7.1: Components of the Carbon Cycle

What is Carbon?

Before we begin to look at the Carbon Cycle in-depth, let’s very quickly cover a few important facts about Carbon:

• Carbon is the fourth most abundant element in the Universe
• Carbon is essential to all living things… we are “carbon-based” lifeforms… in fact, all life on Earth is carbon-based.
  • Proteins, Fats, and Carbohydrates are all carbon-based as well.
  • By mass, Humans are approximately 19% Carbon
• Carbon is “conserved” on Earth, meaning that the total amount of Carbon on Earth doesn’t change appreciably.
  • Rather, Carbon transitions from one component of the Earth System to another.
  • These processes, and the numerous things that contain carbon on Earth, are a part of the Carbon Cycle
• Carbon can be found everywhere… here are just a few examples:
  • Food: Proteins, Fats, and Carbohydrates all contain carbon.
  • Beverages: Any “fizzy” beverage has been carbonated… pumped with Carbon Dioxide to create those pleasant little “bubbles.”
  • Jewelry: A diamond is entirely carbon!
  • Plants and Animals: All living things have carbon.
  • Natural Gas: Mostly made up of Methane, with a small percentage of other hydrocarbons (such as butane and propane)

As mentioned above, Carbon on Earth is “conserved” meaning that its amount on Earth really doesn’t change (yes, there are small amounts of planetary outgassing and incoming meteors and so on, but the contributions of those components to Earth’s Carbon stockpile are negligible, even over a very long period of time). Instead, Carbon is continuously moving through various reservoirs, such as the human body, the deep ground, or the atmosphere. We call processes that add carbon to a reservoir “sources,” and processes that remove carbon from a reservoir “sinks.” We'll investigate these in the next section.

Carbon Sources, Reservoirs, and Sinks

Figure \(\PageIndex{1}\) provides an overview of the various components of the Carbon Cycle. It may feel overwhelming, but this is actually a fairly simplified picture of the Carbon Cycle. Numbers in white represent the amount of Carbon (in Gigatons, or billions of tons) in a reservoir; yellow numbers represent processes that exchange carbon between reservoirs; and red numbers represent excess carbon introduced into the system by human activities (such as burning fossil fuels and land-use change).

Figure \(\PageIndex{1}\): The Global Carbon Cycle and Estimated Values of Carbon Exchange. (Public Domain; U.S. Department of Energy via Wikimedia Commons). Alternative description of image.

1. The largest carbon reservoir, according to Figure \(\PageIndex{1}\) is:
   1. Soil
   2. Atmosphere
   3. Fossil Pool
   4. Deep Ocean
2. Photosynthesis, a commonly understood process of removing Carbon Dioxide from the atmosphere (these are called “carbon sinks”), removes approximately ___________ gigatons of Carbon from the atmosphere in a given year (hint: include both the yellow natural number and the red anthropogenic number).
   1. 123
   2. 60
   3. 92
   4. 90

Plant biomass is itself a carbon reservoir, holding an annual average of 550 gigatons of Carbon. While photosynthesis is a carbon sink for the atmosphere (since it removes carbon from the atmosphere), photosynthesis is a “carbon source” for plant biomass (meaning that it adds carbon to the plant biomass). On the other hand, respiration by both plants and microbes (via respiration and decomposition) is one of the two key carbon sinks for plant biomass.

3. Combined, plant respiration and microbial respiration/decomposition account for __________ gigatons of Carbon being removed from plant biomass.
   1. 60
   2. 90
   3. 120
   4. 3
4. Since your answer for question 2 represents Biomass’ Carbon source while your answer for question 3 represents Biomass’ Carbon sink, the net amount of Carbon into biomass can be calculated by simply subtracting the amount removed by the sink from the amount added by the source. As a result, there is a net increase of approximately _________ gigatons of carbon in biomass.
   1. 3
   2. 6
   3. 9
   4. 12

This excess Carbon is absorbed into the ground, becoming part of Earth’s soil, and may eventually sink all the way into the fossil pool (which will take tens of thousands of years).

5. Written Question: Now consider the human body as a Carbon reservoir. Name one thing that adds Carbon to the human body (source) and one thing that removes Carbon from the human body (sink). It’s okay to use the internet to research your answer, but please put your answer in your own words and cite your source (note: AI-generated answers are unacceptable and will receive zero credit)

Another important component of the Carbon Cycle is the ocean. Because the deep ocean holds a vast amount of Earth’s carbon, while the surface ocean is home to major sources of photosynthesis (such as phytoplankton), the ocean plays an important role in balancing Earth’s Carbon processes. Unfortunately, as with plant biomass, there is a net increase in carbon being added to the ocean.

6. Of the 9 gigatons of carbon added to the atmosphere by Human activities, approximately ___________ giga tons of that excess carbon is absorbed by the ocean:
   1. 1
   2. 2
   3. 3
   4. 4
7. Combining your answer from question 6 with your answer from question 4 (which showed the amount of excess carbon absorbed into the ground), natural processes are only capable of removing ___________ gigaton’s of the 9 gigatons added by human activities.
   1. 2
   2. 3
   3. 5
   4. 9

Unfortunately, the remaining Carbon is absorbed into the atmosphere, enhancing Earth’s greenhouse effect.