Salinity of Seawater and Its Constituents

Study Notes

Average salinity is 3.5% (96.5% water)
Salinity varies between 30-38 psu (Practical salinity units)
Units previously determined by evaporation, now grams per kilogram or parts per thousand

Major Constituents: Primarily salt (NaCl)
Minor Constituents: Lithium, iodine, and other trace elements
Trace Components: Concentrations less than 1 part per billion (e.g., copper, radionuclides)
Nutrients: Nitrogen, phosphorus, silicon compounds
Gases: Nitrogen, oxygen, carbon dioxide, inert gases
Organics: Dissolved and particulate material, biological origin
Other elements: Found in trace amounts (e.g., Alkali metals like sodium, magnesium, calcium, potassium, strontium; halogens like chloride, bromide, fluoride)
Sodium concentration is 10.76
Potassium concentration is 0.39
Strontium concentration is 0.008
Borate is removed and replaced by boron concentration 0.004
Sodium chloride (NaCl) accounts for 85% of dissolved salts.
Salts dissociate into cations and anions.

Heavy metals (zinc, chromium, copper)
Concentrations in the nanomolar or picomolar range
Elements typically found in rows 5, 6, and 7 of the periodic table
Nutrients (various forms, examples given include nitrate, nitrite, ammonia, and urea)
Organic compounds (dead and living matter): Contains elements C, H, O, N, P, S and other nonmetals.
Inorganic particles
Pollutants : xenobiotic and natural compounds.

All elements are present in seawater, some at very low concentrations.
These elements may be dissolved or in particles
Concentrations in seawater can fluctuate globally, depending on inputs and outputs of water systems, but major salts' concentrations remain constant.

Rivers (weathering of rock, carrying dissolved cations)
Atmosphere (volcanic emissions, biogenic materials, windblown dust, rainfall, dry depositions)
Hydrothermal vents.

Key elements in order of abundance in the crust include Silicon, Aluminum and Iron.

Some salts (conservative like sodium and magnesium) are unchanged by biological/chemical processes
Other salts (non-conservative like nitrate, sulphate) undergo chemical and biological reactions (e.g., precipitation, oxidation-reduction)