Carbon cycles, global sedimentary rocks

If oxygen is produced by the reduction of carbon dioxide, then atmospheric C02 levels must have declined over the life of the Earth, and there is evidence that this has indeed occurred.The climatic implications of changes in atmospheric C02 levels and how the global carbon cycle operates are addressed in Chapter 6, after the various elements of the carbon cycle have been introduced in the intervening chapters. For the moment, we continue with a more detailed look at the evolution of life in terms of the organisms that are most important by way of their contributions to the organic matter that is eventually buried in sedimentary rocks. 

The vast majority of carbon is found in sedimentary rocks as inorganic carbonate minerals and solid organic compounds, and in the deep interior as carbon dioxide and methane. The global carbon cycle can therefore be envisioned as the long-term transfer of deep crustal and mantle carbon to shallow crustal sediment reservoirs via the ocean/atmosphere system, where carbon is parsed into several reservoirs through reactive pathways that modify the initial carbon isotopic value of the outgassed carbon. The... 

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