from volcanoes; carbonate (CaCO 3 + CO 2 + H 2 . The sinks are carbonate rock weathering + SiO2. Ca HCO

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1 The Carbon Cycle Chemical relations We would like to be able to trace the carbon on Earth and see where it comes and where it goes. The sources are CO 2 from volcanoes; carbonate (CaCO 3 ) formation in oceans 2HCO Ca ++ CaCO 3 + CO 2 O; and human-generated CO 2. The sinks are carbonate rock weathering CaCO 3 O + CO 2 Ca HCO - 3 ; and silicate rock (we use CaSiO 3 for general silicate rock) weathering 2CO 2 O + CaSiO 3 Ca HCO SiO2. While the atmosphere takes in 52 ± 4% of the carbon emitted each year (Ch. 15), the ocean takes in 37 ± 4%, and phosphorus matching accounts for 2%. Atmospheric CO 2 levels were higher 65 Myr BP, when temperatures were roughly 10 C warmer than now. (132) Figure E shows various processes described by the chemical equations above. The net result of carbonate weathering and formation is zero change. The net result of silicate weathering and carbonate formation may be summarized as CaCO 3 + SiO 2 CaSiO 3 + CO 2. The cycle operates in long-term balance the rock part of the cycle lasts about 500,000 years. For the early Earth, the CO 2 concentration was 100 to 1,000 times present values. (132) Silicate weathering decreased the concentration to near the present levels. For all gases present in the atmosphere in large amounts, geological cycles work to stabilize concentrations.

2 Energy, Ch. 16, extension 3 The carbon cycle 2 Fig. E An illustrated version of the carbon cycle: atmospheric carbon dioxide erodes rocks, carbon put into shells of marine organisms, sinks to bottom to form rock, rock is melted and carbon dioxide is released to the atmosphere by volcanoes. C 3 and C 4 photosynthesis The amount of O 2 stored in rock every year, for example, is about the amount added from weathering. (133) In 400 kyr, all the atmosphere s CO 2 would be gone without the recycling of 12 to 27 gigatonnes per year (Gt/yr) from volcanoes and seafloor spreading. (132) Even so, silicate weathering is so efficient that eventually, life will no longer be possible; 150 ppm of CO 2 is needed for C 3 photosynthesis (this type proceeds with three carbons). (134) Using this information, one estimate had life on Earth ending in one to several hundred million years. Others noted that C 4 plants (using four carbons) can

3 Energy, Ch. 16, extension 3 The carbon cycle 3 persist down to concentrations of a mere 10 ppm. (134) Thus, C 4 plants can survive for another 1.2 ± 0.3 billion years. (134) The difference between C 3 and C 4 plants is the two differing ways they photosynthesize. Two competing processes occur in C 3 plants: photorespiration (fixing O 2 ) and photosynthesis. In a sense, C 3 plants are undoing some of what they do because of their use of the enzyme known as Rubisco (see Ch. 22), so the C 3 plants are less efficient at photosynthesis than C 4 plants. C 4 plants fix CO 2 and jam it into cells, blocking the oxygen reaction. The C 3 plants are more effective below 28 C, while C 4 plants are more effective above it. (135,136) Similarly, C 4 plants do better at low CO 2 concentrations. (136) For some time, there have been puzzles about which plants would do better in a carbondioxide-richer future, and people looked back to see no knockout changes, with one plant getting a definitive upper hand. It became clear that water had something to do with plant success as well. Water-use efficiency increases as carbon dioxide concentration rises, so plants in a richer carbon dioxide environment do not so much water to flourish as they would in a carbon dioxide poor environment. (137) C 4 grasses flourished in the mountains even in glacial times, making tree line a dangerous proxy for local temperature. (137) An analysis of the late Miocene grassland showed that despite differences in C 3 and C 4 successes, there was no major change in carbon dioxide concentration; they inferred that the reason must have to do with water abundance. (138) Finally, the story became clear: in two sites only 2,000 km apart at the same time, C 3 plants took over one ecosystem and C 4 grasses the other. The carbon dioxide concentration was the same, but neither type of plant was the victor. The answer was that the wetter ecosystem supported the takeover of the C 3 plants, while the drier ecosystem allowed the C 4 grasses to flourish. (139)

4 Energy, Ch. 16, extension 3 The carbon cycle 4 It appears that the drier ecosystems will be subject to invasion by C 4 grasses when the carbon dioxide concentration increases. In an experiment in a Mojave Desert ecosystem, it was found that in the wet years, local plants did well, but in drier years, they suffered relative to invading species, and the species composition lost natives in favor of invaders. (140) Carbon cycling Because the concentration of CO 2 in the air is so small, though, the concentrations can change substantially over small times. The CO 2 concentrations in rock also change in response to increased CO 2 concentrations in the air, but there is a rather long delay. Carbon is stored in the various reservoirs in large amounts. Plants on land contain 500 to 900 Gt of carbon, and the soil contains twice as much. ( ) Forests contain about 10 to 20 times more carbon than cropland. (145,146) The atmosphere contains around 700 Gt of carbon. ( ) The residence time in the atmosphere is only 50 to 300 years. Of this amount, about 140 Gt of carbon is from burning of fossil fuels. (141) The concentration of CO 2 is kept within a rather narrow range. If it were less than one-third its present value, most photosynthesis would likely cease. (133) During the last ice age (see Fig ) about 20,000 years ago, CO 2 was present at about 200 ppm. (133,142) During the last warm period, it was about 400 ppm. The hundred-thousand-year record shows that it was possible to have changes in concentration of 50 to 80 ppm over a period less than a century. (150,151) The sea takes up carbon dioxide in a surface layer. The amount of carbon in the ocean depends on the temperature. At higher temperature, liquid water can hold less carbon

5 Energy, Ch. 16, extension 3 The carbon cycle 5 dioxide gas in solution. The oceans contain about 50 petatonnes (Pt) of sedimentary carbonates and 38 teratonnes (Tt) of dissolved inorganic carbon. They also contain at least 20 Pt of dissolved organic carbon, (145,146) perhaps more. (152) Most carbon in the ocean is in the form of ions of carbonate or bicarbonate, not dissolved carbon dioxide. (145) As atmospheric CO 2 increases, however, the oceans take up only one-ninth the increase. (141,145,146,153) The residence time in the ocean abysses is several thousand years. Some carbon dioxide is used in marine biomass (total mass about 3 Gte); the CO 2 is then cycled into sediments or into the carbon reservoir of the deeper ocean at the rate of 2 to 3 Gt per year. (154) In the oceans, a biological pump operates (145,146, ) to take CO 2 from the atmosphere into the ocean: The plants and animals taking up the carbon convert it to skeletal material, live out their life cycles, and die (see Figure 14.12). Their skeletons sink to the ocean depths, where the carbonate is partly deposited in sediment and partially dissolves but cannot mix due to temperature and salinity differences. On the shallow continental shelves, some carbon is cycled into sediment along with nitrogen and oxygen. (147) The current surface water is more acidic than the water in the ocean deeps. As this water circulates, it will cause calcium carbonate to dissolve, releasing more carbon dioxide into the seawater; much of this will be released to the atmosphere over a time period of several hundred years. (159)