By Ian Pringle
Carbon (C), the fourth most abundant element in the Universe, after hydrogen (H), helium (He), and oxygen (O), is the building block of life. It is the element that anchors all organic substances, from fossil fuels to DNA. On Earth, carbon atoms pass through (cycle through) the land, the oceans, the atmosphere, and even the Earth’s interior in a major biogeochemical cycle. This global carbon cycle can be divided into two categories: the geological carbon cycle, which operates over large time scales (millions of years), and the biological carbon cycle, which operates at shorter time scales (days to thousands of years).
In the geological carbon cycle, carbon moves between rocks and minerals, seawater, and the atmosphere. Carbon dioxide in the atmosphere reacts with some minerals to form the mineral calcium carbonate (limestone). This mineral is then dissolved by slightly acidic rainwater and carried to the oceans. Once there, it can precipitate out of the ocean water, forming layers of sediment on the sea floor. As the Earth’s plates move, through the processes of plate tectonics, these sediments are subducted underneath the continents. Under the great heat and pressure far below the Earth’s surface, the limestone melts and reacts with other minerals, releasing carbon dioxide. The carbon dioxide is then re-emitted into the atmosphere through volcanic eruptions.
The balance between weathering, sub-duction, and volcanism controls atmospheric carbon dioxide concentrations over time periods of hundreds of millions of years. The oldest geologic sediments suggest that, before life evolved, the concentration of atmospheric carbon dioxide may have been one-hundred times that of the present, providing a substantial greenhouse effect during a time of low solar output. On the other hand, ice core samples taken in Antarctica and Greenland have led scientists to hypothesize that carbon dioxide concentrations during the last ice age (20,000 years ago) were only half of what they are today, ie about 200 ppm compared to the present CO2 concentration approaching 400 ppm.
During photosynthesis, plants absorb carbon dioxide and sunlight to create fuel—glucose and other sugars (carbohydrates)—for building plant structures. This process forms the foundation of the biological carbon cycle. Plants and animals effectively “burn” these carbohydrates (and other products derived from them) through the process of respiration, the reverse of photosynthesis. Respiration releases the energy contained in sugars for use in metabolism and changes the carbohydrate “fuel” back to carbon dioxide and other waste products. Together, respiration and decomposition consume organic matter (the latter mostly by bacteria and fungi) returning the biologically fixed carbon back to the atmosphere. The amount of carbon taken up by photosynthesis and released back to the atmosphere by respiration each year is about 3 orders of magnitude greater than the amount of carbon that moves through the geological cycle in the same time.
Carbon storage sites redress the vast time-scale differences between the above carbon cycle categories and the shear volume of carbon in the system. Carbon is stored in four major storage sites and the carbon cycles described above are combined into a biogeochemical cycle by which carbon is exchanged between these reservoirs. As one of the most important cycles on Earth the carbon cycle allows for this critical element to be recycled and reused throughout the biosphere and all of its organisms. These reservoirs are:
The importance of the carbon cycle, one of many cyclic phenomena found on Earth – like moving continents and continental drift resulting in the sub-duction and recycling of continents, annual seasons, lunar cycles, daily tides – cannot be over emphasized. They all play a role in our continued survival on Earth, they all recycle stuff. Without them Earth would become a dead planet like Mars; where little or no recycling takes place!
Earth is a planet where recycling is a matter of life and survival – do your bit!
Geological Carbon Cycle
All the carbon atoms (12C & 13C; excluding radio-active 14C) that cycle through the Earth’s systems today were present at the birth of the solar system 4.5 billion years ago. Individual 14C atoms present today are radio-active atoms and were not present at the birth of the solar system but they do form part of the global carbon cycle today. Since those times, carbonic acid (a weak acid derived from the reaction between atmospheric carbon dioxide and water) has slowly but continuously combined with elements like calcium and magnesium in the Earth’s crust to form carbonate rocks and minerals.In the geological carbon cycle, carbon moves between rocks and minerals, seawater, and the atmosphere. Carbon dioxide in the atmosphere reacts with some minerals to form the mineral calcium carbonate (limestone). This mineral is then dissolved by slightly acidic rainwater and carried to the oceans. Once there, it can precipitate out of the ocean water, forming layers of sediment on the sea floor. As the Earth’s plates move, through the processes of plate tectonics, these sediments are subducted underneath the continents. Under the great heat and pressure far below the Earth’s surface, the limestone melts and reacts with other minerals, releasing carbon dioxide. The carbon dioxide is then re-emitted into the atmosphere through volcanic eruptions.
The balance between weathering, sub-duction, and volcanism controls atmospheric carbon dioxide concentrations over time periods of hundreds of millions of years. The oldest geologic sediments suggest that, before life evolved, the concentration of atmospheric carbon dioxide may have been one-hundred times that of the present, providing a substantial greenhouse effect during a time of low solar output. On the other hand, ice core samples taken in Antarctica and Greenland have led scientists to hypothesize that carbon dioxide concentrations during the last ice age (20,000 years ago) were only half of what they are today, ie about 200 ppm compared to the present CO2 concentration approaching 400 ppm.
Biological Carbon Cycle: Photosynthesis and Respiration
Biology plays an important role in the movement of carbon in and out of the land and oceans through the processes of photosynthesis and respiration. Nearly all forms of life on Earth depend on the production of sugars from solar energy and carbon dioxide (photosynthesis) and the metabolism (respiration) of those sugars to produce the chemical energy that facilitates growth and reproduction.During photosynthesis, plants absorb carbon dioxide and sunlight to create fuel—glucose and other sugars (carbohydrates)—for building plant structures. This process forms the foundation of the biological carbon cycle. Plants and animals effectively “burn” these carbohydrates (and other products derived from them) through the process of respiration, the reverse of photosynthesis. Respiration releases the energy contained in sugars for use in metabolism and changes the carbohydrate “fuel” back to carbon dioxide and other waste products. Together, respiration and decomposition consume organic matter (the latter mostly by bacteria and fungi) returning the biologically fixed carbon back to the atmosphere. The amount of carbon taken up by photosynthesis and released back to the atmosphere by respiration each year is about 3 orders of magnitude greater than the amount of carbon that moves through the geological cycle in the same time.
Carbon storage sites redress the vast time-scale differences between the above carbon cycle categories and the shear volume of carbon in the system. Carbon is stored in four major storage sites and the carbon cycles described above are combined into a biogeochemical cycle by which carbon is exchanged between these reservoirs. As one of the most important cycles on Earth the carbon cycle allows for this critical element to be recycled and reused throughout the biosphere and all of its organisms. These reservoirs are:
- The atmosphere carbon store where carbon is stored primarily as the gas carbon dioxide (CO2). Although it is a small percentage of the atmosphere and is measured in parts per million, it plays an important role in supporting life. Other gases containing carbon in the atmosphere are minute quantities of methane (CH4) and man made chlorofluorocarbons (Strahler, 1965). Since the Industrial Revolution, humans have greatly increased the quantity of carbon dioxide found in the Earth’s atmosphere and oceans. Atmospheric carbon dioxide levels have increased by nearly 40%, from about 275 parts per million (ppm) in the early 1700s to just over 380 ppm today. How do we know this? Modern technology and analytical techniques allow scientists to travel to the tops of glaciers all over the world and drill down into the ice to collect samples. The tiny air bubbles trapped in the ice cores are analysed to measure how much CO2 was in the Earth’s atmosphere at the time the snow fell to form the ice layer that captured the air pockets. Scientist now have accurate records which go back more than 650 000 years (Gore, 2007). The current upward trend shows that future atmospheric levels of carbon dioxide could reach an amount between 450 to 600 ppm by the year 2100. The major source of this increase is due to human activities including fossil fuel combustion and the modification of natural plant cover found in grassland, woodland, and forested ecosystems. Some carbon dioxide is released from the interior of the lithosphere by volcanoes. Carbon dioxide released by volcanoes enters the lower lithosphere when carbon-rich sediments and sedimentary rocks are subducted and partially melted beneath tectonic boundary zones.
- The terrestrial biosphere carbon store, which is usually defined to include fresh water systems and non-living organic material, such as soil carbon. Carbon is released from ecosystems as carbon dioxide gas by the process of respiration. Respiration takes place in both plants and animals and involves the breakdown of carbon-based organic molecules into various by-products including carbon dioxide and methane gas. The detritus food chain contains a number of organisms whose primary ecological role is the decomposition of organic matter into its biotic components.
- The ocean carbon store and its aquatic biomass, dissolved carbon dioxide and marine sediments including calcium carbonate shells. Carbon dioxide enters the waters of the ocean by simple diffusion. Once dissolved in seawater, the carbon dioxide can remain as is or can be converted into carbonate (CO3-2) or bicarbonate (HCO3–). Certain forms of sea life biologically fix bicarbonate with calcium (Ca+2) to produce calcium carbonate (CaCO3). This substance is used to produce shells and other body parts by organisms such as coral, clams, oysters, some protozoa, and some algae. When these organisms die, their shells and body parts sink to the ocean floor where they accumulate as carbonate-rich deposits. After long periods of time, these deposits are physically and chemically altered into sedimentary rocks (limestone and dolomite).
- The lithosphere carbon store in the form of sedimentary limestones and fossil fuel deposits. Carbon is stored in the lithosphere in both inorganic and organic forms. Inorganic deposits of carbon in the lithosphere include carbon bearing minerals like calcite, ankerite, magnesite and siderite but none of these surpass the carbon content of the limestone deposits of the world. Fossil fuels like coal, oil, and natural gas form vast stores of organic carbon, while other organic forms of carbon in the lithosphere include litter, organic matter, and humic substances found in soils.
The importance of the carbon cycle, one of many cyclic phenomena found on Earth – like moving continents and continental drift resulting in the sub-duction and recycling of continents, annual seasons, lunar cycles, daily tides – cannot be over emphasized. They all play a role in our continued survival on Earth, they all recycle stuff. Without them Earth would become a dead planet like Mars; where little or no recycling takes place!
Earth is a planet where recycling is a matter of life and survival – do your bit!
