Life is composed of many chemical elements, and these are required in the right amounts, the right concentrations, and the right ratios to each other.  If these conditions are not met, then life is limited.  The study of chemical availability, called in the most general form biogeochemical cycles, is important to the solution of many environmental problems.  This section should foster your understanding of concepts covered in the text specific to the factors that control these chemical cycles, how each major component of the Earth system are involved in chemical cycles, which biogeochemical cycles are most important to life, and how to analyze biogeochemical cycles.  

ENVIRONMENTAL ISSUE  

   How Has Life Affected the Carbon Cycle?  Carbon dioxide enters the biosphere through photosynthesis and is released through respiration and decay.  Although this biological carbon cycle is important over short time periods, the geochemical cycle is far more important in regulating atmospheric carbon dioxide in the long run.   

   Living organisms contribute to the geochemical cycle when they die and are buried in the soil, leaving organic matter behind.  During chemical weathering of the soil, the organic material reacts with oxygen in the atmosphere to form carbon dioxide, which is slowly released back to the atmosphere.  As presented by the graph in the text, a computer model has been used to demonstrate that the chemical weathering of organic material over the past 100 million years has resulted in a drastic reduction in the level of carbon dioxide in the Earth's atmosphere.  This conclusion is based on a number of assumptions and uncertainties, which gives rise to the need for critical thinking and investigation of scientific questions relative to this issue.