A simple evolutionary model for water and salt in the Black Sea

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Abstract
This paper presents a time‐dependent variable‐size box model for the total fluid and salinity balances of the Black Sea. In addition to the advective flows due to riverine and Mediterranean inputs of water, the model accounts for entrainment flows which are parameterized using the theory of Ellison and Turner (1959) and employs a modified version of the Gargett (1984) formulation for eddy diffusivity in describing cross‐pycnocline mixing flows. The variable volume of the boxes permits explicit simulation of the replacement of the preexisting late Pleistocene lake water by the inflow of denser Mediterranean water. Our results indicate that the time needed to displace the old deep water with new saltier water is 2000 years or less. Because of the effects of mixing, the salinity of both the surface and deep waters requires a greater time interval to obtain present‐day values, i.e., ∼4000 years if the inflow of Mediterranean water is constant in time at its present value and ∼6000 years if this input changes with time in approximation to the rate of sea level rise. The occurrence of the marine coccolithophorid Emiliania huxleyi in the Holocene sapropel layer requires a minimum surface salinity of 11‰, which takes some 1600 to 3100 years to develop. This range is a result of the uncertainty in the history of the Mediterranean water input. The model is also employed to investigate the freshening of the Black Sea after cutoff of Mediterranean input. The salinity of both surface and deep waters is found to fall below 11‰ in less than 3000 years, and the freshening process is complete in ∼9000 years.