Chapter 6 - The Sulfur Cycles

Abstract

The scientists' knowledge about details of the sulfur transfer is still limited. The sulfur concentrations in the air are so small that new, analytical methods had to be developed before accurate concentration measurements and rate transfer measurements became possible. In the ocean, sulfur concentrations are equally small and methods have yet to be improved before changes in the concentrations of intermediates can be tracked. In the sediment, problems are caused by inhomogeneity of strata and deposits, which exhibit erratic changes in total sulfur content and in the ratio of the various sulfur forms. Some steps of the sulfur cycle involve geological time periods, which are far beyond the meaningful extrapolation range of laboratory experiments. The residence time of sulfate in the ocean, for example, is believed to be about 40 million years; that of pyrite in sedimentary rocks is over 250 million years. The estimated values of the different steps of the global sulfur cycle vary considerably from author to author. The geochemical data of this type is based on difficult-to-test assumptions about global processes, which are obtained by computing the sum of a variety of different regional and local sulfur cycles, comparing processes in different areas, weighing the relative importance of different models, and adding, that is, extrapolating, data. Thus, only few steps in the sulfur cycle can be directly determined; the remaining links are deduced by balancing the cycle.