Fundamental aspects of quantitative models for geochemical cycles

The modern treatment of geochemical cycles requires a truly quantitative framework. This paper addresses various aspects of such quantification. The concepts of residence time and response time are reviewed. The important concept of mass balance in geochemical cycles and its dynamical implications are discussed, with particular stress on the need for feedbacks in the carbon cycle. Particular emphasis is also placed on understanding the behavior of non-linear systems. The use of new and more encompassing global rate laws in models of geochemical cycles requires that (a) proper attention be given to the statistical averaging and extrapolation involved in constructing the particular sets of cycle reservoirs (for example, the flux of river water to the sea) and (b) that the knowledge about the fundamental physical chemical and biological processes operating within reservoirs and giving rise to fluxes between reservoirs be incorporated into any global dynamics. As an example of the latter, the problem of rates of dissolution during weathering is presented to illustrate the importance of knowing more about the surface properties and reactivities of minerals.