Carbon is sequestered in several different reservoirs. The reservoirs vary in size (total volume of carbon) and relative sequester time period (how long any given carbon atom is maintained in the reservoir). I find that the easiest organization of stores is into four categories: Earth’s crust, oceans, the atmosphere, and terrestrial ecosystems. These stores will be further divided into smaller categories. The amount of carbon in each of these reservoirs is given in gigatonnes (GT) with numbers acquired from NASA. A gigatonne is equal to 1 x 10
kilograms. These numbers are recent estimates of the mass of carbon but are not exact. Nonetheless the numbers are useful when comparing one reservoir to another when comparing reservoirs.
Carbon reservoirs are defined as being long and short term. Long term carbon stores find carbon being held for lengths of time on the order of millions to hundreds of millions of years. When defining a carbon reservoir as being short term the time period of sequestration is on the order of years to hundreds of years. In order to grasp the scope of the carbon cycle, and ultimately man's impact on it, it is important to note the approximate holding periods of carbon as it passes through reservoirs. The impact of man on the carbon cycle, as long term stores are being manipulated by current activities, is now a necessary component of understanding the cycle as a whole.
The processes that move carbon from reservoir to reservoir will be discussed in a subsequent section. This section will define the reservoirs and discuss current estimates of the mass of carbon contained in each as well as discussing the relative amount of time any given carbon atom is expected to spend in the store.
Carbon Reservoirs: The Earth’s Crust
This reservoir encompasses sedimentary rocks and hydrocarbons. It is estimated that the mass of carbon in this store is roughly 100,000,000GT. Limestone and shale are the sedimentary rocks that amass a significant portion of this store, approximately 99.996%. Hydrocarbons such as petroleum, natural gas, and coal account for the remaining portion. These two groups are considered separately because of their different uses to man.
Limestone is a chalky, white rock and shale is typically a dark brown to red or black color. Both are relatively soft as is the nature with sedimentary rocks. Sedimentary rocks are formed as layers of material are piled on top of each other over the years. As they are buried by still more layers of sediment they are compacted by pressure of the subsequent layers. Over the course of millions of years and just as many layers of sediment the material is compacted into rock.
Limestone is primarily composed of calcium carbonate deposits that are the remains of shell forming organisms and corals. Innumerous amounts of ocean dwelling creatures form shells by combining carbon and calcium. When these organisms die their remains drift to the bottom of the ocean and this carbon source is deposited. These deposition layers, of organic source calcium carbonate, are lithified into limestone. Limestone forming events occur primarily in shallow warm seas, where the conditions for the necessary organisms are best. These shallow sea environments are found in shallow between 30 degrees north latitude and 30 degrees south latitude. Areas of importance today are the Caribbean Sea, Indian Ocean, Persian Gulf, Gulf of Mexico, around Pacific Ocean Islands and within the Indonesian archipelago.
Limestone is a general term for sedimentary rock composed primarily of calcium carbonate and thus limestone has many forms. Tufa, chalk, coquina, fossiliferous limestone, lithographic, travertine, and oolitic limestone are all common forms of limestone.
Carbon trapped in limestone is subjected to release when the limestone has been exposed to the surface. Rain, which is naturally acidic, weathers these rocks releasing the carbon back into the environment. This process will be discussed in subsequent sections of this unit. A significant point of discussion is the use of limestone across the globe. Ultimately the use of limestone as a commercial commodity results in the movement of carbon from the reservoir wherein it will find its way back into the cycle. Limestone is used a dimension stone in construction for such items as tiles and sills to structural blocks or even statues. Asphalt shingles are embedded with crushed limestone as it acts as an effective weather and heat resistant layer to the shingle. Limestone, along with shale and sand, is the key ingredient in Portland cement. The metal refining industry uses limestone as a flux. Aglime is used to reduce the acidity of soils worldwide, as well is lime. Limestone is found in animal feed as an easy form of calcium supplement for chickens, to form strong eggs, and dairy cows, for milk production.
Shale is formed from the deposition of organic matter that has been covered and mixed with mud and silt. As is the process with other sedimentary rocks layers accumulate over eons and combined with pressure and other tectonic forces the rock is formed. Shale is used to produce clays and common building materials such as brick and cement. Recent discoveries of hydrocarbons with shale have diversified the importance of shale in the overall carbon cycle discussion. We now extract natural gas from shale through hydraulic fracturing (hydrofracking).
Certain shale deposits have high amounts of natural gas trapped within them in tiny pores. Although this was known about for years, it was not until the 1990’s that drilling companies developed an effective method of liberating the hydrocarbon. Essentially they drill down to the shale and then pump down water at high pressure to fracture the rock and allow the gas to flow out into the well. The natural gas that is trapped in shale shows an effective connection between the two reservoirs of carbon and why it is responsible to consider the Earth’s crust as single reservoir and simplify discussions in the classroom.
The latest estimate for the amount of carbon in hydrocarbons within the Earth’s crust is 4,000GT. Hydrocarbons, also called fossil fuels, are the product of ancient organic matter that was trapped underground and subjected to intense pressure and temperatures. The intent of this unit is to discuss the movement for carbon from a hydrocarbon to another reservoir, mainly the atmosphere, and not to address the world of controversy surrounding the extraction, control, and use of hydrocarbons across the globe. This unit will approach the math for calculating the amount of carbon that is moving from hydrocarbons to the atmosphere as it is important for students to realize that the amounts are quantifiable. By sharing these calculations with students the connection between energy use and their impact becomes more salient.
Carbon Reservoirs: Oceans
This reservoir includes all of the world's ocean water and flora and fauna living within. Discussing the ocean as a reservoir requires that ocean surface, or epipelagic zone, and all depths below are considered separately. Depths below the epipelagic will be referred to as “deep ocean” within this unit. Note that there are several layers or zones, but differentiating factor is light penetration. Light penetration is limited to the epipelagic. The best estimate for total carbon in the ocean is 39,000GT, with 38,000GT of that in the waters of the deep ocean. The epipelagic zone contains the remaining balance of carbon in the ocean, accounting for the biota that live in the ocean, primarily in this zone of water. Thus the majority of carbon found in the ocean is bonded within inorganic carbon molecules. A small portion is dissolved carbon dioxide and carbon that is being used in organic molecules mostly limited to the epipelagic zone.
The ocean surface, or epipelagic zone, extends about 200 meters in depth. Sunlight penetrates this zone supporting a rich array of life. The ocean’s primary producers, phytoplankton up and through to the apex consumers live within this zone. The enormity of the ocean is hard to substantiate and it is fascinating that the majority of all oceanic life is found in just the top 200 meters, considering that the average ocean depth is around 3700 meters, with a maximum depth of 11,000 meters.
The carbon accounted for in the epipelagic zone, just about 3% of the carbon in this reservoir, the oceans, is significant to the carbon cycle because of its role in short term carbon cycling. This carbon is bound in either organic molecules or as inorganic carbon dioxide. The biota of this zone is utilizing carbon for growth and maintenance. This biological carbon is then released through death and decay. The flora of the ocean utilizes dissolved carbon dioxide to drive photosynthesis producing carbohydrates. Plants and algae are consumed and the carbohydrates are metabolized being incorporated into proteins and other biological molecules and being released back into the water through respiration. Consider all of the fish, coral, crustaceans, mammals, plants, and algae utilizing, either directly or indirectly, the dissolved carbon dioxide to drive life. The inorganic component of this this faction is in the form of dissolved carbon dioxide. Carbon dioxide reacts with ocean water to form carbonic acid and bicarbonate. Bicarbonates are used by plants and animals to form calcium carbonate shells. This calcium carbonate will often find its way to the bottom of ocean where it will deposit and eventually move to another reservoir, the earth’s crust, as limestone.
The deep ocean, for the purposes of this unit, accounts for the other 97% of carbon found in this reservoir. Here carbon is found bound in organic molecules such as carbonic acid and bicarbonates. The majority is found in bicarbonates. Another point of separation between deep ocean and ocean surface, in addition to sunlight penetration, is the relative time that carbon can spend in the reservoir. The ocean surface is a short term reservoir and carbon is cycling in and out of the ocean surface rapidly. The deep ocean is a long term reservoir wherein carbon can be stored for thousands to millions of years.
Carbon Reservoirs: Atmosphere
There is just 750GT of carbon in the atmosphere. Though this number is low compared to the massive store in the Earth’s crust and the oceans, it is of significant importance to human activity. The carbon molecule of interest in this store is carbon dioxide which accounts for the bulk of carbon in the atmosphere. The remaining balance of atmospheric carbon is in molecules of methane and other molecules. The amount of carbon dioxide in the atmosphere is believed to have an influence on global climate. Though the debate is ongoing as to the effect of carbon dioxide on climate, there is solid evidence that global atmospheric carbon has risen 34% over the past few hundred years. Atmospheric carbon is bound in carbon dioxide primarily. Less significant amounts are accounted for in methane and various other molecules. The atmosphere is a short term store of carbon, with carbon cycling in and out of the atmosphere relatively rapidly, on the order of years, months, and days.
Atmospheric carbon is being released and sequestered through biological processes and physical processes. Photosynthesis by terrestrial plants is sequestering carbon dioxide which subsequently liberated through respiration and decay. Physical processes release carbon through weathering of rocks and gas exchange on the ocean’s surface. Subsequently carbon is sequestered from the atmosphere via gas exchange, photosynthesis, and deposition.
Carbon Reservoirs: Terrestrial Ecosystems
This reservoir encompasses soil and land plants and totals to approximately 2100GT. Included in this store are all the terrestrial animals as well, though their total mass of carbon is insignificant compared to the plants and soil. This carbon is primarily found in organic molecules. Carbon is fundamental to every living thing. This is the carbon making up every plant, animal, and microorganism. Organic molecules found in living, dead, and decaying organic matter.
The soils hold the bulk of the carbon in this reservoir accounting for roughly 75%. Carbon is processed in the soil by microbes which through decomposition release the carbon back to the atmosphere as carbon dioxide. On land woody plants are the most significant players in this store because of their ability to store enormous amounts of carbon in the cellulose that they are comprised of.