Seawater is a source of calcium, and it looks like desalination concentrate is an option for limited scale production: https://www.nature.com/articles/s41545-0

Seawater is a source of calcium, and it looks like desalination concentrate is an option for limited scale production: https://www.nature.com/articles/s41545-022-00153-6 https://www.nature.com/articles/s41545-022-00153-6 "Seawater desalination concentrate—a new frontier for sustainable mining of valuable minerals" (2022) > "The metallic elements found in the highest concentration are sodium, magnesium, calcium and potassium, which have been commercially extracted as the chlorides, sulfates, and carbonates while magnesium has been extracted as the hydroxide."