Carbonate rocks distribution

The content of the material in a carbon reservoir is a measure of that reservoir s direct or indirect exchange rate with the atmosphere, although variations in solar also create variations in atmospheric content activity (Stuiver and Quay, 1980, 1981). Geologically important reservoirs (i.e., carbonate rocks and fossil carbon) contain no radiocarbon because the turnover times of these reservoirs are much longer than the isotope s half-life. The distribution of is used in studies of ocean circulation, soil sciences, and studies of the terrestrial biosphere. 

Effect of Rock Permeability. The effect of rock permeability has been investigated by comparison of mobility measurements made with Baker dolomite and Berea sandstone. Mobility measurements carried out with Rock Creek sandstone (from the Big Injun formation in Roane County, W.Va) is also reported. Rock Creek sandstone has a permeability of 14.8 md. A direct comparison was made with Berea sandstone and Baker dolomite measured with 0.1% AEGS. As mentioned in an earlier section, the permeability of Baker dolomite (a quarried carbonate rock of rather uniform texture with microscopic vugs distributed throughout) was 6.09 md, and of Berea sandstone was 305 md. The single phase permeabilities were measured with 1% brine solution. 

When submitting a carbonate rock to the flov of an acidic solution, the attack, most of the time, leads to the preferential grovth of large pores. The dissolution of the rock is not uniform, and the final pore size distribution is much broader than the original one. This fact has been recognized and described in detail more than fifteen years ago (4j 5). Macroscopic pores, which are the end result of such an unstable attack, have received the name of wormholes. 

Paleoproterozoic Hurwitz Group siliciclastic and lesser carbonate rocks, which are currently distributed as erosional remnants of regional scale syn forms. 

Over the last 30 years the study of the stable isotope composition of carbonates has been one of the more active areas of research in carbonate geochemistry. These studies have particular application to later discussion of carbonate diagenesis and historical geochemistry of carbonate rocks. Many of the same considerations involved in understanding elemental distribution coefficients apply to the fractionation of stable isotopes. Consequently, we have included a discussion of the chemical principals associated with isotope behavior in this chapter. Only a relatively brief summary of these basic chemical considerations will be presented here, because recent books and extensive reviews are available on this topic (e.g., Arthur et al., 1983 Hoefs, 1987). Also, our discussion will be restricted to carbon and oxygen isotopes, because these isotopes are by far the most important for the study of carbonate geochemistry. The principles, however, apply to other stable isotopes (e.g., sulfur). 

Kinetic and thermodynamic considerations show that the mass transfer leading to extensive alteration of carbonates in the meteoric realm is difficult to account for by transport of chemical components over long distances. This conclusion has important implications for the formation of regionally-distributed cements in carbonate rocks. These implications and further problems of mass transfer are subjects of the next chapter. 

Figure 10.5. Mass-age distribution of carbonate rocks and other sedimentary rock types plotted as survival rate (S) versus age. Total rock mass data from Gregor (1985) and estimates of carbonate rock mass from Table 10.1.

In Figure 10.30 the survival rate of the total sedimentary mass for the different Phanerozoic systems is plotted and compared with survival rates for the total carbonate and dolomite mass distribution. The difference between the two latter survival rates for each system is the mass of limestone surviving per unit of time. Equation 10.1 is the log linear relationship for the total sedimentary mass, and implies a 130 million year half-life for the post-Devonian mass, and for a constant sediment mass with a constant probability of destruction, a mean sedimentation rate since post-Devonian time of about 100 x 1014 g y 1. The modem global erosional flux is 200 x 1014 g y-1, of which about 15% is particulate and dissolved carbonate. Although the data are less reliable for the survival rate of Phanerozoic carbonate sediments than for the total sedimentary mass, a best log linear fit to the post-Permian preserved mass of carbonate rocks is... 

Grantham P. J. and Wakefield L. L. (1988) Variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time. Org. Geochem. 12, 61-73. 

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