How does water help move carbon dioxide through Earth’s carbon cycle?

Water is essential to the movement of carbon through Earth’s carbon cycle by facilitating both chemical and physical transport processes.

Chemical Weathering and Transport

When atmospheric carbon dioxide dissolves in water, it forms carbonic acid. This weak acid chemically weathers continental rocks, releasing calcium and other ions. These dissolved products are then transported by rivers into the oceans. In the ocean, these ions combine to form calcium carbonate, which settles on the seafloor to create limestone deposits.123

Exchange and Biological Cycling

Water acts as a medium for the physical exchange of carbon dioxide between the atmosphere and the hydrosphere. Once dissolved, this carbon becomes accessible to aquatic organisms, such as phytoplankton, which use it for photosynthesis. When these organisms die, they sink, effectively sequestering carbon in the deep ocean.34

Tectonic Recycling

Over geologic time, carbonate rocks on the seafloor can be drawn into Earth’s interior via plate tectonics. The subsequent melting of these rocks releases carbon dioxide back into the atmosphere through volcanic activity, completing the cycle.12