Main reservoirs of carbon

The carbon cycle is the continuous movement of carbon among different parts of Earth. Carbon exists in various forms and is stored in several main reservoirs, which are large stores where carbon accumulates. These reservoirs include the atmosphere, ocean, soil, living things, and rocks. Understanding these helps explain how carbon flows through the environment.

Key carbon reservoirs and their characteristics

• Atmosphere - Carbon is primarily stored here as carbon dioxide (CO₂), a gas that makes up a small but crucial portion of air; this reservoir is dynamic and influences climate through greenhouse effects.
• Ocean - The largest carbon reservoir, holding carbon as dissolved CO₂, bicarbonate ions (HCO₃^-), and carbonate ions (CO₃^2-); it absorbs and releases carbon through surface interactions.
• Soil - Carbon is stored in organic matter from decayed plants and animals, as well as in minerals; this reservoir acts as a medium-term store influenced by biological activity.
• Living things - Carbon forms the backbone of organic molecules in plants, animals, and microorganisms; this is an active reservoir where carbon is constantly taken up and released through life processes.
• Rocks - Carbon is locked in sedimentary rocks like limestone (calcium carbonate, CaCO₃) and in fossil fuels; this is a long-term reservoir that stores carbon for millions of years.

These reservoirs are interconnected, with carbon moving between them through various processes. The amount of carbon in each can change over time due to natural and human activities.

Key processes that move carbon between reservoirs

Several key processes transfer carbon from one reservoir to another, maintaining the balance of the carbon cycle. These include photosynthesis, respiration, decomposition, ocean exchange, weathering, volcanism, and combustion. Each process involves specific mechanisms that either remove carbon from the atmosphere or add it back, affecting global carbon levels.

These processes create a dynamic system where carbon is recycled. For example, carbon taken up by living things through one process is often released back through another, ensuring continuous circulation.

Photosynthesis as a carbon-moving process

Photosynthesis is the process by which green plants, algae, and some bacteria convert carbon dioxide from the atmosphere into organic compounds, using sunlight as energy. This moves carbon from the atmosphere into living things and eventually into soil or oceans when organisms die.

Steps of photosynthesis

1. Light energy from the sun is absorbed by chlorophyll in plant cells, exciting electrons and splitting water molecules to release oxygen (O₂).
2. The excited electrons provide energy to convert CO₂ and hydrogen from water into glucose (C₆H₁₂O₆), a sugar that stores chemical energy.
3. The glucose is used to build other organic molecules, incorporating carbon into the plant's biomass.

Chemical equation for photosynthesis:

$6C{O}_2+6H_{2}O\to C_6{H}_{12}{O}_6+6O_2$

This equation shows how atmospheric CO₂ is fixed into organic matter. As a result, photosynthesis reduces CO₂ levels in the atmosphere and builds up carbon in living things.

Respiration and decomposition as carbon-moving processes

Respiration and decomposition are processes that release carbon back into the atmosphere or other reservoirs. Respiration occurs in living organisms, while decomposition happens after death, both breaking down organic matter to produce energy or recycle nutrients.

Respiration

Respiration is the breakdown of glucose in cells to release energy, moving carbon from living things back to the atmosphere as CO₂.

Steps of respiration:

1. Glucose is oxidized in the presence of oxygen, releasing energy stored in its chemical bonds.
2. The carbon from glucose combines with oxygen to form CO₂, which is exhaled or released.
3. Water (H₂O) is also produced as a byproduct.

Chemical equation for respiration:

$C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O$

This process occurs in all living things, balancing the carbon removed by photosynthesis.

Decomposition

Decomposition is the breakdown of dead organic matter by bacteria and fungi, transferring carbon from living things to soil or the atmosphere.

Steps of decomposition:

1. Microorganisms secrete enzymes that break down complex organic compounds into simpler substances.
2. Carbon is released as CO₂ through microbial respiration, or it remains in soil as humus (stable organic matter).
3. Nutrients are recycled, but some carbon may enter the ocean if washed away by water.

This process ensures carbon from dead organisms does not accumulate indefinitely, instead returning it to the cycle.

Ocean exchange, weathering, volcanism, and combustion as carbon-moving processes

Beyond biological processes, physical and geological processes also move carbon between reservoirs. These include ocean exchange, weathering, volcanism, and combustion, each operating on different timescales and scales.

Ocean exchange

Ocean exchange involves the transfer of CO₂ between the atmosphere and ocean surface.

Steps of ocean exchange:

1. CO₂ from the atmosphere dissolves into ocean water, forming carbonic acid (H₂CO₃).
2. This acid dissociates into bicarbonate and carbonate ions, storing carbon in the ocean.
3. Conversely, warmer water or upwelling can release CO₂ back to the atmosphere.

This process acts as a buffer, regulating atmospheric CO₂ levels.

Weathering

Weathering is the chemical breakdown of rocks that removes CO₂ from the atmosphere and stores it in rocks or oceans.

Steps of weathering:

1. Rainwater absorbs atmospheric CO₂, forming weak carbonic acid.
2. The acid reacts with minerals in rocks, such as silicates, releasing ions like calcium (Ca²⁺).
3. These ions combine with bicarbonate to form carbonate rocks like limestone, locking away carbon.

This long-term process contributes to carbon storage in rocks.

Volcanism

Volcanism releases carbon from rocks back into the atmosphere through volcanic eruptions.

Steps of volcanism:

1. Heat from Earth's interior melts rocks containing carbonates.
2. During eruptions, CO₂ is released as a gas along with lava and ash.
3. This adds ancient carbon to the atmosphere, balancing losses from weathering.

Combustion

Combustion is the burning of organic matter, releasing carbon as CO₂.

Steps of combustion:

1. Fuel (like wood or fossil fuels) reacts with oxygen at high temperatures.
2. Carbon in the fuel combines with oxygen to form CO₂.
3. Energy is released as heat and light, with CO₂ entering the atmosphere.

This process occurs naturally (e.g., wildfires) or through human activities (e.g., burning fossil fuels).

Short-term fluxes and seasonal changes in CO₂ levels

Short-term fluxes are rapid movements of carbon that cause fluctuations in atmospheric CO₂ levels over days to months. These are mainly driven by seasonal variations in photosynthesis and respiration, leading to predictable yearly patterns.

Factors controlling seasonal CO₂ changes

• Northern hemisphere dominance - More landmass in the Northern Hemisphere means greater plant growth in summer, increasing photosynthesis and reducing atmospheric CO₂.
• Summer decrease - High photosynthesis rates remove CO₂ from the atmosphere, storing it in living things.
• Winter increase - Reduced photosynthesis and ongoing respiration/decomposition release more CO₂ back into the atmosphere.
• Ocean influence - Cooler winter oceans absorb more CO₂, but this is outweighed by land-based biological fluxes.

These fluxes create a sawtooth pattern in CO₂ measurements, with levels dropping in summer and rising in winter.

Long-term carbon storage and its release

Long-term storage keeps carbon out of the active cycle for thousands to millions of years, primarily in sediments and fossil fuels. Sediments form when organic matter or carbonates settle on ocean floors, compacting into rocks. Fossil fuels (coal, oil, natural gas) develop from buried ancient organisms under heat and pressure.

Release of long-term stored carbon

• Natural processes - Weathering slowly erodes sediments, releasing carbon; volcanism melts and releases carbon from rocks as CO₂.
• Human processes - Mining and burning fossil fuels (combustion) rapidly release stored carbon into the atmosphere as CO₂, accelerating the cycle.

This release can disrupt the natural balance, leading to increased atmospheric CO₂ over time.