What type of deposits form directly from seawater in the locations where they are found?

Authigenic deposits are formed directly from seawater through processes such as precipitation of minerals. This occurs in environments where conditions allow for the crystallization of minerals from dissolved substances in the seawater. These deposits typically consist of minerals that have formed in place, rather than being transported from other locations. An example of this is the formation of salt deposits through the evaporation of seawater, which leaves behind minerals like halite and gypsum.

In the context of marine geochemistry, understanding authigenic deposits is important as they can provide insights into past environmental conditions, as well as the biochemical processes occurring in the ocean. They differ fundamentally from biogenic deposits, which are formed from the remains of living organisms, and terrigenous deposits, which originate from terrestrial sources. Allogenic deposits arise from external sources but are transported and deposited in the marine environment, rather than precipitating directly from seawater.