How Carbon Dioxide Decreased

• Solubility of Gases: Carbon dioxide is a highly water-soluble gas. Its ability to dissolve in liquid water provided the primary physical mechanism for its initial removal from the atmosphere.

• Chemical Precipitation: Once dissolved in water, $CO_2$ can react with metal ions (like Calcium) to form insoluble solid carbonates. This converts a gas into a stable mineral form.

• Biological Conversion: Photosynthesis is a biochemical process that uses light energy to convert inorganic $CO_2$ into organic glucose, effectively trapping carbon in biological tissues.

1. Physical Dissolution

• Ocean Formation: As the Earth cooled, water vapor condensed to form oceans. Because $CO_2$ is soluble, large volumes moved from the gas phase into the liquid phase of the new oceans.
• Equilibrium: This process continued until the partial pressure of $CO_2$ in the air balanced with its concentration in the water.

2. Chemical Precipitation

• Carbonate Rocks: Dissolved $CO_2$ reacted with minerals in the water to form carbonate precipitates. These solids settled on the seabed, eventually forming sedimentary rocks like limestone and chalk.
• Marine Organisms: Later, primitive animals used these dissolved carbonates to build shells and skeletons, which also became rock after the organisms died.

3. Biological Sequestration

• Evolution of Algae: Roughly 2.7 billion years ago, algae evolved and began using $CO_2$ for photosynthesis. This process actively 'scrubbed' the gas from the atmosphere.
• Fossil Fuel Formation: When these plants and algae died in swamps or oceans, their carbon-rich remains were buried under high pressure, eventually forming coal, crude oil, and natural gas.

• Carbonates vs. Fossil Fuels: While both 'lock up' carbon, carbonates are formed from mineral reactions and shells, whereas fossil fuels are formed specifically from the decayed remains of plants and microorganisms.

• Sequence Matters: Always describe the cooling of the Earth and the formation of oceans before discussing the dissolution of $CO_2$. The liquid water was the prerequisite for the first major drop in gas levels.

• The 'Locked Up' Concept: Use the term 'locked up' to describe how carbon was removed from the atmosphere and stored in rocks or fossil fuels. This is a key phrase in many marking schemes.

• Photosynthesis Equation: Be prepared to write the balanced equation: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$. Note that this process simultaneously decreases $CO_2$ and increases $O_2$.

• Check the Source: If a question asks why $CO_2$ decreased, ensure you mention both the formation of sedimentary rocks and the role of photosynthesis to get full marks.

• The 'Escape' Myth: A common mistake is thinking $CO_2$ escaped into space. In reality, Earth's gravity is strong enough to hold $CO_2$; it was removed by moving into the crust and oceans, not by leaving the planet.

• Nitrogen Confusion: Do not assume $CO_2$ turned into Nitrogen. Nitrogen was released by volcanoes and built up because it is unreactive, while $CO_2$ decreased because it is highly reactive and soluble.

• Plant-Only Error: Many students credit plants as the only reason for $CO_2$ decrease. However, the formation of oceans and carbonate rocks removed a significant portion of $CO_2$ long before complex plants evolved.