Carbonate precipitation vadose zone

Both authors calculations also indicated that it is possible for solutions of reasonable compositions for natural waters to produce mixtures of freshwater and seawater that were undersaturated with respect to calcite but supersaturated with respect to dolomite. This observation is a cornerstone for some dolomitization models that are discussed later in this chapter. It is also important to note that the extent of undersaturation which results from mixing is strongly dependent on the initial Pco2 °f the dilute water when it is in equilibrium with calcite. Waters high in CO2 can cause more extensive dissolution. If these waters enter a vadose zone where CO2 can be degassed, they will become supersaturated and calcium carbonate can precipitate. This process provides an excellent mechanism for cementation near the water table. Because the water table can oscillate vertically, a considerable zone of cementation can result. 

Carbonate precipitation inthe vadose zone of hot arid to semi-arid regions is enhanced by a decrease in Pqo and / h,o due to increasing temperature and evaporation. Conversely, carbonate leaching is enhanced by a humid climate, which prevents the evaporative concentration of dissolved Ca " " and Mg " ". Loss of water through uptake by plants was argued by Klappa (1980) to be a likely mechanism for the precipitation of carbonates around roots. Carbonate precipitation around roots (rhizocre-tions) may also be enhanced by microbial activities (Krumbein, 1968) and an increase in alkalinity due to the decay of dead plants. 

Carbonate cements are often among the dominant components of diagenesis and hence are of decisive importance in determining the reservoir quality of sandstone sequences. Despite this, the timing, the geochemical conditions of precipitation and dissolution, as well as the source and fate of these cements are not fully understood. In continental and near-shore sediments, cements commonly precipitate as calcretes and dolocretes in the vadose and phreatic zones, and attain a variety of mineral-ogical, textural and distribution patterns as well as elemental and isotopic compositions. These cements form lenses and layers of densely cemented... 

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