Over millions of years these carbon-bearing rocks can be exposed to sufficient heat and pressure to melt, causing them to release their carbon back into the atmosphere as carbon dioxide via volcanism. As the successive layers of sediment are compressed and cemented they are turned into limestone rock. This carbon from land, as well as carbon atoms in CO 2 absorbed by the ocean from the atmosphere, can become incorporated into calcium carbonate (CaCO 3) shells made by algae, plants, and animals. The resulting sediments, along with organic material, can be transported ( eroded) from the land to enter the ocean where they sink to the bottom. For example, the weathering of rocks removes carbon dioxide from the atmosphere. On longer timescales, significant amounts of carbon are transferred between rocks and the ocean and atmosphere, typically over thousands to millions of years. The combustion of biomass during wildfires also release large amounts of carbon stored in plants back into the atmosphere.

When organisms die and decay carbon also returns to the atmosphere, or is integrated into soil along with some of their waste. Meanwhile, the respiration of plants, animals, and microbes returns carbon to the atmosphere as carbon dioxide (CO 2). As plants are eaten by herbivores and herbivores are eaten by carnivores, carbon moves up the food web. For example, photosynthesizing plants on land remove carbon dioxide directly from the atmosphere, and those carbon atoms become part of the structure of the plants. Carbon is transferred between the ocean, atmosphere, soil, and living things over time scales of hours to centuries.

Easy carbon cycle diagram