How does it enter the Ocean

However, once dissolved in the ocean, a carbon atom will stay there at an average of more than 500 years.

Inorganic Carbon Dioxide Absorption

CO2 enters water by dissolving into it from the air. CO2 from the atmosphere dissolves more easily into cold water, where it can sink to deep levels of the ocean and remain there for thousands of years. Much of the CO2 dissolved from the atmosphere stays in the upper levels of the Ocean, and can easily re-enter the atmosphere as a result of evaporation.

Organic Carbon Dioxide Absorption

Organisms like phytoplankton absorb carbon dioxide as a result of photosynthesis, the larger a population of phytoplankton is, the more carbon dioxide it will store. Dead organisms like whales, fish, and other marine life that sink to the bottom of the ocean release dissolved CO2 into the water as they decay.