Redox cycle, global

Kump L.R. and Garrels R.M. (1986) Modeling atmospheric O2 in the global sedimentary redox cycle. Amer. J. Sci. 286, 337-360. 

It looks reasonable to hypothesize that globally ubiquitous iron oxides were incorporated into life development from the early evolutionary stage, but subsequent to the synthesis of simple amines and organic acids. The diffusion of sulfide across primitive membranes is also meaningful for the maintenance of the thioester world . An intracellular iron redox cycle, driven possibly by light and sulfide, could have supported chemosynthesis. Iron respiration and photometabolism could have processed on the external surface of vesicles. This would have contributed to a vectorial transport of thioesters and protons into vesicles and accordingly to the development of the early protonmotive transport system. 

Therefore, polysulfide ions play a major role in the global geological and biological sulfur cycles [1, 2]. In addition, they are reagents in important industrial processes, e.g., in desulfurization and paper production plants. It should be pointed out however that only sulfide, elemental sulfur and sulfate are thermodynamically stable under ambient conditions in the presence of water, their particular stabihty region depending on the redox potential and the pH value [3] ... 

Global geochemical cycle of solar redox energy. Note that no reaction stoichiometry Is shown In this schematic depiction. 

Arsenic moves between ditferent environmental compartments (rock-soil-water-air-biota) from the local to the global scale partly as a result of pH and redox changes. Being a minor component in the natural environment, arsenic responds to such changes rather than creating them. These changes are driven by the major (bio)geochemical cycles. 

In this chapter I will propose a kinetic estimate for the thermodynamics of reactions like Eq (lb). The solid phases listed in Table 1 may act as a reductant or an oxidant. One of the prominent geochemical electron donors is pyrite. From an estimate of global pyrite weathering of 36 Tgy"1 (Garrels et al., 1973) we may deduce an average electron flux on the land surface in the order of 0.02 mol m 2 y1. At redox boundaries in salt marshes and in lake sediments microbial sulfate reduction will intensify this electron cycling. Luther (Chapter 6, this volume) discusses the details of sulfide redox mechanisms. 

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