Convective layer

If an external body is engulfed, it can enrich the star with the original interstellar medium abundances of 6Li, 7Li, 9Be and 10,11B (written here in increasing order of hardness to be destroyed by thermonuclear reactions). This mechanism is then supposed to produce stellar enrichment of these elements up to the maximum meteoritic value. Also, the engulfing star will suffer a rotational increase due to the gain of the planet momentum and a thermal expansion phenomenon due to the penetration of the body provoking mass loss phenomena (Siess Livio 1999). An extension to this scenario has been proposed by Denissenkov Weiss (2000) in order to explain supermeteoritic Li abundance values, via a combination of stellar rotation and activation of the 7Be mechanism at the base of the convective layer produced by the penetration of the external body. 

Figure 2.20 summarizes the role of inversions and the boundary layer in terms of typical changes in mixing of the atmosphere close to the earth s surface at various times of the day (Stull, 1988). At midday, there is generally a reasonably well-mixed convective layer... 

The mode of the mantle convection, layered or mantle-wide, is one of the most fundamental problems in current earth science. As we discussed earlier, 4He-heat systematics appears to suggest that the lower mantle (apart from the exact locale) is essentially isolated from the upper mantle by a barrier that impedes He migration between the layers. Other noble gas characteristics, for example much higher 40Ar/36Ar and 129Xe/130Xe in the upper mantle than in the lower mantle, also appear... 

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. 

Szewczyk, A.A.. "Stability and Transition of the Free Convection Layer Along a Vertical Flat Plate", lnt. J. Heat Mass Transfer, Vol. 5, pp. 903-914, 1962. 

Fig. 7-11 Schematic diagram and flow regimes for the vertical convection layer, according to Ref. 18.

Solomatov V. S. and Stevenson D. J. (1993) Suspension in convecting layers and style of differentiation of a terrestrial magma ocean. J. Geophys. Res. 98, 5375—5390. 

In the derivation of Eq. (152), we neglected the quantity of the species produced or consumed at the electrode. In practice this is true for microscale electrochemical experiments but is necessarily inexact for macroscale or exhaustive electrolysis. To derive the general equation, let us consider the total equation for the bulk solution. The chemical contribution is given in Eq. (152). On the other hand, a flux of the species at the convection layer limit (5conv must be considered. This flux corresponds to an algebraic number dNgiec of moles of the species produced, given in Eq. (154),... 

Compare Eq. (187).] For a bulk solution containing only R, the boundary condition at the extremity of the convection layer is then... 

As explained earlier, in transient electrochemical methods an electrical perturbation (potential, current, charge, and so on) is imposed at the working electrode during a time period 0 (usually less than 10 s) short enough for the diffusion layer 8 (2D0) to be smaller than the convection layer (S onv imposed by natural convection. Thus the electrochemical response of the system investigated depends on the exact perturbation as well as on the elapsed time. This duality is apparent when one considers a double-pulse potentiostatic perturbation applied to the electrode as in the double-step chronoampero-metric method. 

One of the most fundamental, contemporary debates about the nature of the Earth s mantle centers on the subject of mantle convection. There are two conflicting views which are commonly described as "layered convection" and "whole-mantle convection" (see Fig. 3.17). The layered convection model is championed by geochemists who prefer to see the mantle as two separate convecting layers. In this model the upper and lower mantle are geo-chemically isolated from each other and convect separately. Whole mantle convection is advocated by geophysicists, who that believe there is evidence for a significant exchange of mass between the upper and lower mantle. 

Nnccio PM, Paonita A (2000) Investigation of the noble gas solubility in HjO-COj bearing silicate liquids at moderate pressure II the extended ionic porosity (EIP) model. EarthPlanet Sci Lett 183 499-512 O Nions RK (1987) Relationships between chemical and convective layering in the Earth. J Geol Soc Lond 144 259-274... 

The lower boundary layer model avoids some of the geophysical problems of larger convective layers, but must resolve how such a layer is initially stabilized with gas-rich material. Also, it must be combined with other mechanisms to account for the mantle evolution of all the noble gases. However, the model illustrates how a deep boundary may be important. 

O Nions RK (1987) Relationships between chemical and convective layering in the earth. J Geol Soc Lond 144 259-274... 

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