Bottom convective layer

Keywords Black Sea Bottom convective layer Budget Hydrogen sulfide ... 

Hydrogen sulfide, H2S (J]H2S = [H2S] + [HS ] + [S2 ], where [HS ] represents ca. 80% at pH 7.5-7.65 in the Black Sea anoxic interior), is the key chemical compound that defines the direction and origin of many biogeo-chemical cycles in the anoxic zone of the Black Sea. The main goal of this review is to present the contemporary inventory of hydrogen sulfide and sulfur intermediate species, the results of recent physiochemical studies of the bottom convective layer, and to discuss the sulfur isotopic composition of dissolved sulfide and sulfate. This review concludes by presenting the sulfur budget of the Black Sea. 

The H2S vertical distribution is quasilinear above 500-600 m. Dissolved sulfide concentration increases gradually with depth and has an average vertical gradient of about 0.5 mmol m above 500 m, decreasing with depth (Table 1). The vertical sulfide gradient at the boundary between the entire anoxic water mass and the bottom convective layer (ca. 1700-1750 m) increases sharply and is only two times less than the vertical gradient in the upper 500 m. 

The existence of a homogeneous bottom water mass - bottom convective layer (BCL) - at water depths below 1740-1800 m was first reported, based on detailed CTD profiling, by Murray et al. [42] and since then it has been intensively studied [43-52], Based on the data obtained in 1999-2002 in the north-eastern Black Sea, the bottom water mass was characterized by the following parameters potential temperature 0 = 8.883-8.888 °C, salinity S = 22.330-22.334psu, and potential density oq= 17.233-17.236kgm 3 [51]. On average, the water column below 500 m is about 0.01 °C warmer and 0.003-0.005 psu saltier in the western part than in the eastern part of the sea. Recent detailed studies have shown that not only the thermohaline characteristics, but also concentrations of several chemical species in bottom waters... 

The existence of the Black Sea bottom convective layer (BCL) has important implications for the physical and chemical exchange at the sediment/water interface and at the interface between intermediate and bottom water masses. Two-fold increased vertical gradients of dissolved sulphide at the upper boundary of the BCL suggest the presence of the anoxic interface separating entire anoxic water mass dominated by turbulent diffusion from underlying waters of the BCL where double diffusion is the main mixing mechanism. 

However, in the spring of 2004, the entire column was nearly uniform in salinity at about 86 ppt (see Table 3), indicating that winter convection was likely to had been able to destroy the meromictic conditions, and the enhanced stratification was an intermittent rather than permanent feature. By the late summer of 2004, a new type of stratification had arisen, which persisted in aummns until the fall of 2006. This stratification was mainly three-layered, involving two salinity maxima, one at the surface, and the other one at the bottom, separated by a relatively fresh intermediate layer (Fig. 6). Despite the salinity inversion just below the upper mixed layer, the column was maintained in a stable state by a steep thermocline. The deeper salinity maximum was best developed in 2005, but a hint of it was also observed in the fall of 2006, when there was a marked bottom mixed layer, indicating the presence of strong currents in the bottom layer (which was also confirmed by direct current measurements). 

How can this convective cooling process be stopped, thus allowing a pool of water to be an effective heat sink The answer lies in adding common salt (NaCl). When sufficient salt is added to a pool of water 2 to 3 m deep, a salinity gradient is established. That is, the salt concentration is not uniform in the pool. In fact, three distinct layers can be identified. The top layer, called the convective layer, has a salt concentration of about 2% by mass and is only about half a meter thick. The bottom layer has a very high salt concentration (about 27% by mass) and serves as the heat storage layer. The middle, so-called nonconvective layer is about 1.5 m thick. Being intermediate in density between the top and bottom layers, the nonconvective layer is trapped between these layers and acts as an insulator. 

The central part of the RDE theory and technique is the convection of electrolyte solution. Due to the solution convection, the reactant in the solution will move together with the convection at the same transport rate. Let s first consider the situation where the flow of electrolyte solution from the bottom of the electrode edge upward with a direction parallel to the electrode surface to see how the diffusion— convection layer can be formed and what is its mathematic expression. 

As shown in Fig. 21, in this case, the entire system is composed of an open vessel with a flat bottom, containing a thin layer of liquid. Steady heat conduction from the flat bottom to the upper hquid/air interface is maintained by heating the bottom constantly. Then as the temperature of the heat plate is increased, after the critical temperature is passed, the liquid suddenly starts to move to form steady convection cells. Therefore in this case, the critical temperature is assumed to be a bifurcation point. The important point is the existence of the standard state defined by the nonzero heat flux without any fluctuations. Below the critical temperature, even though some disturbances cause the liquid to fluctuate, the fluctuations receive only small energy from the heat flux, so that they cannot develop, and continuously decay to zero. Above the critical temperature, on the other hand, the energy received by the fluctuations increases steeply, so that they grow with time this is the origin of the convection cell. From this example, it can be said that the pattern formation requires both a certain nonzero flux and complementary fluctuations of physical quantities. 

The El Paso Solar Pond was the first in the world to successfully use solar pond technology to store and supply heat for industrial processes. It was built with three main layers a top layer that contains little salt, a middle layer with a salt content that increases with depth, and a very salty bottom layer that stores the heat. Which layer has the greatest density The least density Why doesn t the storage layer in the El Paso Solar Pond cool by convection ... 

The shallow-water character of the sea provides rapid propagation of wind and convective mixing down to the bottom, which leads to equalizing the vertical temperature distribution in most cases, the temperature difference is less than 1 °C. Meanwhile, during summertime calm periods, the thermocline is formed which prevents the near-bottom layer from water exchange. 

Sorption by sediment and suspended solids Sedimentation and resuspension of solids Aerosol formation at the air-water interface Uptake and release by biota Transport within water bodies Turbulent dispersion and convection Diffusion between upper mixed layer and bottom layer Transformation Biodegradation Photochemical degradation... 

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