Diagenetic reactions

Initial U concentration. If uranium concentration has changed as a result of diagenetic reactions, one may, in principle detect this by comparison between uranium concentrations in modem corals and their fossil counterparts. Early work documenting and studying uranium concentrations in corals is extensive (e g., Barnes et al. 1956 Tatsumoto and Goldberg 1959 Veeh and Turekian 1968 Schroeder et al. 1970 Thompson and Livingston 1970 Gvirtzman et al 1973 Amiel et al. 1973 Swart 1980 Swart and Hubbard 1982 Cross and Cross 1983). This broad body of data shows that primary surface coral uranium concentrations lie between 1.5 and 4 ppm (see Fig. 1). Concentrations appear to be species dependent (Cross and Cross 1983). Furthermore, uranium concentrations vary within individual coral skeletons (Schroeder et al. 1970 Shen and Dunbar 1995 Min et al. 1995). 

In this chapter we consider how reaction modeling applied to open systems might be used to study the nature of diagenetic alteration. We develop examples in which modeling of this type can aid in interpreting the diagenetic reactions observed to have occurred in sedimentary rocks. 

If the solute imdergoes any chemical changes, a reaction term must be added to Eq. 12.4. In the absence of specific rate law information, diagenetic reactions are generally assumed to be first-order with respect to the solute concentration. Thus, the one-dimensional advection-diffusion equation far a nonconservative solute is given by... 

For chemicals that do not undergo diagenetic reactions, concentration profiles in the sediment can be used as records of changes in accumulation rate (Figure 12.6a). In... 

In the marine environment oxygen and hydrogen isotope compositions of pore waters may be inherited from ocean water or inflnenced by diagenetic reactions in the sediment or nnderlying basement. Knowledge of the chemical composition of sedimentary pore waters has increased considerably since the beginning of the Deep-Sea-Drilling-Project. From numerous drill sites, similar depth-dependent trends in the isotopic composition have been observed. 

For oxygen this means a decrease in from an initial 5-valne very near 0%c (ocean water) to abont -2%o at depths aronnd 200 m (Perry et al. 1976 Lawrence and Gieskes 1981 Brnmsack et al. 1992). Even lower 5 0-values of about -4%c at depths of around 400 m have been observed by Matsumoto (1992). This decrease in 0 is mainly dne to the formation of anthigenic 0-enriched clay minerals such as smectite from alteration of basaltic material and volcanic ash. Other diagenetic reactions inclnde recrystallization of biogenic carbonates, precipitation of... 

One of the most sensitive tracers recording the composition of ancient sea water is the isotopic composition of chemical sediments precipitated from sea water. The following discussion concentrates on the stable isotope composition of oxygen, carbon, and sulfur, but in recent years other isotope systems have been included such as Ca (De La Rocha and De Paolo 2000 Schmitt et al. 2003 Fantle and de Paolo 2005 Farkas et al. 2007) and B (Lemarchand et al. 2000, 2002 Joachimski et al. 2005) and Li (Hoefs and Sywall 1997). One of the fundamental questions in all these approaches is which kind of sample provides the necessary information, in the sense that it represents the ocean water composition at its time of formation and has not been modified subsequently by diagenetic reactions. 

EBERL (D.D.), 1971. Experimental diagenetic reactions involving cJay... 

At this junction it seems appropriate to consider some further thermodynamic and kinetic arguments that bear on carbonate diagenetic problems discussed in this chapter and in Chapters 7 and 8. Because most carbonate diagenetic reactions take place in an aqueous environment (seawater, freshwater, brines) over a range of temperature and pressure, it is rapidly becoming... 

The chemistry of waters in which diagenetic reactions occur is of central importance in understanding massive carbonate diagenesis. Topics that have received the most attention are the influence of mixing waters of dissimilar chemistries and the ability of waters to transport the necessary masses of diagenetic reactants and products. 

Lafon G.M. and Vacher H.L. (1975) Diagenetic reactions as stochastic processes Application to the Bermudian eolianites. Geol. Soc. Amer. Memoir 142, 187-204. 

Despite the importance of water and organic compounds in diagenetic reactions, and although organic compounds and water in various forms (liquid and bound 0 and OH ) are known to be... 

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