Dynamics of the Caspian Sea: Preliminary results of isotope studies

The water level of the Caspian Sea, the world's largest inland water body, rose by approximately 2 m within the last 20 years. To explore the potential of isotope techniques for studying the water balance and dynamics of the Caspian Sea and aspects related to the recent sea level rise, two research-training cruises were carried out in September 1995 and 1996 within the framework of an IAEA technical co-operation project. The paper presents a preliminary evaluation of the results obtained so far. By correlating sea level fluctuations and freshwater inflow variations observed in previous studies, it has been found that changes of the sea level are caused by changes in the water budget, mainly in the river input. The water of the Caspian Sea is enriched in oxygen-18 with respect to the total inflow (rivers plus precipitation) by about 9 per mille. The vertical distribution of δ18O observed in the southern and central basins of the Caspian Sea indicates that the main water body of the sea (between 200 m and 800 m depth) is enriched in oxygen-18 with respect to the surface layer by approximately 0.3 per mille. This enrichment suggests that water at depth most probably represents the previous low level stand of the Caspian Sea (1950-1977) and that the total inflow exceeded the evaporation by about 10% during the recent sea level rise. Vertical temperature and salinity gradients observed during the 1995 and 1996 expeditions suggest that deep water exchange in the central and southern basins of the Caspian Sea cannot be triggered by cooling of surface water alone but also requires an increase of salinity. The presence of bomb tritium and tritiogenic helium-3 in the deep water of the central and southern basins points to rather intense vertical water exchange in both basins. The water ages calculated from tritium and helium-3 increase from shallow to deep water in both basins; near the bottom, a maximum model age of some 20 to 25 years was. (author) obtained