In the carbon cycle, which reservoir stores most carbon on geologic timescales?

• A. Atmosphere
• B. Hydrosphere
• C. Geosphere
• D. Biosphere

Answer: (C)

The main idea is that long-term carbon storage happens most effectively in rocks. Over geologic timescales, carbon gets locked away when it is buried as organic matter and when carbon is precipitated as carbonate minerals, forming sedimentary rocks in the geosphere. Fossil fuels and other carbon-rich rocks act as enormous reservoirs that hold carbon for millions of years. While the atmosphere, hydrosphere, and biosphere exchange carbon rapidly and store substantial amounts in the short term, they don’t accumulate carbon to the same massive extent as rocks do over millions of years. Tectonic processes can shuffle carbon among reservoirs, but the geosphere remains the largest long-term sink for carbon on geological timescales.