Decomposition anoxic

Figure 5.5 Electron balances during anoxic decomposition of soil organic matter to CH4 and CO2 in eight rice soils. Soil properties given in Figure 5.3 (Yao and Conrad, 2000). Reproduced by permission of Blackwell publishing...

Based on the pE value, redox environments are classified as follows (a) pE > 7 indicates an oxic environment, (b) at pE values between 2 and 7, the environment is considered suboxic, and (c) pE < 2 indicates an environment considered anoxic. The occurrence of redox reactions in the subsurface environment is limited by the decomposition and reduction of water ... 

Hydrogen is almost exclusively present in combined form, as in water and organic compounds. Free hydrogen is brought into the anoxic environment by the decomposition of organic matter and in a few cases by volcanism. 

Decomposition Products of (Dimethylsulfonio)propionate in Anoxic Marine Sediments... 

Lee, C. (1992) Controls on organic carbon preservation the use of stratified water bodies to compare intrinsic rates of decomposition in oxic and anoxic systems. Geochim. Cosmochim. Acta 56, 3323-3335. 

Nguyen, R.T., and Harvey, H.R. (1997) Protein and amino acid cycling during phytoplankton decomposition in oxic and anoxic waters. Qrg. Geochem. 27, 115-128. 

Sun, M.Y., Zou, L., Dai, J., Ding, H, Culp, R.A., and Scanton, M.I. (2004) Molecular carbon isotopic fractionation of algal lipids during decomposition in natural oxic and anoxic seawaters. Org. Geochem. 35, 895-908. 

Wang, X.C., and Lee, C. (1995) Decomposition of aliphatic amines and amino acids in anoxic marine sediment. Geochim. Cosmochim. Acta 59, 1787-1797. 

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. 

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