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Processes Controlling Seawater Composition
Salts dissolved in seawater come from three main sources:
• volcanic eruptions
• chemical reactions between seawater and hot, newly formed volcanic rocks of spreading zones ( mid-oceanic ridges )
• chemical weathering of rocks on the continents
Volcanic eruptions produce large volumes of gases that eventually reach the oceans -- most important are sulphate and chloride. Submarine eruptions at spreading ridges inject gases directly into the oceans; gases from sub-aerial volcanoes are dissolved in rainfall.
Chemical reactions between hot seawater and recently formed basaltic ocean crust lead to removal of magnesium and some sulphate from the seawater, while other elements like lithium and rubidium are added. Ocean water is circulated through the ocean crust by this exchange mechanism. The entire volume of all the ocean’s water circulates every 5-10 million years. This is probably the main reason that the composition of seawater has been nearly constant over billions of years.
Many salts in seawater originate from weathering of rocks on land. As rocks are weathered to form soils, they release soluble constituents like silica and elements like sodium, calcium, potassium and magnesium. River waters also carry bicarbonate (HCO3-) - a by-product of weathering of silicate rocks or dissolution of limestone. Once they enter the oceans the dissolved salts remain, while the water continues to move through the hydrological cycle.
Most reactions occur between soil waters and silicate ( feldspars, quartz, etc. ) or carbonate ( e.g. calcite, dolomite ) minerals in rocks:
Silicate minerals + CO2 + H2O => Clay mineral + dissolved silica + ions ( sodium, calcium, bicarbonate, etc. )
Example: weathering of feldspar:
2 NaAlSi3O8 + 2 CO2 + 11 H2O = Al2Si2O5(OH)4 + 2 Na+ + 2 HCO3- + 4 H4SiO4
Limestone ( composed of the mineral calcite ) is completely dissolved by acidic waters:
CaCO3 + H2O + CO2 = Ca+ + 2 HCO3-
Because, there is geological evidence to suggest that seawater has retained the same salinity for billions of years ( at least 3.4 Ga ) other processes must be removing dissolved constituents from seawater. Some are removed by chemical reactions between seawater and the sediment ( e.g. manganese nodules. ) Others ( e.g. silica, nitrate, calcium carbonate ) are removed by organisms, including skeletal material. Other constituents, like sodium and chloride are removed as salts ( evaporite deposits ) when some of the seawater evaporates. This occurs when an arm of the sea is partially or totally cut-off from the open ocean and happens to lie in an arid climatic zone. The potash deposits of Saskatchewan were formed when an arm of the sea was isolated from the open ocean about 400 million years ago. Most of the water evaporated, leaving behind a thick deposit of salts. Wind blowing sea-spray also removes salt as aerosols which may be deposited along the coastline. Also, some minerals in seafloor sediments, such as clays, adsorb ( “grab” ) metallic ions from seawater onto their surfaces. When evaporites and seafloor sediments are uplifted, associated with collision of lithospheric plates, they again become subject to weathering and the dissolved salts eventually return to the sea.
