Topic 1.4 Notes – The Carbon Cycle

What the Carbon Cycle Is

Carbon moves among reservoirs, which are places where carbon is stored. It can be part of organic molecules in living things, like carbohydrates and proteins, or inorganic forms like carbon dioxide $\text{CO}_2$.

A core APES idea here is conservation of matter. Carbon atoms are not created or destroyed in the cycle. They are transferred and transformed.

Key vocabulary

• Reservoirs are storage locations for carbon.
• Fluxes are transfers of carbon from one reservoir to another.
• Source means more carbon is released than taken in over a certain time.
• Sink means more carbon is taken in than released over a certain time.
• Residence time is the average time carbon stays in a reservoir.

One common mistake is mixing up sink and reservoir. A reservoir is where carbon is. A sink describes net movement. The ocean is a reservoir all the time, but it is only a sink when it takes in more carbon than it releases.

Major Carbon Reservoirs

Here’s the full set you need to know. This carbon cycle diagram gives you the big-picture map of where carbon is stored before the table breaks the reservoirs down one by one.

Carbon cycle reservoirs and major fluxes

The atmosphere, living things, and surface ocean are part of faster movement. Rocks, deep ocean, and fossil fuels hold carbon much longer.

How Carbon Moves Through the Fast and Slow Cycles

The fast cycle moves carbon quickly among the atmosphere, organisms, soils, dead matter, and surface ocean.

Photosynthesis

Plants, algae, and phytoplankton take in $\text{CO}_2$ and fix that carbon into organic molecules. This moves carbon from air or water into biomass.

Feeding and food webs

When organisms eat producers or other consumers, carbon moves through the food web. Feeding transfers carbon. It does not remove it from the cycle.

Cellular respiration

Plants, animals, and decomposers release carbon back as $\text{CO}_2$. Photosynthesis and respiration are complementary. One stores carbon in organic matter, the other returns it to air or water.

Death, waste, and decomposition

Dead matter and waste become detritus and soil carbon. Decomposers break this down and release $\text{CO}_2$. In low-oxygen conditions, they can produce methane.

• Faster decomposition happens in warm, moist, oxygen-rich places.
• Slower decomposition happens in cold, dry, or waterlogged places.

Ocean-atmosphere exchange

$\text{CO}_2$ diffuses into and out of the surface ocean. Once dissolved, it can become bicarbonate and carbonate.

The slow cycle stores and moves carbon over thousands to millions of years.

Burial and fossil fuel formation

If dead organic matter is buried before full decomposition, heat and pressure can eventually form fossil fuels.

• Coal comes mostly from terrestrial plants.
• Petroleum and natural gas come largely from marine organisms.

Sedimentation and carbonate rock formation

Marine shells and skeletons settle, are buried, and can become carbonate rock such as limestone.

Geological release

Weathering, uplift, and volcanic activity slowly return stored carbon to the cycle.

Why Timescale Matters

Fast and slow cycles are connected. They are not separate systems. The same carbon atom can move quickly through a leaf, soil, and atmosphere, or stay locked in rock for millions of years.

This is where diagrams matter on tests. Boxes show stored carbon. Arrows show movement and process. A huge reservoir does not always mean a fast flux.

How Humans Alter the Carbon Cycle

Fossil-fuel combustion burns coal, petroleum, and natural gas and rapidly moves long-stored carbon into atmospheric $\text{CO}_2$. That carbon formed slowly but is released quickly.

Deforestation and land clearing do three things at once:

• reduce carbon stored in biomass
• reduce photosynthetic uptake of $\text{CO}_2$
• increase release through burning, decomposition, and disturbed soils

The bottom line is simple. Atmospheric carbon rises when carbon release exceeds uptake by natural sinks.