Carbon Cycle

Biota

During their formation from protoplanetary dust cloud, Earth, Venus, and Mars captured similar percentages of carbon. Due to differences in their history, however, the concentrations of carbon in the modern atmospheres of these terrestrial planets vary widely. The atmospheres of Venus and Mars consist mostly of CO2, with concentrations of 96.5% and 93.5% by volume, respectively. The atmosphere of Earth, in turn, is a 99% mixture of nitrogen and oxygen; the concentration of carbon dioxide is very low (∼0.0000289%, or 289 ppm [parts per million] by volume in the middle of the 19th century and 383.86 ppm in 2007). Unlike Mars and Venus, most of the carbon on Earth is found not in the atmosphere but in the lithosphere in the form of carbonate deposits and organic carbon (Table 1). This striking difference in carbon distribution between Earth and other terrestrial planets can be explained only by the activity of living organisms.

In photosynthesis, green plants (including marine phytoplankton) produce carbohydrates (organic matter). The net primary productivity (the rate of annual biologic fixation of carbon by continental and oceanic biota) is about 85 to 90 Gt/yr. (gigatons per year) (Table 2). More than 99% of this organic matter is decomposed and returned to the atmosphere (Table 2). Depending on the molecular weight of specific organic compounds, however, the rate of their decomposition varies from less than a year to hundreds and even thousands of years.

The biomass of marine organisms is two orders of magnitude smaller than the biomass of terrestrial organisms, but the productivity of marine biota is only two times smaller than the productivity of all terrestrial organisms (see Tables 1 and 2).

Lithosphere

A common form of organic carbon in geologic sedimentary rocks is kerogen, which is insoluble in water, a carbohydrate of irregular structure and very high molecular weight. In the past few hundred million years, a rather small portion of kerogen was transformed inside the lithosphere into various types of fossil fuel: coal, oil, and natural gas. Overall, however, sedimentary geologic rocks contain most of their carbon not as organic matter but as carbonates (Table 1). Carbonates are mineral sedimentary rocks formed in the process of weathering of original igneous (volcanic) rocks. The process of chemical weathering is regulated by the reaction of carbon dioxide with silicate minerals in soil. This reaction can be accelerated by root respiration of vascular plants as well as by activity of soil bacteria. As a result of chemical weathering, calcium and/or magnesium ions are released from soil into water, while carbon dioxide is transformed into

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