Weathering of carbonates

Dissolved carbonate in fresh waters may exhibit an extremely variable isotopic composition, because it represents varying mixtures of carbonate species derived from weathering of carbonates and that originating from biogenic sources like freshwater plankton or CO2 from bacterial oxidation of organic matter in the water column or in soils (Hitchon and Krouse 1972 LonginelU and Edmond 1983 PaweUek and Veizer 1994 Cameron et al. 1995). 

This process consumes oxygen and produces C02. As a result, the oxygen content of air in soil may be as low as 15%, and the carbon dioxide content may be several percent. Thus, the decay of organic matter in soil increases the equilibrium level of dissolved C02 in groundwater. This lowers the pH and contributes to weathering of carbonate minerals, particularly calcium carbonate. 

Skidmore M. L., Sharp M. J., and Tranter M. Fractionation of carbon isotopes during the weathering of carbonates in glaciated catchments I. Kinetic effects during the initial phases of dissolution in laboratory experiments. Geochim. Cosmochim. Acta (in press). 

The mean concentrations of constituents of seawater are determined not by simple distillation of river water but by their various mechanisms of removal from the ocean. The dominant cation in river water, e.g., is calcium from weathering of carbonate and silicate rocks, whereas the dominant cation in the ocean is sodium, because there are no efficient loss mechanisms for sodium analogous to the formation of CaC03 as a loss... 

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 ... 

The Wiggly vertical line indicates particulate C and DOC transport from the ocean euphotic zone to deep water. Symbols W, weathering of carbonates (CaCOj-hCOj + HjO ... 

The solubility and rate of dissolution of mo.st minerals is strongly pH-dependent. Weathering of carbonate, silicate, and alumino-silicate minerals consumes protons and releases metal cations. (See Chaps. 7 and 9.)... 

An important consequence of acid rain and dry deposition of SO2 and HNO3 is the rapid weathering and destruction of exposed carbonate stone buildings and monuments (cf, Baedecker et al. 1990). Lipfert (1989) has proposed a theoretical damage function equation to describe the weathering of carbonate stone between pH 3 and 5, which is... 

In the weathering of carbonates, 1 mole of rock CO2 is mobilized for each mole of atmospheric CO2... 

The major sink for atmospheric C02 involves the subaerial weathering of carbonates and silicates in sedimentary rocks. Physical weathering (e.g. freeze-thaw and tectonic action) increases the surface area available for chemical weathering by the carbonic acid formed upon dissolution of C02 in rain water ... 

While the weathering of carbonate rocks does not affect long-term atmospheric CO2 concentration because of carbonate burial in the ocean (which releases CO2), the process is different when silicate rocks are weathered to provide ions for marine carbonate formation (e.g. Berner 1990), because there is a net consumption of CO2 ... 

In this reaction, there is a net transfer of Ca " " (or Mg + in the case of magnesium carbonates) from the continental crust to the ocean, a creation of two equivalents of alkalinity and a transfer to the river system of two moles of carbon per mole of Ca " ", one coming from the crust, the other one from the atmosphere. Once the weathering products reach the ocean, they will increase the saturation state of surface waters with respect to calcite and induce rapidly (within 10 years) the biologically driven precipitation of one mole of CaC03 followed by its deposition on the seafloor. The precipitation-deposition reaction is the reverse of reaction [1]. The net carbon budget of the weathering of carbonate minerals followed by deposition of sedimentary carbonate is thus equal to zero. 

Moreover, this calculated inorganic carbon concentration does not incorporate the fact that carbon is continuously supplied into the atmosphere and oceans by degassing from metamorphism and magmatism and by the weathering of carbonate minerals and organic carbon, and is continuously consumed by the production of carbonate and organic carbon sediments (Fig. 2.9). Flence, the total DIG load of the ocean can be expected to vary over time. Finally, the uptake of atmospheric GO2 by the sea surface is a dynamic process that depends on oceanic and wind circulation interlinked. Observations suggest that the Southern Ocean sink of GO2 has weakened between 1981 and 2004 by 0.08 10 g decade" relative to the... 

Inorganic C may exist in natural water systems in a number of different chemical forms. These are carbonate (COf"), hydrogencarbonate (HCOi ), and dissolved CO2 (free and hydrated form). In natural waters, the inorganic forms of carbon are derived from the chemical weathering of carbonate rocks, the decomposition of biotic material (respiration), and solvation of atmospheric CO2. The dissolved inorganic C species distribution is a function of pH and is described by equations given below (i.e., the carbonate system) ... 

The weathering of carbonate rocks and the precipitation and resolution of CaCO3 in natural waters are problems of intense interest. It is estimated that over 99 % of the earth s carbon is present in CaCO3 minerals limestone, calcite, aragonite, vaterite, marble, etc. To the equilibria of the previous problem, we now add... 

Reactions (5.38)-(5.41) indicate that CO2 removes from the atmosphere to the hydrosphere by weathering of carbonate and silicate dissolution. 

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