How does water participate in the long-term carbon dioxide cycle?

In hydrologic sciences, water plays a critical role in the long-term carbon dioxide cycle by facilitating the chemical weathering of rocks and the subsequent sequestration of carbon in marine sediments.12

When atmospheric carbon dioxide dissolves in water, it forms carbonic acid. This weak acid reacts with rocks exposed to precipitation and groundwater, a process known as weathering. This reaction converts solid minerals into soluble products, including bicarbonate ions (HCO₃⁻), which are eventually transported to the ocean via fluvial processes.123

Once in the ocean, these bicarbonate ions exist in equilibrium with other dissolved forms of carbon dioxide. In specific conditions, carbonate ions can combine with calcium ions—also derived from weathering—to precipitate as solid calcite (CaCO₃), the primary mineral in limestone. When this calcium carbonate and organic matter sink to the ocean floor, they are incorporated into sediments. Over geologic time, these sediments can become part of the Earth's crust, effectively storing carbon for millions of years until uplift and erosion or volcanic activity eventually return the carbon to the atmosphere.142