The global cycles of biologically active elements are an important part of basic and advanced Earth Science, Ecology and Biogeochemistry courses. An understanding of biogeochemical cycles and anthropogenic impacts on them is also fundamental in studies of global climate change. Unfortunately, most presentations of biogeochemical cycles occupy one of two extremes: they are either presented so simply that they contain information on pathways only, or in such detail that they defy comprehension and are us eful only to specialists. Further, most workers have specialized in facets of individual cycles, and broad perspectives and an understanding of interactions between cycles is lacking.

The figures presented here are an attempt to fill the middle ground and allow comparisons between element cycles. Figures based on current literature values showing the global cycles of C, O, N, S, P, and Si, as well as H20 and CH4, are presented in a uniform format that shows pool or reservoir sizes, significant natural and anthropogenic transfers or fluxes between pools, and residence times estimated from the ratio of pool size and the major fluxes. The uniform format and use of color to distinguish pools, fluxes, and turnover times makes comprehension of individual element cycles and comparison between element cycles straightforward.

The figures were developed as class assignments in a graduate-level course in Earth Systems at the University of California Irvine and have been used in graduate as well as undergraduate courses. Discussion surrounding these figures has evolved from "what do we include and which numbers do we use?" as they were being prepared to a current "where did these numbers come from?" To permit easy comparison with the original literature, we have used the units as published. Conversion factors are supplied in the captions.

The colors used for pool sizes (yellow), turnover rates (white), and residence times (cyan) are the same in all figures and are identified in the caption. The arrows associated with the fluxes denote the direction of the flux. Questions, corrections, updates, and suggestions should be addressed by e-mail to W. S. Reeburgh at