Question

How do seashells like coquina show that rocks and water store carbon?

Seashells are made of carbon dioxide gas frozen into a solid.

Seashells are made of minerals that contain carbonates (carbon-oxygen compounds).

Seashells release carbon into the atmosphere.

Seashells only exist in places that don’t have carbon.

Answer 1

Correct answer:
- Seashells are made of minerals that contain carbonates (carbon‑oxygen compounds).

Why:
- Carbon dioxide (CO2) from the atmosphere dissolves into seawater and reacts to form bicarbonate (HCO3–) and carbonate (CO32–) ions.
- Many marine organisms (clams, oysters, corals, etc.) combine calcium ions (Ca2+) with carbonate ions to build shells of calcium carbonate (CaCO3). Coquina is a rock made mostly of broken shells cemented together — so it’s largely CaCO3.
- That process moves carbon from the dissolved CO2 pool in the water into solid mineral form in shells and, when buried, into sedimentary rock. In that way both water (as dissolved carbonate/bicarbonate) and rocks (as carbonate minerals) act as carbon reservoirs.

Quick notes on the incorrect choices:
- Seashells are not “frozen” CO2 gas. The carbon is chemically bound in carbonate minerals (CaCO3), not trapped as gas.
- Shells can later release carbon if they dissolve or are metamorphosed, but their formation generally removes carbon from the short‑term atmosphere/ocean pool.
- Shells do not only exist where there’s no carbon; they form because carbon is present in water as carbonate/bicarbonate.