Follow the Carbon--An Exercise in Geochemical Cycling
Earth Science Extras
by Russ Colson
Introduction to Geochemical Differentiation and Cycling
Although Carbon (C) is only the 11th most abundant element in Earth's crust, it holds an important place because it is the primary building block for life as we know it, and because of its importance in planetary-scale processes such as greenhouse warming (as discussed in a climate-change lesson posted at https://EarthSci4Teachers.com/ESE/greenhouse_warming.html)
Carbon is a major component present in living things, in coral reefs, in diamonds, in soils, in rock (including limestone, coal, black shales, and others). It is a minor but important component in the atmosphere and the oceans.
Thinking about how carbon moves through multiple earth system is a good way to think about geochemical cycling. Understanding any aspect of local or planetary-scale geochemistry, including the cycling of carbon, depends on understanding of two key ideas:
1) Geochemical partitioning and differentiation (The chemical balance or movement between different earth reservoirs through chemical and physical processes). You can pursue the idea of geochemical partitioning and differentiation further if you wish in the following two lessons)
https://earthsci4teachers.com/ESE/gold_pollution_farmland-1.html
https://earthsci4teachers.com/ESE/gold_pollution_farmland-2.html
and
2) Geochemical Cycling (how elements from one reservoir move to another reservoir, and another, until eventually returning to the original reservoir)
These two processes aren't completely independent of each other--geochemical cycling involves many small steps of geochemical partitioning and differentiation. Below is the differences between them as presenting in the book Learning to Read the Earth and Sky (2016) by Russ and Mary Colson, National Science Teacher Association Press. Study the schematic illustrations below to get a better understanding of the two processes, with an important Earth-based example of each.
Geochemical differentiation and cycling emphasize different aspects of the complex processes involved in movement of matter on a planetary scale. The word differentiation comes from the idea of "make different" and emphasizes the ever-changing character of a differentiating world. In contrast, the word cycle emphasizes the steady-state nature of geochemical processes. Both processes can be understood through chemical partitioning and mass balance. A schematic illustration of each of these ideas is shown below.
One of these ideas is not "wrong" and the other "right," but rather each focuses our attention on a different aspect of geochemical processing. On shorter time frames, many chemical components maintain a rough steady state, like water cycling from ocean to mountains and back again to the ocean, or carbon cycling from atmosphere, to life, and back to the atmosphere through metabolic processes. On longer time frames, compositions change, like the changing composition of the Great Salt Lake during cycling of water and evaporation, or the change in carbon concentration in the atmosphere and other reservoirs that contributes to climate change
Inferring Processes that Affect the Carbon Cycle
The Carbon Cycle is not ony an important part of the cycle of life on Earth, but also has implications for world climate change. In working to understand geochemical cycles like the C cycle, it is important to realize that the goal is not to memorize a diagram with lots of arrows (although one is provided at the top of this lesson) but rather the goal is to understand the processes by which an element like carbon moves, and how those processes, and the residence times in different reservoirs, affect the overall earth systems.
The exercises below engage you in thinking about the processes involved in the movement of carbon. These exercises are set up a bit like a 'guessing game", but you can use your understanding of earth processes to do more than just guess. Think about what you know of how the earth works and what process(es) might possibly move carbon from one reservoir to another (for example, everyday experience can suggest to you that carbon present in a tree trunk can get into the atmosphere either by burining of the wood, thus oxidizing the carbon to carbon dioxide, or by decay, which does the same thing). You can also use logic to place constraints on which processes might move carbon into a particular reservoir--for example, ask yourself whether the process would move the carbon in the correct direction to answer the question. Most of us know that photosynthesis produces oxygen, so it doesn't make sense that this process would move oxygen out of the air into plant life. Again, depending on how much you understand the chemistry of common earth processes, these exercises might feel somewht like a guessing game. If you don't want to try to figure these out based on your experience with earth processes, you can read the answer sheet at the end of the lesson. However, I suggest first working through the exercises without that so you can engage in thinking about what processes might be important.
The exercises below also engage you in thinking about how much carbon (including carbon released into our atmosphere by people) might be sequestered in different reservoirs. This is probably even more of a guessing game than the questions on "processes", because the amounts of carbon present in different reservoirs probably isn't something that you know. However, even in this case, a bit of thinking can provide some constraints (for example, which would have more carbon, the atmosphere, or all the rock of the earth?) You can either work through the lessons below first, 'guessing at' the proportions of carbon in each reservoir, or you can look at the 'answer sheet' that I provide at the end of this lesson first, to get an understanding of the rough amounts of carbon in each reservoir. Since there isn't really a way to 'solve' the "relative amount of carbon" questions through reasoning alone, I am giving those particular questions only 'for fun' and they do not count as points against you if you miss them. These questions are shown as 'pop up' questions whereas the "process" questions are imbedded within the lesson.
Proportions of Carbon in Reservoirs and Processes that Move Carbon between Reservoirs:
For your information, here is a diagram that I put together a few years back illustrating in a conceptual way the processes (arrows) by which Carbon moves among different Earth reservoirs (Text). This diagram shows an estimate for how much of the Earth's total carbon is in each reservoir (shown in parentheses). For example, only about 0.001 % (or 0.00001) of the Earth's total carbon is presently in the atmosphere. About 0.0021% (or 0.000021) is in soil and permafrost. So this means that there is about 2.1 times as much in the soil and permafrost as in the atmosphere.
An estimate of how much human produced carbon has managed to find its way into each of those reservoirs is also shown [in brackets]. So, for example, about 23% of human-produced carbon dioxide is in the upper ocean, but only about 1% in the lower ocean. This makes sense because it takes a longer time for the human-produced carbon to work its way down into the deeper ocean.
The arrows indicate different process by which carbon moves among these reservoirs. So, for example, carbon dioxide can move out of the rock of the Earth's interior (the mantle) through volcanism (arrow number 5)--gases, including carbon dioxide, are emitted into the atmosphere during a volcanic eruption. Or the movement of carbon dioxide between the atmosphere and rivers, lakes, and oceans depends on the chemical balance, or partitioning, of carbon dioxide between water and air (arrow D).
Now that we've done our lesson on C-cycling, here is a short primer on global warming:
This lesson examines the historical progress of our understanding of the potential to change world climate through production of carbon dioxide and is a supplement to the theoretical explanation of greenhouse warming posted at https://earthsci4teachers.com/ESE/greenhouse_warming.html
Some historical context on research into the effects of CO2 on global warming
Earth is much warmer than might be expected if one thinks of the Earth as a rock in space, both absorbing energy from the sun and re-emitting that energy back into space. People have known about this 'warmer than expected' concept for a long time. In 1824, Fourier considered a number of possible causes for Earth's warmth, including the possibility that the atmosphere blocks escaping infrared radiation. He compared this "trapping" of heat to a much earlier experiment by de Saussure (1767) in which multiple panes of glass trap heat in a "greenhouse". This is the likely origin of the expression 'greenhouse' warming.
Following up on that earlier work, Tyndall in 1859 found that water vapor and carbon dioxide gas block infrared radiation. "As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial rays, produces a local heightening of the temperature at the Earth's surface." 1862 John Tyndall
In 1896, Arrhenius derived his greenhouse law determining that a change of a factor of 2 in CO2 concentration would change world temperature about 2 to 4°C. Modern estimates are between about 2 and 4.5°C.
In 1897 Chamberlin, a geologist interested in what caused the ice ages and aware of the potential effect of CO2 on world climate, calculated how much of Earth's carbon was in rocks, oceans, and living things, showing that only a tiny fraction is in the air. The take away message for many of Chamberlin's scientific contemporaries was that with so little carbon in the atmosphere, it would be buffered (that is, roughly 'held constant') by the oceans--meaning that humans couldn't change it much.
However, in 1957, Revelle published new work on the chemistry of the oceans which revealed that the simple reaction between air and ocean (CO2 in air ↔ CO2 in ocean) was incomplete and so the oceans might not be able to buffer CO2 concentrations after all. Whereas the simple reaction implied that a 2-fold increase in the atmosphere would correspond to a 2-fold increase in the oceans, meaning that it would take an immense amount of carbon to change either the ocean or atmosphere, the new reaction, shown in part below, suggested that a 2-fold change in the atmosphere would correspond to only a 10% change in the oceans--meaning bigger changes in the atmosphere were possible.
What Revelle realized was that carbon in the oceans exists in many different chemical forms, and when CO2 dissolves in the oceans, it starts a cascade of reactions that will ultimately resist more CO2 going into the oceans. One key idea of chemistry is that if you increase the chemical activity of a component, it will drive reactions in such a way as to get rid of some of that chemical component--that is, increasing the concentration of components on one side of reaction drives a reaction to the other side (Le Chatelier principle). Adding CO2 to the ocean produces H ions and bicarbonate (shown by the left red arrow in the illustration above). That same H ion (shown in red circles in the illustration above) also reacts with carbonate ions in the ocean water, which makes more bicarbonate (shown by the red arrow on the right-hand side of the illustration), which creates a chemical "backpressure" that prevents more CO2 from entering the ocean. Consequently, the oceans wouldn't be able to sop up all of the human-made CO2 after all!
It was also realized that more CO2 in the ocean makes the ocean water more acidic (those H ions in the equation above) and causes more limestone and coral to dissolve, releasing more CO2, and doubling-up the effect of CO2 in the atmosphere. Also, rising temperature makes CO2 less soluble in water --just like the carbonation in your coke goes away faster when it's warm than when it's cold--thus, the warmer it gets, the less CO2 will go into the oceans. In fact, once global warming starts, then the oceans might actually spew more CO2 into the atmosphere, possibly causing a runaway greenhouse event.
So, the theoretical potential for global warming was in place by 1957. But was the earth actually warming?
1938: Callendar argued YES, global warming caused by CO2 was already underway based on his work shown below. Do you see the trends born out over 147 temperature measurement stations beginning in the late 1800s?
In the 1960s, Mitchell said "wait, not so fast!" World temperature took a dip downward in the 1940s, as shown in his data below. Do you see the reason for his doubtfulness about global warming? What would you think if you lived in the 1960s?
In the 1970s, new and better measurements of past climate change yielded the graph below, correlating past temperatures (middle curve) with CO2 and methane in the atmosphere. 'Today' is on the left side of this graph and the right side is over 400000 years ago. The sharp rise in each curve about every 100,000 years corresponds to steep climbs out of Ice Ages! Whoa! Based on this pattern, it looked like we might have hit the peak of the 'warm time' and be due to start sinking back into the next ice age! What would you think? If you lived in the 1970s, you might have thought that maybe we should do all the "greenhouse warming" that we can!
By the 2000s, world temperatures had resumed their strong upward trend (see graph below), and changes in global ice cover were becoming visible even from space (see picture below). At this point, a scientific consensus emerged that, yes, world temperatures were rising beyond the level seen between past ice ages. What would you think?
last updated 3/22/2025. Text and pictures are the property of Russ Colson except graphs from research papers indicated in the text and NASA pictures from space.