Carbonic acid weathering

Fig. 5.10. Conceptual model of carbonic acid weathering of ferromagnesian minerals on Mars. Reprinted from Marion et al. (2003a) with permission...

For example, if the 6 C values suggest that the dominant source of carbon is from carbonic acid weathering of silicates, the activities should be high reflecting the young age of the soil CO2 (Schiff et at, 1990). For more information, see Chapter 5.15. 

The efficiency of the weathering of rocks in using carbonic acid produced in the carbon cycle is affected by various hydrologic, environmental, and cultural controls. The fact that the principal anion in fresh surface water worldwide almost always is bicarbonate attests to the overriding importance of this process. Exceptions are systems in which evaporite minerals are available for dissolution by groundwater or where human activities are major sources of sulfate or chloride inflow. 

In all cases, water and carbonic acid, the latter of which is the source of protons, are the main reactants. The net result of the reaction is the release of cations (Ca " ), Mg ", K", Na" ) and the production of alkalinity via HCO. When ferrous iron is present in the lattice, as in biotite, oxygen consumption may become an important factor affecting the weathering mechanism and the rate of dissolution. 

Soil reaction (pH) The relationship between the environment and development of acid or alkaline conditions in soil has been discussed with respect to formation of soils from the parent rock materials. Soil acidity comes in part by the formation of carbonic acid from carbon dioxide of biological origin and water. Other acidic development may come from acid residues of weathering, shifts in mineral types, loss of alkaline or basic earth elements by leaching, formation of organic or inorganic acids by microbial activity, plant root secretions, and man-made pollution of the soil, especially by industrial wastes. 

Acidolysis is a similar weathering reaction to hydrolysis in that is used to weather minerals, but in this case the source of is not water but organic or inorganic acids. Humic and fulvic acids (discussed in Section 8.3.2), carbonic acid, nitric or sulfuric acid, and low-molecular-weight organic acids such as oxalic acid can all provide H to weather minerals. All of these acids occur naturally in soils in addition nitric and sulfuric acid can be added to soil by acid pollution. The organic acids are prevalent in the... 

The action of carbonic acid on limestone produces a calcium bicarbonate solution that is exceedingly soluble in water. (For comparison, at 20°C the solubility of calcium carbonate in water is only 0.0145 g per liter while the solubility of calcium bicarbonate is 166 g per literJ ) Magnesium ions from dolomite are also released into aqueous solution according to the same mechanism. The weathering of gypsum, calcium sulfate, also releases calcium ions into natural water supplies. 

The chemical weathering of crustal rock was discussed in Chapter 14 from the perspective of clay mineral formation. It was shown that acid attack of igneous silicates produces dissolved ions and a weathered solid residue, called a clay mineral. Examples of these weathering reactions were shown in Table 14.1 using CO2 + H2O as the acid (carbonic acid). Other minerals that undergo terrestrial weathering include the evaporites, biogenic carbonates, and sulfides. Their contributions to the major ion content of river water are shown in Table 21.1. 

The terrestrial weathering of organic matter derived from shales and soils results in the oxidation of carbon, which generates CO2. Dissolution of this CO2 in water produces carbonic acid. This weak acid serves to enhance chemical weathering reactions... 

Because sulfuric acid is a strong acid, it is a more potent weathering agent than carbonic acid. By serving as a source of H (aq), sulfuric acid is transformed into SO Caq). The oxidation of pyrite is responsible for 11% of the sulfete in river water, with pollution now contributing 54%. The latter is largely associated with the burning of sulfur-rich coal. 

As the rock cycle continues, the calcium silicate minerals are eventually uplifted onto land where they imdergo chemical weathering. This reaction involves acid hydrolysis driven by carbonic acid. The latter is derived from the dissolution of the magmatic CO2 in rainwater ... 

Weathering is the process by which rock is broken down into smaller and smaller particles. It involves both mechanical and chemical breakdowns. The mechanical breakdown into smaller and smaller pieces occurs as a result of exposure to freeze-thaw cycles and to the action of wind and water. Chemical breakdown occurs as a result of exposure to air and water and other chemicals that may be dissolved in water, such as acids. Weathering by exposure to atmosphere results in some of the carbon dioxide being removed from the atmosphere along with the broken-down rock and eventually washed into the ocean. 

Berthelot,1 in pointing to the nature of the reactions occurring under its influence, which are particularly similar to those of plants, advanced the following views In clear weather there exists between two strata of air only one metre apart a potential difference of 20-30 volts which, in rainy weather, can increase to about 500 volts. Reactions can already take place under the influence of such tensions thus at 7 volts a fixation of nitrogen by carbohydrates can already occur the decomposition of carbonic acid requires higher tensions. 

The four basic processes involved in chemical weathering are (i) formation of carbonic acid from water and carbon dioxide, (ii) ionization, (iii) hydrolysis and (iv) oxidation. 

Acid Hydrolysis. The water that enters soil as rain or snow is in equilibrium with CO2 in the atmosphere, which dissolves to form carbonic acid. Unpolluted rainwater has a pH of approximately 5.7, whereas water in soil pores may be exposed to air containing a higher partial pressure of CO2 than the free atmosphere, and hence soil water may be more acidic (see Section 5.4). It is the attack on soil minerals by this weak carbonic acid that is the major chemical weathering process in most soils. For example, acid hydrolysis of calcium carbonate yields calcium and bicarbonate ions ... 

The present chapter has described the presence of EMF where soil minerals occur and their formation of intimate contact with soil minerals. It seems clear that EMF can dissolve at least some of these minerals when exposed to them in laboratory conditions. Furthermore, EMF obtain large amounts of carbon from their host trees and convert much of it to organic and carbonic acids, making them potentially important in dissolving minerals in natural ecosystems. The main questions for future research are to quantify the contribution of EMF to total weathering of minerals in the soil and to determine whether this contribution will change as a result of more intensively managed forest ecosystems and increased... 

The appearance of acid solutions in the weathered layer cannot be substantiated physicochemically even if a high content of carbon dioxide in the atmosphere is assumed with =1 bar the pH of the saturated aqueous solution (pure water with no impurities of other anions and cations) is only 3.9—a value clearly insufficient for intensive removal of ferric iron in the form of stable Fe FeOH, or Fe(OH)2. Moreover, when pure (rain) carbonic acid waters react with rocks the pH value increases as the saturation with dissolved cations (mainly Mg, Na" ", K, and Ca ) increases. The constant presence of other volatiles of the acid fume group in the Precambrian atmosphere is not very likely, due to their high solubility... 

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