Disequilibrium

M. Gascoyne, in Uranium Series Disequilibrium Applications to Environmental Problems, ed. M. Ivanovich and R. S. Hannon, Clarendon Press, Oxford, 1982, pp. 32-55. 

There are two main sources of Rn to the ocean (1) the decay of sediment-bound "Ra and (2) decay of dissolved "Ra in a water column. Radon can enter the sediment porewater through alpha recoil during decay events. Since radon is chemically inert, it readily diffuses from bottom sediments into overlying waters. The diffusion of radon from sediments to the water column gives rise to the disequilibrium (excess Rn) observed in near-bottom waters. Radon is also continuously being produced in the water column through the decay of dissolved or particulate "Ra. 

In a recent paper [11] this approach has been generalized to deal with reactions at surfaces, notably dissociation of molecules. A lattice gas model is employed for homonuclear molecules with both atoms and molecules present on the surface, also accounting for lateral interactions between all species. In a series of model calculations equilibrium properties, such as heats of adsorption, are discussed, and the role of dissociation disequilibrium on the time evolution of an adsorbate during temperature-programmed desorption is examined. This approach is adaptable to more complicated systems, provided the individual species remain in local equilibrium, allowing of course for dissociation and reaction disequilibria. 

Un-wucht, /. unbalance, imbalance, disequilibrium. -zahl, /. immense number. unz Lhlbar, unzahlig, a. innumerable. 

The last chapter in this introductory part covers the basic physical chemistry that is required for using the rest of the book. The main ideas of this chapter relate to basic thermodynamics and kinetics. The thermodynamic conditions determine whether a reaction will occur spontaneously, and if so whether the reaction releases energy and how much of the products are produced compared to the amount of reactants once the system reaches thermodynamic equilibrium. Kinetics, on the other hand, determine how fast a reaction occurs if it is thermodynamically favorable. In the natural environment, we have systems for which reactions would be thermodynamically favorable, but the kinetics are so slow that the system remains in a state of perpetual disequilibrium. A good example of one such system is our atmosphere, as is also covered later in Chapter 7. As part of the presentation of thermodynamics, a section on oxidation-reduction (redox) is included in this chapter. This is meant primarily as preparation for Chapter 16, but it is important to keep this material in mind for the rest of the book as well, since redox reactions are responsible for many of the elemental transitions in biogeochemical cycles. 

We cover each of these types of examples in separate chapters of this book, but there is a clear connection as well. In all of these examples, the main factor that maintains thermodynamic disequilibrium is the living biosphere. Without the biosphere, some abiotic photochemical reactions would proceed, as would reactions associated with volcanism. But without the continuous production of oxygen in photosynthesis, various oxidation processes (e.g., with reduced organic matter at the Earth s surface, reduced sulfur or iron compounds in rocks and sediments) would consume free O2 and move the atmosphere towards thermodynamic equilibrium. The present-day chemical functioning of the planet is thus intimately tied to the biosphere. 

The latter reaction must involve a large number of molecular steps and may be a much slower process. The mechanisms of a few inorganic electron transfer processes have been summarized by Taube (1968). The presence of very slow reactions when several redox couples are possible means that the Eh value measured with an instrument may not be related in a simple way to the concentrations of species present, and different redox couples may not be in equilibrium with one another. Lindberg and Runnells (1984) have presented data on the extent of disequilibrium... 

Church, M. and Slaymaker, O. (1989). Disequilibrium of Holocene sediment yield in glaciated British Columbia. Nature 337,452 54. 

Thermodynamic Disequilibrium and Microbial Catalysis of Oxidation Reactions... 

Palaisa, K. A. et al., Contrasting effects of selection on seqnence diversity and linkage disequilibrium at two phytoene synthase loci, Plant Cell 15, 1795, 2003. 

Woodruff, L.G. and Shanks, W.C. Ill (1988) Sulfur isotope. study of chimney minerals and vent fluids from 21°N East Pacific Rise Hydrothermal sulfur sources and disequilibrium sulfate reduction. J. Geophys. Res., 93, 4562-4572,... 

Processes that fractionate nuclides within a chain produce parent-daughter disequilibrium the return to equilibrium then allows quantification of time. Because of the prescribed decay behavior, U-series disequilibria can be used for geochronology or for examining the rates and time scales of any dynamic processes which induces fractionation. In many cases, the direction of disequilibrium (activity ratios above or below one) provides a powerful means of tracing specific processes. 

Based on Equation (3), in the case of a system where there is an initial disequilibrium in the chain (namely A,jNj A,2N2), it is generally stated that the system returns to secular equilibrium after -six half-lives of the daughter. The wide variety of parent-daughter pairs allows disequilibria to provide temporal constraints over a wide range in time scales (Fig. 3). 

Figure 3. Parent daughter disequilibrium will return to equilibrium over a known time scale related to the half-life of the daughter nuclide. To return to within 5% of an activity ratio of 1 requires a time period equal to five times the half-life of the daughter nuclide. Because of the wide variety of half-lives within the U-decay-series, these systems can be used to constrain the time scales of processes from single years up to 1 Ma.

Fame G (1986) Principles of Isotope Geology, Second Edition. John Wiley and Sons, New York Fleischer RL, Raabe OG (1975) Recoiling alpha-emitting nuclei. Mechanisms for uranium-series disequilibrium. Geochim Cosmochim Acta 42 973-978 Goldstein SJ, Murrell MT, Williams RW (1993) Pa and h chronology of mid-ocean ridge basalts. Earth Planet Sci Lett 115 151-159... 

Holden NE (1990) Total half-lives for selected nuclides. Pure Appl Chem 62(5) 941-958 Ivanovich M (1992) The phenomenon of radioactivity. In Uranium-series Disequilibrium Applications to Earth, Marine, and Environmental Sciences. Ivanovich M, Harmon RS (eds) Clarendon Press, Oxford, p 1-33... 

Ivanovich M, Harmon RS (1992) Uranium-series Disequilibrium Applications to Earth, Marine, and Environmental Sciences. Clarendon Press, Oxford... 

Huang M, Hirabayashi A, Shirasaki T, Koiznmi H (2000) A multimicrospray nebulizer for microwave-induced plasma mass spectrometry. Anal Chem 72 2463-2467 Ivanovich M, Murray A (1992) Spectroscopic methods. In Uranium-Series Disequilibrium Applications to Earth, Marine, and Enviromnental Sciences, 2" Ed. Ivanovich M, Harmon RS (eds) Orfbrd Univ. Press, Oxford... 

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