Terry M. Bella
The carbon cycle can be broken down into two categories, short term and long term. The short term cycle far exceeds the long term cycle in mass of carbon movement annually. Consider though that the bulk of carbon is sequestered in long term store it makes sense that long term storage is much more stable. Short term cycling accounts for ten to one hundred times more volume of carbon movement annually then does long term. Of note though is human activity that moves carbon from long term storage to short term primarily through fossil fuel combustion and cement making. Humans are upsetting the balance of the cycle, consequences of which are yet to be fully substantiated but are believed to be causing climate change.
Short term cycling is referring to the fluxes that occur annually within the atmosphere, terrestrial ecosystems and the ocean surface. This is considered short term because massive amounts of carbon may move into and out of these three reservoirs within minutes, hours, days or years. Conversely, the bulk of carbon that enters into a long term reservoir, the deep ocean and the Earth’s crust, it will be sequestered there for thousands to hundreds of millions of years.
Carbon Exchange: Short Term Cycling
Short term cycling involves the following movements of carbon: ocean surface to the atmosphere, atmosphere to the ocean surface, atmosphere to terrestrial ecosystems, and terrestrial ecosystems to the atmosphere.
Carbon Exchange: Short Term Cycling: Ocean Surface and the Atmosphere
Annually 92GT of carbon is moves into the ocean surface from the atmosphere and vice versa, from the ocean surface to the atmosphere just 90GT annually. The variance is due to human activity releasing carbon dioxide into the atmosphere and ultimately upsetting the balance. This is resulting in an immediate change to the volume of carbon found in the ocean surface which is having dire consequences for ocean life because this increased dissolved carbon dioxide is ultimately driving the pH of the ocean down. This phenomenon is called ocean acidification and will be discussed following a brief description of the processes involved in carbon movement between these two short term stores.
The dissolving of carbon dioxide into the ocean and the release of carbon dioxide from the ocean is controlled by the pressure of carbon dioxide in the two reservoirs. The carbon dioxide pressure is constantly equalizing between the two stores through the passage of carbon dioxide between them. One can account for this flow of carbon knowing that cold water can dissolve more carbon dioxide than warm water.
Gaseous carbon dioxide dissolves readily into the ocean. As carbon dioxide (CO
2
) dissolves into ocean water it forms carbonic acid (H
2
CO
3
). This carbonic acid then breaks down to form hydrogen ions (H
+
) and bicarbonate ions (HCO
3
-
). Bicarbonates break down to H
+
and carbonate ions (CO
3
-2
). Carbonate ions are used by shell forming organisms through binding with calcium ions to form calcium carbonate (CaCO
3
). Many of the shell forming organisms are microscopic and as they are consumed their shells are likely metabolized and the carbon is incorporated into the consumer. Subsequent consumption by larger and larger animals will ultimately result in this carbon being released through respiration, excreted and dissolved, or excreted and deposited on the ocean floor. Ultimately, the carbon may be involved in numerous chemical and biochemical reactions throughout the year. Following the release of carbon dioxide via respiration it may easily diffuse out of the water and back into the atmosphere as a gas.
Secondly, carbon dioxide is used in photosynthesis by phytoplankton, marine algae, and marine plant life. Photosynthesis in the marine environment is identical to that which occurs on land. Driven by the light energy, or photons, the chloroplasts within the cells of photosynthesizing organisms form carbohydrates. The reaction is as follows:
Carbon Dioxide (CO
2
) + Water (H
2
O) + Light Energy -> Carbohydrate (C
6
H
12
O
6
) + Oxygen (O
2
)
These carbohydrates are used in respiration either by the organism that created them, the producer, or by a consumer. Most all organisms process carbohydrates for their chemical energy in a process called cellular respiration. These carbon based molecules go through the following generalized reaction:
Carbohydrate (C
6
H
12
O
6
) + Oxygen (O
2
) -> Carbon Dioxide (CO
2
) + Water (H
2
O) + Chemical Energy
One can imagine that the released carbon dioxide may now dissolve and form carbonic acid, be used by a producer, or perhaps diffuse out of the ocean and into the atmosphere. Between these two reservoirs carbon is moving back and forth throughout the year resulting in the flow of 90GT of carbon moving both into and out of the ocean surface. Of note is that about 2GT is transferred from the ocean surface to the deep ocean with the shell remains of organisms as the vehicle.
There is currently concern about this balance being upset both directly by the release of carbon dioxide by human activity and indirectly by possibility of anthropogenic carbon dioxide causing climate change. Both of which are leading to an acidification of the ocean wherein the normal chemical reactions involved in the cycle are disrupted by the increase of free hydrogen ions.
Ocean acidification caused by an overabundance of carbon dioxide being dissolved, leading to an excess of carbonic acid, unfortunately leading to the formation of bicarbonate ions. Though it is normal for there to bicarbonate ions as carbonic acid breaks down to carbonate, the excess free hydrogens cause the reaction to shift and favor the formation of bicarbonate. Bicarbonate is not a useable ion for shell forming organisms, they require carbonate ions to make calcium carbonate. This is affecting the entire food web of the ocean and may ultimately have dire consequences.
Climate change, whether anthropogenic or not, is causing the upwelling of carbon rich water from depths beyond the ocean surface. Recall that cooler water can dissolve more carbon dioxide. This upwelling is bringing bicarbonate to the ocean surface and acidifying the waters. The upwelling may be caused by increased wind activity along the coastline, pushing surface water out to sea to be replaced with deep ocean water rich in carbon.
Carbon Exchange: Short Term Cycling: Atmosphere and Terrestrial Ecosystems
Terrestrial ecosystems contain about 2100GT of carbon at any given time. The modern atmosphere contains some 750GT of carbon. Annually 120GT of carbon is being exchanged between and within these two reservoirs through natural activity. An additional 9GT is being released into the atmosphere through human activity. Through respiration and decay 120GT of carbon is released into the atmosphere annually. Conversely through photosynthesis 60GT of carbon is sequestered from the atmosphere annually by terrestrial plants and 60GT moves from plants to soils via consumption and metabolism or death and decay. Though the terrestrial ecosystems and the atmosphere pale in comparison of volume of stored carbon when compared to the Earth’s crust of that of the ocean, the activity of carbon flow between them is of great importance.
Photosynthesis, as discussed earlier, is the biological process by which light energy is used by photosynthetic organisms to convert gaseous inorganic carbon dioxide into organic carbohydrate molecules. Annually this accounts for 60GT of carbon being pulled from the atmosphere to make carbohydrates. This is 6 x 10
13
kilograms of carbon a year. In more understandable terms, if one kilogram is equal to 2.20462 pounds, that is roughly 1.24 x 10
14
pounds of carbon annually. Therefore on a daily basis 3.4 x 10
11
pounds of carbon is being biologically processed by photosynthesis. Consider that the planet’s terrestrial surface is only 29% of the total and furthermore this is not accounting for Antarctic and other snow covered regions as well as deserts.
The terrestrial producers, when considering a food web, produce the food for the consumers. As plants are consumed the carbon rich molecules are metabolized and carbon becomes incorporated into molecules of the producer, liberated as carbon dioxide through respiration, or excreted. Additionally, the producer itself may utilize sugars produced through photosynthesis in cellular respiration, releasing the carbon back to the atmosphere. In any case, carbon is moving through and into and out of the terrestrial ecosystem relatively quickly. At this point it may be useful to reflect on the law of conservation of mass as you imagine a single carbon atom on a journey through the different reservoirs and processes that move it along.
If a plant is not consumed while living, its remains will ultimately fall to the ground and become incorporated into the soil. Soils are teaming with microorganisms that will break down organic materials and the carbon will be moved along through various biological processes until it is ultimately released as carbon dioxide gas. Annually 60GT of carbon moves from the living plants and animals of the Earth’s surface to the soil by means of excretion, death, and decay.
Annually a combined total of 120GT of carbon is released, primarily in carbon dioxide, by land plants, animals, and microorganisms of the soil.
Carbon Exchange: Long Term Cycling
The slower aspects of the carbon cycle, deemed long term cycles, are the formation of rocks and fossil fuels. Both of these processes can take thousands to millions, even hundreds of millions of years. By all accounts, 99.958% of the carbon on this planet is “locked” up in the crust. The majority of which is bound in rock formations with just a tiny, albeit important, fraction amassing to 4000GT of hydrocarbons. Hydrocarbons, such as coal and oil, drive this world’s economy, food production, and transportation. Human activity is intimately associated with the carbon cycle and we have an impact on what is likely the slowest of the cycle processes. Fossil fuels that can take hundreds of millions of years to form are being cycled out of storage and into the atmosphere in minuscule geologic time lengths of hundreds of years. Though this may be a point of discussion later on in this unit, it is always useful to set a perspective along the way.
Rock formation, as discussed previously, occurs because of accumulation of organic matter over time that is then covered with successive layers. As the layers pile up the pressure produced on lower levels is sufficient to compact the material into rock. Heat and other tectonic forces play a role as well. Recall that limestone is primarily the shell remains of aquatic organisms and that shale is organic remains mixed with mud and silt. At some point the carbon that is sequestered in rock was once free in the atmosphere, free to be cycle between ocean, air, and terrestrial ecosystems relatively quickly. Year after year just a fraction of carbon is steered towards a long term cycle, rock formation, wherein it will be bound for millions of years. On average over time, an approximately equal amount is liberated from rock annually to again be an active player in short term cycling. Upheaval of rock layers through tectonic activity, exposure of rock from tectonic events, and volcanoes are all methods that set in the motion the movement of carbon out of the store we call the Earth’s crust. Once exposed to the atmosphere rocks are weathered by acid rain and carbon contained within is gassed off as carbon dioxide or transported to soils, rivers, lakes, and oceans as dissolved carbonates.
The carbon that is stored in hydrocarbons and considered as part of the world’s carbon store amounts to about 4000GT. This is the estimate for all the carbon within all the Earth’s coal, oil, and natural gas. The formation of fossil fuels differs from the formation rock in that fossil fuels are derived almost wholly from organic matter. Though it is often assumed that the fossil fuels that we so enjoy today are composed of the carbon that was once the dinosaurs of yesterday, this assumption is wrong. The majority of fossil fuels available were formed from the remains of flora and fauna from times before the dinosaurs. This idea helps one conceptualize just how old this stored energy is. This essentially ancient sunlight is the product of photosynthesis that occurred upwards of three hundred million years ago.
Fossil fuel formation is similar to sedimentary, carbon rich, rock formation in that the carbon was previously within living organisms. Where fossil fuels differ is that they are formed primarily of organic matter, large pockets and layers of organic matter subjected to the same heat and pressure that forms rocks but so rich in organic content and lack in rock that the outcome is a hydrocarbon, the similarities are only general though. Fossil fuels come from a time of a warmer Earth, tropical plant life flourished throughout the globe on land masses distributed differently than today. Ancient swamps, rich in vegetation are the source. Layers of fauna, from microscopic phytoplankton and protoplankton to massive ferns, deposited on swamp bottoms over generations and slowly decayed. The slow rate of decay is attributed to the swampy environment being saturated with water and creating an anoxic zone for the organic matter to rot. This slows the rotting process and layer upon layer of matter accumulated. Periodic changes in sea level would wash up sediment depositing a layer of sand and silt on top of the thick layer of organics. This resulted in massive pockets of material rich in carbon. Over time this process occurred again and again pushing older layers deeper and deeper as successive layers accumulated. This satisfied the necessary condition of pressure to compact the matter. As this layer is moved deeper into the Earth’s crust it is also subjected to heat. The four conditions needed to produce fossil fuels, large stores of carbon rich organic matter, an aerobic environment, intense pressure, and heat were satisfied repeatedly over time some three hundred millions years ago. If it was not for this period of time when the planet was covered in massive swamps we would likely not be enjoying the benefits of hydrocarbons today.
