Drivers of chemical disequilibrium in the Earth system

We have now dealt with the thermodynamics of mass cycling associated with water in relation to the planetary forcing. The next step in describing the thermodynamic view of the Earth system shown in Fig. 1.5 is to extend and connect the geochemical cycling of mass to this view. As we will see in this chapter, the thermodynamic formulation of geochemical cycling is similar to hydrologic cycling, except that the connections of geochemical cycling to energy fluxes and radiative effects is more subtle. This formulation then allows us to understand how and by which processes chemical energy is generated within the Earth system.We can then understand how chemical disequilibrium is maintained in the Earth system, how biotic activity as a specific geochemical process fits into this description, by how much biotic activity contributes to the maintenance of chemical disequilibrium, and how geochemical cycling interacts with the thermodynamic state of the Earth system.

This thermodynamic description of geochemical cycling in the Earth system relates back to one of the motivations of the introduction, in which it was described that the Earth's atmosphere reflects a notable state of chemical disequilibrium, reflected mostly in the simultaneous presence of methane, CH4, and oxygen, O2 (Lovelock 1965). If left alone, methane would react with oxygen to form carbon dioxide and water, so it requires a continuous exchange of these compounds between the atmosphere and other compartments of the Earth system to maintain the simultaneous presence of these chemical compounds. This remarkable state of chemical disequilibrium has long been recognized as being a unique signature of the Earth system when compared to other planetary atmospheres of the solar system. It possibly serves as a fundamental indicator of a planet with life (Lovelock 1965; Hitchcock and Lovelock 1967; Lovelock 1975), it motivated the development of the Gaia hypothesis (Lovelock and Margulis 1974), and is indicative of geochemical cycling through the Earth's atmosphere as exchange fluxes of chemical compounds are needed to maintain this disequilibrium state.

This chapter deals with geochemical cycling as a thermodynamic phenomenon of the Earth system that reflects the maintenance of geochemical reactions in a state of disequilibrium at the planetary scale. The description of geochemical cycling starts with a thermodynamic formulation of chemical reactions and with relating the concentrations of chemical compounds to states of chemical disequilibrium.