Barriers geochemical

In the process of the global cycle, mobile carbon repeatedly crosses the Earth s surface. Over the surface, playing the role of the geochemical barrier , it circulates mainly in the oxidized form (C02), and under the surface mainly in the restored form (CH4). This fact requires a study of the global cycles of C02 and CEI4. 

Thus the geochemical barrier which causes nearly total precipitation of ferric oxide from water is a change in the reaction of the environment from highly acid (pH = 0-2) to moderately acid (pH = 2-4). In the case where equilibrium is reached the amount of iron hydroxide precipitated from a saturated solution is inversely proportional to the pH, the dependence being nonlinear and the main mass of sediment being deposited from highly acid solutions. 

Thus in highly reducing conditions iron can migrate in a wide pH range (from 0 to 6) and precipitates as sediment in the form of oxides and hydroxides only in neutral environments. The acidity of the environment, as a natural geochemical barrier governing the precipitation of iron, is appreciably reduced. Variation in the redox potential as a result of the overall evolution of the atmosphere, hydrosphere, and biosphere plays a large role. 

The gradient of the redox potential is the only geochemical barrier causing deposition of iron when pH is constant. 

Thus from the thermodynamic data it follows that if acid or neutral silica-bearing solutions arrived in the depositional basin, the action of the presumed geochemical barriers (gradients of pH, Eh, concentration) could not operate in isothermal conditions of chemogenic deposition of silica, to form cherty or cherty-iron sediments. Only a change in concentration due to evaporation of substantial volumes of water in closed basins could have led to deposition of silica. An easy calculation shows that to deposit chert bands 0.3-0.5 cm thick in that way, a 100-m water layer has to be evaporated. Thus the formation of thick piles of Precambrian iron formations would have required the evaporation of a fantastic amount of water from restricted... 

The pH gradient can be an important geochemical barrier controlling the deposition of silica from colloidal solutions. Such deposition would take... 

The geochemical barrier which causes rapid deposition of colloidal iron... 

After passing near-shore geochemical barriers the waters contained hardly any colloidal silica and in the deep parts of the basins the intensity of accumulation of chemogenic cherty sediments would have fallen off sharply. 

In a study of the Waquoit Bay subterranean estuary, a large accumulation of iron (hydr)oxide-coated sediments within the fresh-saline interface was encountered. These iron-oxide-rich sands could act as a geochemical barrier by retaining and accumulating certain dissolved chemical species carried to the subterranean estuary by groundwater and/or coastal seawater. Significant accumulation of phosphorus in the iron oxide zones of the Waquoit cores exemplifies this process (Figure 8). 

A.I. Perelman in 1961 noted the presence in nature of local zones with most intense elimination, which he proposed to call geochemical barriers. The efficiency of such barriers is defined by the ratio of the rock or sediment volume, wherein elimination occurs, and the water volume passed through the volume and lost part of its composition. Geochemical barriers are peculiar natural sieves on the way of ground water flow, which do not let through themselves some components of water composition and thereby facilitate their accumulation. They can exist for a long time, thus facilitating the accumulation of some elements or their compounds in rocks or sediments. That is how ores of many economic minerals form. 

A mandatory condition for the formation of a geochemical barrier in the hydrolithosphere is extended ground water filtration and abrupt change of the environment in its way. As a rule, elimination of elements in such barriers is caused by a whole plethora of aggregate physicochemical processes. Nevertheless, these processes, as a rule, are dominated by one (changes in Eh, pH, temperature, pressure, adsorption ability of the sediments, etc.), which determines the nature of a barrier (Table 3.18). 

Figure 3.35 Schematic of the geochemical barrier and sulphur deposit Shor-su formation with the participation of microorganisms. 1- oil 2 water 3 - sulphur ore. (Kuznetsov S.l. at al., 1962 Perelman A. 1., 1972).

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