Carbon on earth

This treatment of the carbon cycle is intended to give an account of the fundamental aspects of the carbon cycle from a global perspective. After a presentation of the main characteristics of carbon on Earth (Section 11.2), four sections follow 11.3, about the carbon reservoirs within the atmosphere, the hydrosphere, the biosphere... 

There are more than a million known carbon compounds, of which thousands are vital to life processes. The carbon atom s unique and characteristic ability to form long stable chains makes carbon-based life possible. Elemental carbon is found free in nature in three allotropic forms amorphous carbon, graphite, and diamond. Graphite is a very soft material, whereas diamond is well known for its hardness. Curiosities in nature, the amounts of elemental carbon on Earth are insignificant in a treatment of the... 

As mentioned in the Introduction, almost all natural carbon on Earth is represented by the isotope 12C, thus the concentration 14R can be defined as the ratio between 1he number of atoms of 14C and atoms of 12C. 

Learn about the history, chemistry, and industry of diamonds— the hardest form of carbon on Earth—by exploring these Web... 

Obviously this wide distribution of the 14C formed in the atmosphere lakes time it is believed to require a period of 500-1000 years. This time is not. however, a deterrent to radiocarbon dating because of two factors die long half-life of I4C and the relatively constant rate of cosmic-ray formation of l4C in the earth s atmosphere over the most recent several thousands of years. These considerations lead to the conclusion that the proportion of 14C in the carbon reservoir of the earth is constant, and that the addition by cosmic ray production is in balance with the loss by radioactive decay. If this conclusion is warranted, then the carbon dioxide on earth many centuries ago had the same content of radioactive carbon as the carbon dioxide on earth today, Thus, radioactive carbon in the wood of a tree growing centuries ago had the same content as that in carbon oil earth today. Therefore, if we wish to determine how long ago a tree was cut down to build an ancient fire, all we need to do is to determine the relative 14C content of the carbon in the charcoal remaining, using the value we have determined for llie half life of 14C. If the carbon from Ihe charcoal in an ancient cave has only as much 14C radioactivity as does carbon on earth today, then we can conclude that the tree which furnished llie firewood grew 5730 30 years ago. 

Figure 3.6. A block diagram of the global biogeochemical cycle of carbon on Earth. Carbon reservoirs and fluxes are described in Table 3.3.

The total quantity of carbon on Earth is about 41,000 billion metric tons (92% in the oceans, 6% on land, and 2% in the atmosphere). Prior to the Industrial Age, the concentration of C02 in the atmosphere was stable and balanced. Two hundred and ten billion tons of carbon dioxide entered the atmosphere and approximately the same amount was taken from the atmosphere by the photosynthesis of plants. That balance has been upset by fuel combustion, deforestation, and changing land use as the population increased. 

Nearly all of the carbon on Earth is present as the stable isotope carbon-12. A very small percentage of the carbon in Earth s crust is carbon-14. Carbon-14 undergoes decay to form nitrogen-14. Because carbon-12 and carbon-14 have the same electron configuration, they react chemically in the same way. Both of these carbon isotopes are in carbon dioxide, which is used by plants in photosynthesis. 

Figure 26-18 shows the carbon cycle. Carbon dioxide in the atmosphere is in equilibrium with an enormous quantity that is dissolved in oceans, lakes, and streams. Some of this dissolved CO2 was once in the form of calcium carbonate (CaC03), the main component of the shells of ancient marine animals. The shells were eventually converted into limestone, which represents a large store of carbon on Earth. When the limestone was exposed to the atmosphere by receding seas, it weathered under the action of rain and surface water, producing carbon dioxide. Some of this CO2 was released into the atmosphere. This process continues today. 

At the present time the balance of evidence is against a cometary origin for prebiotic carbon on Earth, for the same comets would also have delivered water to the Earth and yet the D/H ratio of the terrestrial oceans is different from that in comets. The more likely extraterrestrial input is from asteroids and meteorites, for there is evidence from both lunar and terrestrial samples that the late heavy bombardment event at 3.9 Ga (Section 6.4.1) contributed meteoritic material to the Earth at this time. 

There are three naturally occurring isotopes of carbon carbon 12, which makes up 98.9 percent of all the carbon on Earth, carbon 13, which makes up 1.1 percent of the carbon, and carbon 14, which is radioactive... 

The dominant fraction of carbon on earth (see Table 2.12) is termed kerogen a mixture of organic chemical compounds that make up a portion of the organic... 

FIGURE 13.1 The two major cycles of organic carbon on earth. Most organic carbon is recycled within cycle I. The crossover from cycle I to cycle II is only a tiny leak that amounts to only 0.01-0.1% of the total primary productivity. (Reprinted with permission from Reference 20, Springer-Verlag, Copyright 1978.)... 

Carbohydrates are found in all living organisms. Indeed, they are the most abundant of the natural organic compounds. It is estimated that well over half the organic carbon on earth is in the form of carbohydrates, the great majority of it in plants. Almost three-fourths of the dry weight of plants is carbohydrate, most of which is in cell walls (structural components). In higher land plants, these carbohydrate components of the cell wall are cellulose, the hemicelluloses, and the pectic substances. The subject of this chapter is the carbohydrates other than those that are constituents of primary or secondary cell walls. 

Cellulose, next to chitin, is the most abundant biopolymer on earth. It is the major constituent of plant cell walls and more than half of the organic carbon on earth is fixed in cellulose. It is composed of unbranched, hnear chains of d-glucose molecules finked through 1,4-P-d glycosidic bonds (Figure 11.2). 

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