Which of the following is a major reservoir of carbon in the Earth's system?

• A. Oceans (dissolved carbonates)
• B. Earth's core
• C. Atmosphere (as CO_2)
• D. Moon rocks

Answer: (C)

Verified step by step guidance

1. Understand what a 'major reservoir of carbon' means: it refers to a large natural storage location where carbon is held for significant periods within the Earth's system.
2. Identify the main carbon reservoirs on Earth, which typically include the atmosphere (mainly as CO_2), the oceans (primarily as dissolved carbonates and bicarbonates), the terrestrial biosphere (plants and soil), and the geosphere (rocks and sediments).
3. Evaluate each option given: Earth's core is not considered a major carbon reservoir because it is mostly composed of metals and does not contain significant carbon accessible to the surface system; the atmosphere contains carbon dioxide but holds less carbon compared to the oceans; moon rocks are not part of Earth's carbon cycle.
4. Recognize that the oceans act as a major carbon reservoir because they dissolve large amounts of carbon dioxide, which forms carbonic acid and subsequently dissociates into bicarbonate and carbonate ions, storing carbon in dissolved forms.
5. Conclude that among the options, the oceans (dissolved carbonates) represent a major reservoir of carbon in the Earth's system due to their vast capacity to store carbon compared to the atmosphere or Earth's core.