowledgments We thank our colleagues of the Limnological Institute of the Siberian Division of the Russian Academy of Sciences in Irkutsk, especially M. Grachev, and the Baikal International Center of Ecological Research (BICER) for providing ship time and support. Special thanks go to Michael Schurter for his reliable technical and experimental work, to T. Khodzher and Laura Sigg for providing chemical data, and to David Licwellyn-Jones for providing satellite images. We also thank N. Granin, A. Zhdanov, and the crew of the RV Vereshchugin. David Livingstone helped to improve the language of the final text. This research was made possible by the financial support of the Swiss Federal Institute of Environmental Science and Technology (EAWAG), the Swiss Federal Institute of Technology (ETH), and the Swiss Federal Office for Science and Education (BB W). column, if it occurs, is caused by the seasonal cooling and heating of the water, often supported by wind forcing. Yet, this mechanism is commonly restricted to the upper few hundred meters. In deeper water bodies, horizontal density gradients are necessary to induce vertical exchange down to the bottom. Often such gradients are produced by a combination of temperature and salinity gradients. The formation of deep water in the North Atlantic is one of the most prominent examples of this (Dickson et al. 1990). In lakes such gradients are often caused by inflows with differing salt concentrations (e.g. Aeschbach-Hertig et al. 1996). In deep freshwater lakes in which salinity gradients are small, the nonlinearity of the equation of state of water gives rise to two special phenomena. (1) The thermobaric instability is linked to the pressure dependence of the temperature of maximum density (Tmd). It has been proposed by Weiss et al. (1991) as the key process for deep-water formation in Lake Baikal during winter, when the surface water is colder than the deep water. (2) Cabbeling (McDougall 1987b) results from the nonlinear temperature dependence of water density, as a result of which the mixing of water masses of different temperature always leads to an increase in the mean density. The best known example is the so-called thermal bar, where two adjacent surface water masses, one warmer and the other colder than 4”C, mix to form water of maximum density that then sinks to greater depth (Imboden and Wtiest 1995). Shimaraev et al. (1993) interpreted the thermal bar that develops in spring along most of the shoreline of Lake Baikal as the main trigger of deep-water formation. The authors suggested that the thermobaric instability is initiated by the spring thermal bar at the southeastern shore of the central basin. Deep-water renewal is then accomplished by a jet of