Carbonates vadose

Organic acid fluorescence. In a similar manner to trace constituents, such as Mg, Sr and P, concentrations of organic acids present in speleothem calcite are sufficient to observe variation at temporal scales of less than annual in some cases (e.g.. Baker et al. 1993, Shopov et al. 1994). Organic acids (humic and fulvic) are formed in the soil by humification, and transported to the cave void by percolating waters where they are entrapped in precipitating carbonates. Under certain circumstances, where precipitation patterns are strongly seasonal and the nature of vadose percolation is such that seasonal mixing is incomplete, bands with different luminescent intensities can be differentiated after excitation with UV radiation. In other cases, bands are not observable but secular... 

Doughty C., PruessK. Modeling supercritical carbon dioxide injection in heterogeneous porous media. 2004 Vadose Zone Journal 3(3) 837-847. 

The often smooth top surface to the calcrete suggests that the calcrete formed by the precipitation of Ca (as carbonate) from sub-surface laterally and vertically flowing vadose water sourced from rainfall, which penetrates easily through the overlying sand. The sub-surface has been plugged with calcrete preventing vertical downwards penetration of vadose water. Instead, vadose water would move laterally until it evaporates or is removed by transpiration. Precipitation of calcium carbonate at the surface of the calcrete would occur, thus contributing to the laminated occurrence of the calcrete. 

A few experiments have been successfully performed at low temperatures to simulate carbonate diagenetic processes for example, cements have been precipitated on skeletal carbonate sands in experimental reaction chambers designed to mimic vadose and phreatic meteoric cementation (Thorstenson et al., 1972 Badiozamani et al., 1977). These cements are remarkably similar in composition and morphology to those found in rocks cemented in the meteoric... 

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. 

There is an extensive literature that exists concerning carbonate diagenesis in the vadose and phreatic meteoric environmental settings (see e.g., Bathurst, 1975, 1980 James and Choquette, 1984 and Moore, 1989 for reviews). Much of this work has dealt with the stabilization of aragonite and magnesian calcite to calcite in the present meteoric diagenetic realm associated with Pleistocene limestones. As... 

Figure 7.29. A scatter plot of 813C and 8180 values of Holocene and Pleistocene carbonate sediments. Open circle, unlithified bulk Holocene sediments + s, lithified bulk Holocene sediment closed circles, Pleistocene limestones altered by meteoric water. For Pleistocene limestones altered in the meteoric environment, temperate climate samples are represented by 8180 values more enriched than -5%o, whereas tropical climate samples have 8180 values more depleted than -5%o. The large depletion in 13C is because the sediment samples are primarily from vadose environments. (After Land, 1986.)...

Because the bulk of the limestone volume of Bermuda has spent most of its existence in the vadose zone, much of the limestone mass shows the imprint of vadose diagenesis, and progressive depletion in 13C with increasing age. However, limestones older than 125,000 years have spent more time in the phreatic meteoric zone. These limestones are more extensively altered by freshwater and freshwater-seawater mixtures. Their relatively light 813C values, however, imply alteration in a relatively open system in which soil carbon dioxide, depleted in 13C because of oxidation of organic matter, was an important source of carbon for replacement reactions. 

The rainwater of Bermuda is in near equilibrium with atmospheric Pc02 = 10-3.5 atm., and contains small amounts of sea salt (0.07 wt. % seawater). The rainfall of 147 cm y1 is seasonally distributed. The rain enters the saturated zone by two main paths direct rainfall on marshes and ponds, and percolation downward from the vadose zone as vadose seepage and flow through rocks during times of soil water excess (Vacher, 1978). Total annual recharge of the saturated zone is about 40 cm y-1 (Vacher and Ayers, 1980). The residence time of the groundwater has been calculated as 6.5 years, and the average age of the sampled water as 4 years (Vacher et al., 1989). Such estimates are necessary for calculations of carbonate mineral stabilization rates, as shown in a later section. 

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