Kinetics of Global Geochemical Cycles

Abstract

Geochemical systems of the Earth’s surface and interior are often studied by means of conceptual models that represent them as geochemical or biogeochemical cycles of chemical elements. Such models usually address the various geological, geochemical, geophysical, and biological processes within the cycle or system, and they focus on the model’s ability to evaluate the system changes at different time scales, often extending from the remote past into the future. The time dimension of changes taking place in the different parts of the Earth System makes it necessary to understand the mechanisms and rates of the numerous processes that control the element interactions in geochemical systems of different physical structures and degrees of complexity. In this chapter, we present first a brief overview of the history of development of geochemical cycles and the concept of the rock cycle that encompasses the system of the Earth’s surface and upper lithosphere. Further, we discuss the essentials of geochemical cycle models and some simple mathematical relationships that can be used in their analysis, such as the rate parameters of the fluxes, residence time, and the time of approach to steady state. After this theoretical section, we address the global phosphorus cycle as an example of a biogeochemical cycle of an element of indispensable importance to life. We focus on the structure and dynamics of the cycle, as well as the uncertainties inherent in some of the cycle data and parameters that can affect the conclusions of even a simple cycle analysis. Mineral-water reactions in the continental environment are an extensively studied subject that is fundamental to the understanding of the material fluxes between the atmosphere, sedimentary and crystalline lithosphere, and the world ocean, and their effects on global climate. In this field, we estimate global average denudation rates of the land surface by physical and chemical processes. Drawing on the published, very