Give an illustrated account of carbon cycle with suitable diagram.

The carbon cycle is essential for sustaining life and regulating Earth's climate. It involves the continuous exchange of carbon between various reservoirs. For analytical purposes, the carbon cycle is broadly divided into two main components: the fast (or biological) carbon cycle and the slow (or geological) carbon cycle.

Reservoirs of Carbon

Carbon is stored in several major reservoirs or sinks:

• Atmosphere: Primarily as carbon dioxide (CO2), but also as methane (CH4).
• Oceans (Hydrosphere): Dissolved CO2, carbonic acid, bicarbonate ions, and carbonates, as well as in marine organisms. The ocean holds about 50 times more carbon than the atmosphere.
• Land (Terrestrial Biosphere and Pedosphere): In living organisms (biomass), dead organic matter, soil carbon, and fossil fuels (coal, oil, natural gas) within the lithosphere.
• Earth's Interior (Geosphere/Lithosphere): Stored in rocks (e.g., limestone) and sediments over geological timescales. Most of Earth's carbon is stored here.

Components of the Carbon Cycle

1. The Fast (Biological) Carbon Cycle

This cycle operates relatively quickly, involving the exchange of carbon between the atmosphere, oceans, and living organisms. It completes within years to decades.

• Photosynthesis: Plants, algae, and some bacteria absorb atmospheric CO2 (or dissolved CO2 in water) and use sunlight to convert it into organic matter (glucose and other sugars), releasing oxygen. This is the primary mechanism by which carbon enters the biosphere.
  Equation: CO2 + H2O + Energy → (CH2O)n + O2
• Respiration: All living organisms (plants, animals, microbes) consume organic matter for energy, releasing CO2 back into the atmosphere or water. This is a continuous process.
• Decomposition: When plants and animals die, decomposers (bacteria and fungi) break down organic matter, returning carbon to the atmosphere as CO2 or methane (CH4) and to the soil as organic carbon.
• Consumption (Food Chains): Animals obtain carbon by consuming plants or other animals, incorporating carbon into their biomass.
• Oceanic Exchange: CO2 dissolves in surface waters, forming carbonic acid, bicarbonate, and carbonate ions. Marine organisms use these forms of carbon for shells and skeletons. The ocean also absorbs a significant amount of atmospheric CO2.

2. The Slow (Geological) Carbon Cycle

This cycle involves the movement of carbon through rocks, soil, ocean, and atmosphere over millions of years (100-200 million years). It includes long-term storage of carbon.

• Sedimentation and Burial: Over geological time, dead organic matter (from plants and marine organisms) can be buried and compressed under layers of sediment. Under specific conditions of heat and pressure, this organic matter transforms into fossil fuels (coal, oil, natural gas). Marine shells and skeletons accumulate to form carbonate rocks like limestone.
• Weathering and Erosion: Carbon stored in rocks is slowly released through chemical weathering. Rainwater, slightly acidic from dissolved CO2, reacts with rocks, dissolving carbon-containing minerals and carrying carbon ions to the oceans via rivers.
• Volcanic Activity: Carbon stored in the Earth's mantle and crust can be released back into the atmosphere as CO2 through volcanic eruptions and degassing from mid-ocean ridges.
• Tectonic Activity: Subduction of oceanic plates carries carbon-rich sediments deep into the Earth's interior, where carbon can be stored or eventually released through volcanic activity.

Human Influence on the Carbon Cycle

Human activities have significantly altered the natural carbon cycle, primarily since the Industrial Revolution, leading to an increase in atmospheric CO2 levels.

• Burning of Fossil Fuels: Combustion of coal, oil, and natural gas for energy, transportation, and industry releases large amounts of CO2 that were sequestered over millions of years.
• Deforestation and Land-Use Change: Clearing forests reduces the Earth's capacity to absorb CO2 through photosynthesis (reduced carbon sinks) and releases stored carbon when trees are burned or decompose.
• Industrial Processes: Cement manufacturing, for instance, releases CO2 as a byproduct.

These activities lead to an imbalance, causing an enhanced greenhouse effect and global warming.

Diagram of the Carbon Cycle

Below is a simplified diagram illustrating the major components and fluxes of the carbon cycle:

(Source: Adapted from NOAA/IPCC diagrams. This diagram depicts the movement of carbon among the atmosphere, land, and oceans. Yellow arrows represent natural fluxes, red arrows show human contributions, and numbers in white denote stored carbon in gigatons.)

Explanation of the Diagram:

• Atmosphere: Contains CO2.
• Photosynthesis (Land & Ocean): Arrows from atmosphere/dissolved CO2 to land plants and marine life.
• Respiration (Land & Ocean): Arrows from land biota, soil, and marine life back to the atmosphere/dissolved CO2.
• Decomposition: Arrows from dead organic matter to soil and atmosphere.
• Fossil Fuels: Buried organic matter transforming into coal, oil, and gas, and arrows indicating their combustion releasing CO2 to the atmosphere.
• Oceanic Processes: Exchange of CO2 between atmosphere and surface ocean, movement to deep ocean, and formation of marine sediments.
• Volcanic Activity: Arrow from Earth's interior to atmosphere.