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In Episode Three of the TV Series, the Carbon Cycle is mentioned in passing. Here we present some expanded information. Where is Carbon? In the universe: Carbon is the fourth most abundant element in the universe (after hydrogen, helium, and oxygen). In earth, air and oceans: In the Earth's mantle, less than 1 percent of the mineral content is carbon. Sometimes it appears as diamonds or graphite. About 0.033 percent or 330 ppm is the average level in the atmosphere of carbon dioxide. The oceans contain a vast amount of dissolved carbon. In life forms: Something like half the dry weight of all living things is carbon. That's why we can be called 'carbon-based' life forms. The unique structure of this element allows it to combine in many ways with many other molecules. The Cycle Carbon cycles through land, ocean, atmosphere, and mineralized deposits in a 'bio-geo-chemical' cycle. Much of this movement is due to the fact that carbon is a part of all life processes. Carbon, as carbon dioxide, is taken up from the atmosphere (or ocean) and incorporated into the bodies of plants by photosynthesis. As the plants live, respire and decay, this carbon is returned to the atmosphere. All life is in fact part of this cycle, for all life forms that don't make their own food through photosynthesis (or rarely, chemosynthesis) must consume energy that is prepared by others that do. And likewise, they give it back at the other end of the food chain, in the detritus ecology, where organic substances are broken down into their elemental components once more. Life Processes Incorporating Carbon Photosynthesis: Photosynthesis is the series of enzyme-moderated chemical changes that allows green plants to use the Sun's energy to create carbohydrates. These simple sugars can then be burned as energy or used to manufacture all manner of organic molecules, including proteins, cellulose, and other components of living things. Chlorophyll can absorb photons straight out of the sky, by allowing sunlight to bump some electrons into higher energy orbits. These electrons are split away with enzymes, and replaced with hydrogen ions split from water by yet more enzymes. Specialized plant membrane channels (and more enzymes) then shuffle molecules and ions around various energy gradients. The original electrons go back to be re-energized by sunlight. 6CO2 + 12H2O + energy -----> C6H12O6 + 6O2 + 6H2O Most of the energy released by these activities is used up fuelling the ongoing system; about one sixth is finally expressed as glucose. Sugar of Life Glucose is a simple carbohydrate that consists of 6 atoms of carbon, 12 atoms of hydrogen and 6 atoms of oxygen (the C6H12O6 in the formula above.) This carbohydrate is the basis for many more biological compounds and processes; it's the very substance your brain is burning as you read this. Protein Synthesis The chloroplast bearing photosynthesizers (green plants) use the intermediate stages of carbohydrate metabolism to create amino acids. Proteins are constructed from these base substances, under the direction of DNA, to provide the building blocks for life's diversity. Geological Systems Incorporating Carbon Fossilized Organics Are Carbonaceous During certain periods in the Earth's past, conditions were very good for the growth of plants. Photosynthesis far exceeded respiration. Vast quantities of plant growth left huge deposits of organic matter that, over millions of years of pressure and slow chemical metamorphosis, became coal, gas and petroleum; our 'fossil fuels.' And they don't call the major components of these deposits 'hydrocarbons' for nothing. My Sediments Exactly Carbon dioxide molecules find their way into the ocean by simple diffusion from the atmosphere. Once dissolved, it may become carbonate or bicarbonate. Many organisms, such a corals, molluscs, and phytoplankton can biologically fix carbonates with calcium to become calcium carbonate, from which they elaborate their shells or other structures. Once the organism is finished with them, these bioengineered parts sink to the ocean floor, eventually to become carbon-rich substrate, and then, finally, after eons of pressure and chemical changes, sedimentary rock. These deposits of limestone are the greatest 'sink' of carbon on the planet. (A sink here refers to carbon that is not at present cycling through the system.) Here are some approximate values for the major sinks or deposits of carbon on Earth, in unit of billions of metric tonnes. Give or take the odd pail full, of course. Planetary Sink | Amount in Billions of Metric Tonnes | | Marine Sediments and Sedimentary Rocks: | 66,000,000 to 100,000,000 | | Oceanic: | 38,000 to 40,000 | | Fossil Fuel Deposits: | 4,000 | | Organic Matter in Soil: | 1,500 to 1,600 | | Atmospheric: | 575 back in AD1700, 765 as of AD1999 | | Terrestrial Plants: | 540 to 610 | Some of these carbonaceous sinks are much more active than others, and so have vastly different effects on our environment in the short term. Slow but Steady The inorganic part of this carbon migration occurs over the geological time frame, millions of years. Mostly it involves stores of carbon being taken out of the cycle and stored in sediment. This had led to a gradual lowering of atmospheric concentrations. Again, over the very long term. As I Live and Breathe Each year the world's ecosystems withdraw carbon from the atmosphere through photosynthesis and add it back through respiration and decay. Respiration and Decomposition Carbon is released back into the atmosphere from ecosystems as carbon dioxide gas in the process of respiration. All living things respire; it involves the breakdown of carbon-based organic molecules into carbon dioxide gas and some other compounds. As well, living things eventually decompose. The so-called detritus food chain employs all kinds of creatures (Saprophytes) whose role in the ecology is disassembly of organic matter into non-biological components. Ephemeral Life The organic portion of the cycle is in a much greater state of flux. Seasonal differences, for example, are quite apparent in atmospheric concentrations of carbon dioxide, in response to the Sun's effect on summer photosynthesis. (See 'the Keeling Curve') The Long and the Short View Over epochs of geological time, the amount of carbon dioxide found in the atmosphere had been steadily decreasing. Now we seem to be putting it back. Atmospheric carbon has been increasing steadily since the beginning of the Industrial Revolution. (Pockets of air trapped in the northern ice sheets that dates from various epochs shows no great variance previous to the last three hundred years.) Air of Change Over the last three hundred years, the amount of carbon dioxide found in Earth's atmosphere and oceans has changed dramatically. Atmospheric levels have increased by over 30 percent, from about 275 parts per million (ppm) in the early 1700s to just over 365 ppm today. The now infamous measurements known as 'the Keeling curve' illustrate this. This increase in atmospheric carbon is generally accepted to be 'anthropogenic,' that is, a result of human activity. A great deal (about 65 percent) of this extra carbon dioxide can be attributed to emissions from combustion of fossil fuels, and the remainder derived from a number of human activities such as deforestation, the conversion of large areas to agricultural production, and the byproducts of certain industries. Some researchers estimate that atmospheric levels of carbon dioxide could attain concentrations of 450 to 600 ppm by the end of the century. This would likely cause repercussions well popularized as the 'greenhouse effect.' Have Some Perspective The amount of carbon taken up by photosynthesis and released back to the atmosphere by respiration each year is about a thousand times greater than the amount of carbon that moves through the geological cycle on an annual basis. The carbon that is sequestered each growing season by photosynthesis and then released, by respiration, back into the atmosphere produces very definite seasonal oscillations in atmospheric carbon dioxide concentrations. This seasonal flux of carbon is more than ten times greater than that released to the atmosphere by our burning of fossil fuels. The Missing Sink As far as we can tell, a very great deal more carbon than can be accounted for has been released by all known sources across the globe. Somewhere on the order of 1.8 Billion metric tonnes extra, each year. We're unsure of where this carbon has gone. Some researchers suggest that vegetation in the northern hemisphere, specifically North American forest cover, is sequestering a greater quantity of carbon than before. Another theory proposes that oceanic changes in currents and nutrients have made it possible for various phytoplankton and other species of plant life to increase to new levels of growth. Researchers have shown that increased carbon dioxide levels leads to greater 'net production' by plants. (Net production is the amount by which growth, in net caloric value, is exceeding respiration. More plant biomass would be a result.) It would be nice to have clearer answers here in the light of all our concerns of global warming but there are many hindrances to collection of good data. We can't measure the exact uptake and respiratory activity of all the terrestrial vegetation in the world, let alone that in the oceans. But somehow and somewhere this large 'missing sink' of carbon is being sequestered from the cycle. We're all still working this out. As with many of the natural processes touched on in The Sacred Balance... we've barely scratched the surface.
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