![[This diagram of the carbon cycle shows the major flows in the "fast" carbon cycle and the main reservoirs of the carbon cycle as a whole (both the "fast" and "slow" carbon cycles). Reservoirs are labeled with white text in units of gigatons of carbon (GtC). Arrows indicate flows of carbon between reservoirs and are labelled with yellow text; flow rates are in gigatons of carbon per year (GtC/y). The sizes of the arrows are roughly proportional to the amount of carbon in the various flows. Red text and numbers indicate changes to the natural carbon cycle caused by human activities, primarily burning of fossil fuels, cement production, and land use changes. Source]](attachment:8407a973-f4c2-413a-9453-79f2380c489a:carbon_cycle_diagram_doe_labeled_900x700.jpg)
[This diagram of the carbon cycle shows the major flows in the "fast" carbon cycle and the main reservoirs of the carbon cycle as a whole (both the "fast" and "slow" carbon cycles). Reservoirs are labeled with white text in units of gigatons of carbon (GtC). Arrows indicate flows of carbon between reservoirs and are labelled with yellow text; flow rates are in gigatons of carbon per year (GtC/y). The sizes of the arrows are roughly proportional to the amount of carbon in the various flows. Red text and numbers indicate changes to the natural carbon cycle caused by human activities, primarily burning of fossil fuels, cement production, and land use changes. Source]
Look at the atmosphere. It contains 800 GtC of carbon. Look at the arrows going in and out. What do you notice?
We often think of "carbon" as an abstract element that moves around, but it's actually always in specific chemical compounds. Here's a breakdown:
Carbon forms in different stores
| Store | Chemical form(s) | Organic or inorganic? |
|---|---|---|
| Atmosphere | Carbon dioxide (CO₂), methane (CH₄) | Inorganic |
| Green plants | Glucose (C₆H₁₂O₆), cellulose, starch, proteins, lipids, nucleic acids | Organic |
| Animals | Proteins, lipids, carbohydrates, nucleic acids | Organic |
| Phytoplankton | Same as plants: glucose, proteins, lipids | Organic |
| Soil | Humus (partially decomposed organic matter), living organisms, some calcium carbonate | Mostly organic |
| Surface ocean | Dissolved CO₂, bicarbonate ions (HCO₃⁻), carbonate ions (CO₃²⁻) | Inorganic |
| Shellfish/corals | Calcium carbonate (CaCO₃) in shells and skeletons | Inorganic |
| Fossil fuels | Hydrocarbons (coal: complex C compounds; oil: liquid hydrocarbons; gas: mainly CH₄) | Organic |
| Limestone | Calcium carbonate (CaCO₃) | Inorganic |
Organic carbon: Carbon bonded to other carbons and hydrogen in complex molecules. Made by living things (or derived from them, like fossil fuels).
Inorganic carbon: Simple carbon compounds like CO₂, carbonates and bicarbonates. Not made by living processes (though living things can use or produce them).
This connects directly to transfers vs transformations:
When we say "the ocean absorbs CO₂", the carbon doesn't just sit there as CO₂. Most of it reacts with water:
CO₂ + H₂O ⇌ H₂CO₃ (carbonic acid) ⇌ H⁺ + HCO₃⁻ (bicarbonate)
This is why increased CO₂ leads to ocean acidification - those H⁺ ions lower the pH.
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Biogeochemical cycles ensure chemical elements continue to be available to living organisms.
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![[The Pearson ESS Textbook]](attachment:499db1d4-fb93-4683-a550-753057139e69:1769422047105-29e8bf1e-5240-41a5-a8a9-0a8242f920b2_1.jpg)
[The Pearson ESS Textbook]
| Colour | Boxes | Notes |
|---|---|---|
| Light blue | CO₂ in air | Atmospheric store (inorganic) |
| Green | Green plants | Terrestrial producers (organic) |
| Yellow | Animals and decomposers | Terrestrial consumers (organic) |
| Teal/cyan | Algae and phytoplankton, shellfish | Marine organisms (organic, though shellfish also have inorganic CaCO₃ shells) |
| Brown | Coal, oil, natural gas; sedimentation of biomass | Fossil carbon (organic) |
| Grey/blue | Calcium carbonate of shells, limestone | Carbonate stores (inorganic) |
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Biogeochemical cycles have stores, sinks and sources.
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Organisms, crude oil and natural gas contain organic stores of carbon. Inorganic stores can be found in the atmosphere, soils and oceans.
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The atmosphere is an inorganic store of carbon. Carbon exists here as carbon dioxide gas (CO₂), a simple inorganic molecule.
Key terminology:
| Term | Definition | The atmosphere is currently... |
|---|---|---|
| Store | In equilibrium; inputs balanced by outputs | Not a store (inputs ≠ outputs) |
| Sink | Net accumulation of the element | Acting as a sink for human emissions |
| Source | Net release of the element | — |
Residence time is the average period that a carbon atom remains in a store. In the atmosphere, residence time is only about 3–5 years. Carbon atoms cycle through quickly, but the total amount is increasing because inputs now exceed outputs.