How do carbonate rocks form on the ocean floor?

Carbonate rocks form on the ocean floor primarily through the accumulation and lithification of carbonate sediments. This process occurs through two main pathways:1

• Biological Accumulation: Many marine organisms, such as foraminiferans and coccoliths, secrete shells or tests made of calcium carbonate. Upon the death of these organisms, their remains settle to the ocean floor, forming biogenic oozes that can eventually lithify into limestone.1
• Chemical Precipitation: Carbonate ions and calcium ions present in seawater can combine to form solid calcite, the primary mineral in limestone. This process is influenced by environmental factors, including water temperature and depth.21

The accumulation of these deposits is highly dependent on depth. Below the carbonate compensation depth—typically around 4,500 meters—the pressure and temperature of seawater cause carbonate to dissolve faster than it can accumulate, preventing the formation of these rocks in the deepest parts of the ocean.1