Analysis of deep-water exchange in the Caspian Sea based on environmental tracers

In order to quantify deep-water exchange in the Caspian Sea, the world's la rgest inland water body, water samples were analyzed for the transient trac ers H-3, He-3, He-4, CFC-11, CFC-12 and atmospheric noble gases. Measuremen ts of temperature, salinity (calculated from conductivity for the ionic com position of Caspian Sea water), and dissolved oxygen were employed to inves tigate the processes responsible for deep-water renewal. The Caspian Sea co nsists of two deep basins, the southern and central basins, separated by a sill, and a shallow northern basin. The deep water (below 200 m) accounts f or almost 60% of the total water mass. Below 200 m the concentrations of 3H and 3He are much lower in the southern basin than at the same depths in th e central basin, but this is not the case for either of the CFCs. However, apparent water ages calculated from H-3-He-3 and from CFC-12 concentrations are the same for the deep water of the southern and central basins, and yi eld deep-water exchange rates of approximately 7% per year for each of the two basins. This implies volume fluxes across the 200-m level of about 2220 km(3) yr(-1) within the southern basin and 770 km(3) yr(-1) within the cen tral basin. Based on the apparent water ages, the oxygen depletion in the d eep water is estimated to be about 0.35 mg l(-1) yr(-1). The processes resp onsible for deep-water exchange have not yet been identified conclusively. However, vertical temperature and salinity gradients observed during two ex peditions, in September 1995 and 1996, suggest that within the southern and central basins large-scale convection cannot be triggered by seasonal cool ing alone, but requires the surface water to be cold/saline or to contain h igh suspended sediment loads. In the central basin the increase in salinity occurring during ice formation in early winter is possibly sufficient to c ause convection. In late summer, the horizontal transport of water from the upper 170 m of the central basin into the southern basin results in mixing down to 400 m. In winter this process might result in convection down to t he maximum depth. Alternatively, the data are also consistent with the hypo thesis that rare events cause large-scale convection down to the maximum de pth in the southern and in the central basin simultaneously, followed by sl ight mixing that mainly affects only the top 500 m. According to apparent w ater ages from below 500 m, the last such major mixing event could have occ urred in 1976 shortly before the water level of the Caspian Sea began to ri se. (C) 2000 Elsevier Science Ltd. All rights reserved.