Redox reactions global cycle

Phosphorous is not the only nutrient that can be retained/removed via reactions in the subterranean estuary. The microbial reduction of nitrate to inert dinitrogen gas, a process known as denitrification, is known to occur in the redox gradients associated with fresh and saline groundwater mixing. Conversely, ammonium, which is more soluble in saline environments, may be released within the subterranean estuary s mixing zone. While the overall importance of SGD on the global cycle of certain chemical species remains to be seen, there is little doubt that SGD is important at the local scale both within the United States and throughout the world. 

Fundamentally, the carbon cycle is a series of linked chemical reactions, both biological and abiotic. Many are redox reactions. Although the principal source of energy that drives the global redox system is sunlight, humans have not only diverted naturally occurring sources of energy and carbon to their own use, but are also... 

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

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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