The seas originally received their salt when a young Earth's atmosphere, filled with hydrogen chloride and other materials, dissolved into the primitive ocean. The majority of the salt, however, gets added to the ocean through the gradual weathering of terrestrial rocks by water (and a smaller percentage through hydrothermal or volcanic inputs). Precipitation is slightly acidic due to carbonic acid that forms when water interacts with atmospheric carbon dioxide. Rain thus not only erodes the rock but its acidity also dissolves minerals and salts, carrying them in solution downstream to the ocean. Additionally, the sodium and chloride ions that are present in freshwater sources are mostly not used by organisms and are transported to the ocean, where evaporation concentrates in flowing water leading to saline conditions. There are also mechanisms by which salt gets removed from the ocean, keeping the system in steady state, such as corals using calcium to build reefs, ions attaching to clay particles, and salts precipitating out of solution.
Inland water bodies receive salts in a similar manner. In arid regions and water bodies that have no outflow, evaporation can lead to salinization and the formation of inland seas. Examples include the Great Salt Lake in Utah, and the Salton Sea and Mono Lake in California (USA), the Dead Sea in Israel, and the Caspian Sea in Eurasia.
Terrestrial systems may also be saline. Salt may be carried inland from the ocean in prevailing winds and deposited in rainfall and dust, creating salt deposits in sediments. Erosion and release of salts from parent rocks, as well as the isolation and evaporation of ancient seas, may also contribute to the formation of saline soils. Often salts remain buried within the sediment profile, but agricultural practices such as irrigation and the removal and replacement of deep-rooted woody plants with shallow-rooted crops tend to increase salinities. Woody plants with roots of varying depths draw up fresh groundwater and shade the soil surface, thereby reducing evaporation and salinization. Shallow-rooted crops, on the other hand, do little to shade surface soils and do not tap into the groundwater, thereby allowing it to rise upward, pushing hovering salt layers to the surface. Furthermore, irrigation water is often high in minerals and salts, further exacerbating salinization. This has caused a shift in some arid regions from native flora and fauna and valuable agricultural species to salt-tolerant natives and exotics and barren areas, such as in the wheat belt in Australia.
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