Diagenetic reactions sediment

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. 

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. 

Aller, R.C., and Aller, J.Y. (1998) The effect of biogenic irrigation intensity and solute exchange on diagenetic reactions rates in marine sediments. J. Mar. Res. 56, 905-936. 

A more serious consideration for shelf and coastal dynamics would be the creation of significant dams along the mainstem of the Amazon River. Such structures certainly would reduce the delivery of sediment to the continental shelf. Benthic processes such as fluid mud formation and suboxic diagenetic reaction balances could be altered by these activities. Downdrift coastal mud banks and coastal mangrove environments also could be affected. 

Figure 1). The solute exchanges resulting from reactions in surface sediments can be important for the chemistry of oceanic deep water, and to the overall cycles of several elements in the oceans. In addition, the alteration of particles by these reactions must be taken into account when down-core records of the accumulation of sedimentary components are interpreted in terms of past oceanic and atmospheric chemistry. Understanding early diagenetic reactions in the upper few centimeters of the marine sediment column is important both to the study of geochemical cycles in the contemporary ocean and to the reconstruction of past oceanic conditions. 

In these examples as well as for most aquatic sediments, the principal diagenetic reactions that occur in these sediments are aerobic respiration and the reduction of Mn and Fe oxides. Under the slower sedimentation conditions in natural lakes and estuaries, there is sufficient time (years) for particulate organic matter to decompose and create a diagenetic environment where metal oxides may not be stable. When faster sedimentation prevails, such as in reservoirs, there is less time (months) for bacteria to perform their metabohc functions due to the fact that the organisms do not occupy a sediment layer for any length of time before a new sediment is added (Callender, 2000). Also, sedimentary organic matter in reservoir sediments is considerably more recalcitrant than that in natural lacustrine and estuarine sediments as reservoirs receive more terrestrial organic matter (Callender, 2000). 

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