Bicarbonate from weathering

The rate of global production of bicarbonate by weathering can be determined because we loiow approximately the flow of HCO J in the world s major rivers. This represents a drain of the CO2 of the atmosphere, which must be balanced by resupply to maintain a steady One is tempted to estimate the vulnerability of atmospheric/co to the imbalance between the atmospheric CO2 drain by weathering and resupply by CaCOs precipitation by focusing on the CO2 fluxes from and to the atmosphere. This, however, would be incorrect because the ocean and atmosphere carbon reservoirs are approximately in chemical equilibrium on time scales greater than the circulation of the ocean (see Chapter 11). In order to emphasize the severity of the HCOs imbalance estimated in Fig. 2.4 one should focus on the fluxes of DlC and alkalinity between the land and ocean. Because we have not yet discussed alkalinity and DIC relationships (Chapter 4) a simple approximation can be made by considering the fluxes of bicarbonate and calcium. 

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. 

Therefore, as an oxide mineral "weathers," a bicarbonate may result from the reactions of metal oxides with water and C02. Eventually, the presence of OH- can cause part of the metal-containing mineral to be converted into a metal hydroxide, and most metal oxides react with water to produce hydroxides. For example,... 

Calculating the bicarbonate concentrations with (3), it is possible to plot the measured average pH values of Table 2 against the calculated loglHCOs ]. The result is shown in Fig. 5a. For pH >5.5 a linear relationship, very close to that reported in the literature, can be observed (pH = loglHCOa ] + 11.2). However, for acid lakes the calculated bicarbonate concentrations seem to be too low. It is reported that at pH<6 the release of metals from soils or sediments as a consequence of weathering processes becomes more and more important. Consequently aluminium hydroxides can influence alkalinity. In [18] the equation for calculating alkalinity was modified as follows ... 

Figure 19 Arrhenius relationship between temperature and rates for basalt weathering (a) rates based on porosity increases in plagioclase from Hawaiian basalt flows (Dom and Brady, 1995) and (b) rates based on solute bicarbonate fluxes from a global distribution of watersheds underlain by basalt (Dessert et al., 2001).

Table 3 Carbon storage and carbon loss from forests as bicarbonate due to silicate weathering (in g C m yr ).

The most abundant anion delivered by rivers to the oceans is bicarbonate ion (HCO ), and most of the bicarbonate alkalinity in rivers comes from the weathering of carbonate rocks (Meybeck, 1987). The chemical weathering of limestones and dolostones by dissolved CO2 can be represented by the reactions for dissolution of calcite and dolomite ... 

Seawater has nearly equal amounts of alkalinity and DIG because the main source of these properties is riverine bicarbonate ion, which makes equal contributions to both constituents. The processes of CaCOs precipitation, hydrothermal circulation, and reverse weathering in sediments remove alkalinity and DIG from seawater and maintain present concentrations at about 2 mmol (meq) kg . Reconciling the balance between river inflow and alkalinity removal from the ocean is not well rmderstood, and is discussed in much greater detail in Ghapter 2. 

Calcium at 4.8% is the fifth most common elemental constituent of the earth s crust after oxygen, silicon, aluminum, and iron. It is so popular in practical applications because it is found in rocks and minerals which have veiy high concentration of calcium carbonate. Calcium carbonate is the most common deposit formed in sedimentaiy rocks. The process of formation of calcium deposits begins with weathering of land surface due to the changes in heat, frost, rain, and the effect of sun. Calcium carbonate is not readily soluble in water but calcium bicarbonate is. The concentration of carbon dioxide in water is thus important for calcium carbonate transportation from the land to the sea since rain water is the carrier. It is estimated that 500,000,000 tons of minerals are carried by rivers to the seas eveiy year out of which about 10-15% of sedimentaiy rocks containing calcium carbonate are formed. 

In potable waters below about pH 8.3, bicarbonate is usually the only significant base. Bicarbonate alkalinity in water derives from two sources (1) the weathering of silicate and carbonate minerals by carbonic acid, for example... 

Weathering of continental rocks increases their TDS and concentrations of calcium and bicarbonate relative to sodium and chloride. The composition of streams so affected plot to the left of both diagrams and include the Columbia, Mississippi, Yukon, and Thames rivers. Evaportranspiration from arid climate drainage basins and streams such as the Colorado, Pecos, and Jordan rivers, which receive soil runoff and irrigation return waters, further increase the Na and TDS content of streams. Concomitant precipitation of CaC03 further shifts the prevalent chemical character of such streams back toward NaCl and the chemistry of seawater. 

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