Calcium strontium or barium, the oxide–nitride coating that results in air is stable and lathe machining and other standard metallurgical techniques are suitable for calcium. In the United States and Canada, calcium is instead produced by reducing lime with aluminium at high temperatures. Calcium provides a link between tectonics, climate, and the carbon cycle. In the simplest terms, uplift of mountains exposes calcium-bearing rocks to chemical weathering and releases Ca into surface water. These ions are transported to the ocean where they react with dissolved CO to form limestone (), which in turn settles to the sea floor where it is
Calcium citrate sulfuric acid. In many individuals, bioavailability of calcium citrate is found to be equal to that of the cheaper calcium carbonate. However, alterations to the digestive tract may change how calcium is digested and absorbed. Unlike calcium carbonate, which is basic and neutralizes stomach acid, calcium citrate has no effect on stomach acid. Calcium carbonate is harder to digest than calcium citrate, and calcium carbonate carries a risk of "acid rebound" (the stomach overcompensates by producing more acid), so individuals who are sensitive to antacids or who have difficulty producing adequate stomach acid may choose calcium citrate over calcium carbonate