Which of the following is a major carbon reservoir?

• A. Sun
• B. Oceans
• C. Biosphere
• D. Atmosphere

Answer: (B)

The oceans hold the largest amount of carbon on Earth, making them the dominant carbon reservoir. Seawater contains dissolved inorganic carbon in forms like CO2, bicarbonate, and carbonate ions, and the vast volume of the oceans means this dissolved carbon amounts to far more than what’s stored in the atmosphere or in living organisms. Over long timescales, carbon also moves into sediments and carbonate minerals, locking away carbon and buffering atmospheric CO2. This combination of enormous volume and carbonate chemistry gives the oceans a capacity to store and cycle carbon that dwarfs other reservoirs. The Sun isn’t a carbon reservoir in Earth’s carbon cycle, and while the biosphere and atmosphere do contain carbon, their total stores are much smaller than the ocean’s.