| CTD data from surface to the bottom are used to reconstruct a specific thermohaline structure of near-bottom water masses in deep enclosed depressions within Sierra Leone rift valley (of about 100 miles long). Due to topography features near-bottom water masses of three depressions at depths from 3400-3600 m to 5000 m are insulated both from each other and from the ocean, and any interconnection between them at present time is excluded. All cast profiles reveal existence of quasi-homogeneous bottom layer up to 1000-1200 m thick with near-adiabatic temperature distribution and neglected density gradients that show conditions of absence of noticeable vertical instability.The upper boundary of the bottom layer is clearly marked by sharp change of the thermohaline structure, which is associated with a transient layer 200-250 m thick where the bottom water mixes with deep water masses of NADW. The layer is characterized by high gradients and structural nonuniformity (existence of inversions and step-like structures several meters to several tens of meters thick) on profiles of both potential temperature and salinity. Diapycnal type of mixing in the layer follows from the presence of analog structures of density profiles. The most prominent mixing layer, similar to a benthic pycnocline, with developed thermohaline structure and changes in potential temperature with gradients up to 0.1 deg C/m is associated with one of the depressions. The mean estimate of density ratio R ~ 2 that does not exclude conditions of instability to double diffusion.Specific vertical stratification and significant horizontal changes in temperature of the mixing layer along with higher mean temperature and salinity of the uniform bottom layer of this depression, as compared with the two others, are associated also with some geological evidence of active ore formation here and provide indications of hydrothermal activity on the bottom. |
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