The carbon cycle as a closed system

The carbon cycle describes how carbon, a key element in all living things, moves between different parts of Earth. This process links biological, geological, and chemical activities, making it a biogeochemical cycle.

Key features of the carbon cycle

As a closed system, the carbon cycle has inputs and outputs of energy, but the total amount of carbon stays constant. Carbon exists in organic stores, such as living organisms, and inorganic stores, like rocks, gases, and fossil fuels. It transfers between these stores through flows called fluxes.

Carbon budget, sources, and sinks

The carbon budget measures the difference between carbon inputs to and outputs from a subsystem. For example, in the atmosphere, inputs include carbon from volcanic eruptions, burning fossil fuels, respiration, decomposition, and ocean loss, while outputs involve processes like photosynthesis, sequestration (the capture of carbon from the atmosphere by the ocean, vegetation, or sedimentary rocks where it is stored), and chemical weathering.

Carbon sources and sinks:

• Carbon source - A subsystem where carbon outputs exceed inputs, releasing more carbon than it absorbs.
• Carbon sink - A subsystem where carbon inputs exceed outputs, absorbing more carbon than it releases.

Sudden events, such as volcanic eruptions or major wildfires, can release large amounts of carbon and disrupt the cycle's balance, which may take a long time to restore.

Carbon stores in Earth's systems

Most carbon on Earth is held in long-term stores, with over 99.9% locked away. The distribution varies across Earth's systems, each acting as a reservoir for carbon in different forms.

Distribution of carbon stores

These stores interact through fluxes, with some carbon remaining locked for millions of years, such as in fossil fuels or deep underground.

Carbon fluxes and their scales

Carbon flux refers to the flow of carbon between stores. These fluxes vary in size, speed, and location, influencing how carbon moves globally.

Measuring fluxes

Fluxes are measured in petagrams (Pg) or gigatons (Gt) per year, where 1 Pg or Gt equals 1 billion tonnes of carbon. The largest fluxes occur between the oceans and atmosphere, and between land and atmosphere, mainly through photosynthesis (plants converting CO₂ and water into glucose using sunlight) and respiration (organisms breaking down glucose for energy, releasing CO₂).

Spatial scales of fluxes

• Plant scale - Focuses on photosynthesis and respiration.
• Ecosystem scale - Includes combustion (burning biomass, releasing CO₂ and heat) and decomposition (breakdown of dead matter by bacteria and fungi, releasing CO₂).
• Continental scale - Encompasses all fluxes, including sequestration. More photosynthesis and respiration happen in the northern hemisphere due to greater landmass, while more ocean sequestration occurs in the southern hemisphere with its larger sea area.

Time scales of fluxes

Some fluxes are rapid, transferring carbon in minutes, hours, or days, such as photosynthesis, respiration, combustion, and decomposition. Others are slow, like sequestration into sedimentary rocks or deep ocean, which can take millions of years.

Processes releasing carbon into the atmosphere

Carbon enters the atmosphere through natural and human-related processes, balancing the cycle but sometimes causing disruptions.

Chemical weathering

This process transfers carbon from the atmosphere to the hydrosphere and biosphere. Atmospheric CO₂ reacts with water vapour to form weak carbonic acid, creating acid rain. The acid rain reacts with silicate minerals in rocks, releasing calcium ions that wash into rivers and seas, where they form calcium carbonate.

Volcanism

CO₂ stored in magma and Earth's crust is released during eruptions at plate boundaries or hotspots, a process called outgassing. Additional releases occur at ocean ridges, subduction zones, and geysers through metamorphism (sedimentary rocks transforming under heat and pressure, releasing CO₂). Recent eruptions release less CO₂ than human activities, but large ones could significantly disrupt the cycle.

Negative feedback loop between weathering and volcanism

Chemical weathering and volcanism can create a negative feedback loop: volcanism raises atmospheric CO₂ and temperatures, increasing evaporation and acid rain, which enhances weathering and sequestration, reducing atmospheric carbon.

Combustion

Combustion burns living, dead, or decomposed biomass, including peaty soils, transferring carbon to the atmosphere as CO₂ and generating heat for energy. Wildfires release large quantities quickly, reducing vegetation and photosynthesis, which amplifies atmospheric CO₂ increases.

Carbon sequestration into oceans

Oceans act as major carbon sinks through three interconnected pumps that move carbon from the atmosphere to surface and deep waters.

Biological pump

Phytoplankton (single-celled microscopic organisms on the ocean surface) absorb atmospheric CO₂ via photosynthesis. They form the base of the marine food chain, passing carbon to animals. When organisms die, they sink, transferring carbon to the deep ocean. Respiration releases some CO₂ back, but most stays in surface waters for reabsorption.

Carbonate pump

Chemical weathering washes carbon molecules into seas via rivers, where they react with dissolved CO₂ to form calcium carbonate. Calcifying marine life (sea creatures forming shells from calcium carbonate) uses this to build structures. Upon death, they sink and form calcium carbonate-rich sediments through sedimentation.

Physical pump

Cold water holds more CO₂ than warm water and sinks due to density. The thermohaline circulation (ocean currents driven by temperature and salinity differences) moves warm surface water to cooler areas, where it absorbs CO₂ and sinks (downwelling). Cold water rises elsewhere (upwelling), potentially releasing CO₂ or allowing phytoplankton absorption. This circulation acts like a conveyor belt, holding carbon in deep oceans for hundreds of years.

Carbon sequestration into plants and soils

Plants and soils sequester carbon through biological processes, transferring it from the atmosphere to the biosphere.

Photosynthesis and food chain transfer

Photosynthesis by terrestrial primary producers (plants) converts atmospheric CO₂ and water into glucose and oxygen, enabling growth. Carbon passes to herbivores (primary consumers) and carnivores (secondary consumers). Some becomes proteins and fats, while respiration and decomposition release CO₂.

Soil storage processes

Decomposers like bacteria, fungi, and detritivores (animals feeding on dead material) break down waste, storing carbon in soil as organic carbon (from decayed matter) or inorganic carbon (from limestone breakdown). Most cycling occurs in topsoil, where disturbance aids CO₂ release. Deeper layers hold more fixed carbon. Photosynthesis happens only during daylight and varies seasonally; at night or in low light, plants respire, reversing the flux.

Soil carbon cycle steps

1. Bedrock breaks up, releasing nutrients.
2. Roots take up nutrients from the soil.
3. Vegetation grows and photosynthesises.
4. Leaf litter falls to the ground.
5. Dead plant and animal material decomposes.
6. Worms and micro-organisms break down organic matter, returning nutrients and carbon to the soil.

Carbon storage in dead organic matter and influencing factors

Decomposition transfers carbon from dead biomass to the atmosphere and soil, with rates varying by environment and material.

Decomposition process

Bacteria and fungi break down dead organisms, releasing CO₂ to the atmosphere. Some carbon forms humus (organic soil matter). A large, protein-rich mammal decomposes slower than soft leaves.

Factors affecting soil carbon content

• Soil type - Clay-rich soils store high carbon levels; sandy soils store low levels.
• Vegetation cover - Deciduous trees provide regular organic matter, increasing carbon; low-lying, hardy vegetation provides less.
• Human disturbance - Heavily worked soils, like ploughed farmland, lose carbon through gaseous release; undisturbed wilderness areas retain higher carbon.
• Climate - Slow decomposition in dry, low-oxygen Arctic tundra; faster in warm, wet tropical rainforests.

Soil profiles

Soil profiles reveal carbon levels, with most stored in the topsoil layer rich in humus, above layers of leached minerals, accumulated minerals, broken bedrock, and solid bedrock.