Diagenesis anoxic

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

Oxic Diagenesis Metals remobilized from sediments lying in the oxic zone. Remobilization likely occurs in anoxic microzones adjacent to nodules. Bioturbation is an important metal transport agent. Some nodules now found in oxic sediments were likely formed during times when the redox boundary was closer to the seafloor. 10-50 Todorokite (high Cu and Ni content) 32% 5-10 15-20... 

Many studies of the impact of chemical diagenesis on the carbonate chemistry of anoxic sediments have focused primarily on the fact that sulfate reduction results in the production of alkalinity, which can cause precipitation of carbonate minerals (e.g., Berner, 1971). Although the many reactions involved are complex, this process can be schematically represented as ... 

Figure 6.12. Calculated saturation states with respect to aragonite of marine pore waters undergoing anoxic diagenesis as a function of the original C/N ratio of labile organic matter. The extent of diagenesis is represented by the organic C oxidized in the sediment-pore water system, shown as inorganic carbon added to pore waters.

Berner, R.A. (1974) Kinetic models for early diagenesis of nitrogen, sulfur, phosphorus, and silicon in anoxic marine sediments. In The Sea (Goldberg, E.D., ed.), pp. 427 -50, John Wiley, New York. 

Rosenfield, J.K. (1979) Amino acid diagenesis and adsorption in nearshore anoxic sediments. Limnol. Oceanogr. 24, 1014-1021. 

Kristensen, E. (2000). Organic matter diagenesis at the oxic/anoxic interface in coastal marine sediments, with emphasis on the role of burrowing animals. Hydrobiologia 426, 1—24. 

While all of these reactions are favored thermodynamically, they are almost always enzymatically catalyzed by bacteria. It has been observed from the study of pore waters in deepsea sediments (e.g., Froelich et al., 1979) and anoxic basins (e.g., Reeburgh, 1980) that there is an ordered sequence of redox reactions in which the most energetically favorable reactions occur first and the active electron acceptors do not overlap significantly. Bacteria are energy opportunists. Using estimates of the stoichiometry of the diagenesis reactions (Table 2) one can sketch the order and shape of reactant profiles actually observed in sediment pore-water chemistry... 

Teece M. A., Getliff J. M., Leftley J. W., Parkes R. J., and Maxwell J. R. (1998) Microbial degradation of the marine prymnesiophyte Emiliania huxleyi under oxic and anoxic conditions as a model for early diagenesis long chain alkadienes, alkenones, and alkyl alkenoates. Org. Geochem. 29, 863-880. 

Many studies of the impact of chemical diagenesis on the carbonate chemistry of anoxic sediments have focused primarily on the fact that sulfate reduction results in the production of alkalinity, which can cause precipitation of carbonate minerals (see previous discussion). However, during the early stages of sulfate reduction (—2-35%), this reaction may not cause precipitation, but dissolution of carbonate minerals, because the impact of a lower pH is greater than that of increased alkalinity (Figure 4). Carbonate ion activity decreases rapidly as it is titrated by CO2 from organic matter decomposition leading to a decrease in pore-water saturation state. This process is evident in data for the Fe-poor, shallow-water carbonate sediments of Morse et al. (1985) from the Bahamas and has been confirmed in studies by Walter and Burton (1990), Walter et al. (1993), and Ku et al. (1999) for Florida Bay, Tribble (1990) in Checker Reef, Oahu, and Wollast and Mackenzie (unpublished data) for Bermuda sediments. 

Vasconcelos C. and McKenzie J. A. (1997) Microbial mediation of modem dolomite precipitation and diagenesis under anoxic conditions (Lagoa Vermelha, Rio de Janeiro, Brazil). J. Sediment. Res. 67, 378—390. 

Cowie G. L. and Hedges J. 1. (1993) A comparison of organic matter sources, diagenesis and preservation in oxic and anoxic coastal sites. Chem. Geol. 107, 447 -451. 

Emerson (1976) carried out physical-chemical analyses of interstitial waters in cores of Greifensee in north-central Switzerland. The lake has an area of 8.6 km and a mean depth of 19 m. The lake has become increasingly eutrophic, the hypolimnion being anoxic for about six months of the year. Whereas the sulfate concentration in the water is 18 mg l, it is below 2 mg l i (the limit of detection) in the interstitial waters. The pS ranges from 12 near the bottom surface to 15 at a depth of 16 m (log Chs- from —6 to —9 at pH 7.05) and identifies a zone of active sulfate reduction near the water-sediment interface. This also causes a large gradient of ferrous iron with depth because of diagenesis of iron sulfide minerals. 

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