Carbonates weathering

The carbonate weathered mineral is calcium/magnesium and the reaction consumes one CO2 from the atmosphere and liberates one CO2 coming from the matrix. The main weathering equation is ... 

Thus, half of the HC03 released by carbonate weathering originates from atmospheric C02. 

The estimation of the carbonate and evaporite weathering rate requires the quantification of the elements originating from the two components. Roy et al. [10] calculated the carbonate weathering rate (TDS originating from carbonate dissolution) as follows ... 

Fig. 9 TDS derived from carbonate weathering (TDScarb) and from evaporite (gypsum) dissolution ("l DSevaplGY,.,) versus total TDS for all the monitoring stations. All expressed in mg L 1...

Tortosa, where dissolved exportation derived from gypsum weathering is about 1.5 times derived from carbonate weathering. 

To highlight one example of a surface process of global significance, we repeat here (Chapter 5.1) a brief account on carbonate weathering and CaC03 precipitation ... 

To begin the discussion, we will present briefly a view of the modern carbon cycle, with emphasis on processes, fluxes, reservoirs, and the "CO2 problem". In Chapter 4 we introduced this "problem" here it is developed further. We will then investigate the rock cycle and the sedimentary cycles of those elements most intimately involved with carbon. Weathering processes and source minerals, basalt-seawater reactions, and present-day sinks and oceanic balances of Ca, Mg, and C will be emphasized. The modern cycles of organic carbon, phosphorus, nitrogen, sulfur, and strontium are presented, and in Chapter 10 linked to those of Ca, Mg, and inorganic C. In conclusion in Chapter 10, aspects of the historical geochemistry of the carbon cycle are discussed, and tied to the evolution of Earth s surface environment. 

Ritsema, C. J. and J, E. Groennenberg. 1993. Pyrite oxidation, carbonate weathering, and gypsum formation in a drained potential acid sulfate soil. Soil Sci. Am. J. 57 968-976. 

The calcium concentration of Lake Mary (a lake in New Hampshire) is 4 x 10-4 M. Estimate the pH of this lake. Assume that calcium inputs to this lake are exclusively from calcium carbonate weathering. 

The cations and bicarbonate released by calcium and magnesium silicate and carbonate weathering are carried rapidly by rivers to the oceans where they are removed as Ca-Mg carbonates ... 

In the dissolved load, the preferential dissolution of the different types of lithology or mineral can cause large variations in trace-element abundances in rivers. A classical tracer of rock dissolution is strontium. The isotopic ratio of strontium in river waters is, to a first approximation, explained by mixing between different sources. Rain, evaporite dissolution, and carbonate and silicate weathering are the predominant sources of strontium in rivers (see Chapter 5.12). Examples of the use of Ca/Sr, Na/Sr, and Sr isotopic ratios to quantify the proportion of strontium derived from carbonate weathering in large basins can be found in Negrel et al. (1993)... 

The groundwaters are also predominantly oxidizing with low dissolved iron and manganese concentrations. There is no indication of reductive dissolution of iron oxides or of pyrite oxidation. Under the arid conditions, silicate and carbonate weathering reactions are pronounced and the groundwaters often have high pH values. Smedley et al. (2002) found pH values typically in the range of 7.0-8.7. 

Magnesium and, under reducing conditions, iron carbonates can form by analogous reactions. The continental part of this cycle has been extensively studied (e.g. Walker 1977 Holland 1978, 1984 Franck ei al. 1999). Weathering of rocks on land dissolves carbonates and releases Ca + from silicates. Both the dissolved carbonate and calcium are carried by rivers to the oceans, where they react to form CaC03. Reaction (1) is reversed when carbonates are metamorphosed deep in the crust. Carbonate weathering,... 

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