One of the most important geochemical cycles is the sodium cycle. sodium is one of the major constituents of crustal rocks, sediments, and ocean water, and moves from each of these reservoirs to the other over long geological times. sodium is dissolved from crustal rocks such as granite by rainwater, then streams and rivers carry it in solution to the sea. sodium (Na) and chlorine (Cl) are the two most abundant elements carried in solution in ocean water. They combine to form the mineral halite (NaCl) that remains following evaporation of sea water. The conversion from dissolved sodium to sodium in the mineral halite is ongoing in many areas of strong evaporation along seashores around the world. At times in the geological past large sections of ocean basins (the Mediterranean Sea, Red sea, juvenile Atlantic ocean) have evaporated, leaving thick deposits of salts. Stream waters re-erode some of these deposits and carry them back to the sea, completing one circuit of the geochemical cycle.
When the salt deposits get buried on the seafloor the salt may be interlayered with oceanic muds, then the sodium is removed from the salts and transferred into clay minerals. Replenishing the amount of sodium in the ocean by river flow from the continents takes an estimated 65-100 million years. If the concentration of sodium (or other element in other geochemical cycle) remains the same in one reservoir such as the ocean basins, over time, then there is a balance between the input of that element to the system and its extraction to other systems. The amount of time it takes to replenish that amount reflects this balance, and is known as the residence time, obtained by dividing the mass of the element in the reservoir by the rate of input to the system.
Sodium in the seafloor sedimentary deposits can react with the basalt of the ocean crust, forming veins, and can replace other elements in the basalt. Ultimately these basalts and sediments containing sodium get subducted into the mantle, where some remelt to form igneous rocks that rise to the surface, containing minerals with sodium. These then are prone to erosion by rivers, leaching away sodium to be carried back to the ocean. other atoms of sodium are carried deeper into the mantle, forming the longest residence time arm of the sodium cycle.
Continue reading here: Carbon Cycle
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